A small improvement to the DEMI LNA and a UHF J-Pole Antenna

I recently built two more UHF preamplifier kits made by Down East Microwave (DEMI) and a simple reference antenna to go with them.

The antenna and two preamplifiers feed a USRP radio with a WBX front end. The USRP uses a 6V power supply. This created a slight logistical problem for the preamplifiers. They contain a 78M05 linear voltage regulator that drops the input voltage down to 5V. The 78M05 requires an input voltage higher than 6V, which means that the power supply of the USRP cannot power the preamplifiers; I would need a separate power supply for them, one that provides 7V or higher.

This seemed a bit silly when so many low-dropout regulators are available. After searching a bit I found a 5V low-dropout regulator in the same package as the 78M05 that came with the kits (a TO-252 surface-mount package) and with the same pinout; a direct replacement. This regulator, TL720M05 by Texas Instruments, works down to an input voltage of 5.5V and can tolerate even higher input voltages than the 78M05. It does require a larger output capacitor of 22μF or higher; I used 47μF 10V SMD tantalum capacitors which fit on the printed circuit board without a problem. The parts were not expensive and I am happy with the improved functionality of the preamplifiers and the ability to use a single power supply for them and for the USRP.

Incidentally, I ordered these preamplifier kits with the optional enclosure and connectors; they fit together beautifully, as you can see below.

The antenna I built is a simple J-pole, which means it’s a dipole fed from one end by a quarter length matching stub. I computed the length of the stub and the radiating element using an on-line calculator. I left the radiating element a little long for tuning, soldered the coax to the matching stub at the point suggested by the calculator, and tested the antenna for low SWR with the FT-857D. The SWR was a little high. I tried the antenna up and down the 430MHz band trying to see in which direction to tune the antenna (hoping that tuning would require cutting the antenna rather than extending it). I eventually managed to tune it, but I discovered in the process that a J-pole is not an easy antenna to tune with an SWR meter alone. There are 3 tuning parameters: the length of the radiating element (which you can tune by chopping pieces off the top), the length of the matching stub (chopping pieces off its free end makes the stub shorter and the radiating element longer), and the connection point of the coax to the stub. The first two parameters should bring the antenna to resonance and the third should bring the resistive impedance to 50Ω. Eventually I got it close enough. In the picture on the right you can see the antenna with the coax connected. This short piece of coax is meant to connect the antenna to the preamplifier. Once it was tuned, I added a ferrite sleeve as a choke, put the antenna inside a PVC pipe with some foam to hold it in place (I also tested for resonance inside the PVC sleeve in case it affects the tuning a bit), and the antenna was ready. In the pictures below you can see the coax connection and the two halves of the ferrite sleeve, as well as the antenna mounted with a preamplifier on the roof.

  

Using a Motorola PageTrac and a VHF Antenna Tuner in an APRS iGate

My APRS iGate has been using the old Kenwood TR-2500 that you see in the picture for a few months. The radio was connected to an old commertial VHF antenna. This antenna is not resonant on 144.800MHz, but it is sturdy and high, and it receives very well. So I used it in receive-only mode; the iGate relays APRS messages from RF to the internet, but not the other way around. But when I got two old Motorola VHF radios, I decided to replace the TR-2500 by one of them, a Motorola PageTrac. This was supposed to bring two advantages. First, the Motorola radio would start up on 144.800 after a power loss, whereas the TR-2500 had to be programmed manually for 144.800 after a power loss because its memory battery has long died. Second, the PageTrac puts out 45W whereas the TR-2500 puts out only 2.5W, so if I start transmitting messages, the PageTrac would have a much better range.

The plan proved more difficult to execute than I had thought.

A Matching Network

The existing antenna is sturdy, high, and mounted on an old mast that I had no desire to climb. So I decided to keep using it. To transmit using this antenna, I needed a matching network: a single-purpose antenna tuner. I borrowed an MFJ-269 antenna analyzer from Nir Israeli and measured the impedance of the antenna and coax at the radio-side connector (at 144.800 MHz). I then designed an L match circuit using an online calculator.

I didn’t completely trust the analyzer and I was not sure that the air inductor I prepared had the correct inductance. So I used a variable capacitor in the L match, so that I could tune it. This worked out well. By adjusting the variable capacitor I was able to bring the SWR to a low value that is good enough for transmitting (I think it was 1.2:1 but I didn’t record it). The L match is narrow band and the SWR is reasonable only across about 0.5 MHz, but this is of no significance for APRS, which uses a single frequency.

The online calculator gave me not one appropriate L match but two. The other solution had a tiny series inductance and a capacitor across the transmission line. I tried it (with no inductance at all) and could not get the antenna to match. I am not sure why this matching network did not work.

Antenna tuners of this sort are almost never used at VHF, because it is pretty easy to build resonant antennas. But in my situation, with a good antenna that works well even though it is not resonant, this special-purpose antenna tuner is a good solution.

Power Supply Troubles

The two Motorola radios I got are a DeskTrac, which is a desktop version of the more common mobile MaxTrac, and a PageTrac. The DeskTrac is documented very well on the web. I didn’t find any useful documentation on the PageTrac, but it is a very simple radio, so I didn’t really need much documentation. Both radios worked when I got them, but I quickly discovered that the power supply of the PageTrac was failing. Two large electrolytic capacitors were burned, and they actually charred the PCB beneath them. They were buzzing and burning when the radio was turned on. I tried to replace them, but I didn’t find replacement 15,000μF capacitors. I decided to replace the entire power supply. It was rated at 13.8V and 10A, so I needed a power supply with similar specs.

The first candidate was a power supply from a dead computer. Its 12V output was rated at 16A, so I thought it would work. I spent a bit of effort trying to raise the output from 12V to 13.8V, but this caused buzzing in the PC power supply, so I left it alone. I assumed that running the radio at 12V rather than 13.8V would reduce output a bit, but this was acceptable to me.

The radio worked with the computer power supply, but whenever I tried to transmit, the power supply shut down. I am not sure why it shuts down, but my guess is that the roughly 10A that the radio draws from the supply on transmit causes the switching regulator in the supply to generate wider pulses. This maintains regulation on the 12V line, but it raises all the other outputs that have almost no load on them (5V, 3.3V, etc.; all are generated from the same switching waveform). The over-voltage protection circuit of one of these outputs might be what is shutting down the supply.

Next, I tried an old 12V/11A Lambda switching supply that I thought at a surplus store for about $5. I adjusted it to 13.8V. It works without a problem, and it is small enough to fit inside the PageTrac enclosure. It just sits in the enclosure without being bolted to it, but since the radio just sits on a shelf, this does not cause problems.

The Computer Interface

To use the radio as an APRS iGate, I needed to connect it to the soundcard of the computer running the iGate software. The interface that I used with the TR-2500 was not really an interface at all: just a cable connecting the TR-2500′s earphone output to the computer’s microphone input. But with the PageTrac, I needed some kind of interface, if only to activate the transmitter. I built a simple interface with (1) DC blocking and level adjustment on both the audio input and audio output directions, and (2) a PTT activation circuit. The PTT circuit is activated by the RTS signal of a serial port, and is isolated from the radio by an optocoupler. But the audio lines are not isolated, so the radio and computer are not really isolated. So far this did not cause any problems.

Initially, the interface did not work. When I would connect it, the radio would go into transmit mode, even if only audio cables were connected, not a serial cable. It took me a long time to debug, but eventually I discovered that I made a mistake in the wiring of the cable connecting the interface to the radio’s front speaker-mic connector. Once I fixed the mistake, the interface started working.

Limitations of the PageTrac

The PageTrac has some limitations. One is the fact that the only audio/PTT connector available is the RJ45 speaker-mic connector on the front. It has an RJ11 connector in the back, but it is undocumented. In contrast, the backThe DeskTrac has a DB-25 jack on the back designed for connecting it for to computers and other equipment. The second is that it has no “monitor” button to turn off the squelch. APRS works a bit better without a squelch, and the DeskTrac has a button that allows you to turn it off. I also think that unsquelched audio is available from the DeskTrac’s DB-25 connector. But even through the squelched speaker-mic connection, the PageTrac works fine. Another limitation of both radios is that the speaker is never completely muted (even at the lowest volume level). To prevent the radio from sounding the APRS packets, I simply disconnected one of the speaker’s wire.

Software Modem Issues

Soundmodem, the soundcard-modem that I have been using with the TR-2500, did not decode packets received by the PageTrac. I thought of adding a high-pass filter to the audio interface, to compensate for a possibly too-aggressive de-emphasis, but eventually wrote a new software modem that decodes packets from both radios without a problem. But this is a topic for another post.

After resolving all of these issues, the new iGate configuration is up and running. The iGate beacons on both the internet and on 144.800 MHz, and it relays text messages and other packets from the internet to mobile stations. In the screenshot below you can see me exchanging text messages with a mobile station (which uses a Kenwood D700). You can see both stations on the map, the text-message window of APRSIS32, and the actual packets that are received and transmitted by the iGate, some via RF and others via the internet.

A Lego Enclosure for an APRS Tracker with a Built-In Antenna

My APRS tracker seems to work quite well, but it was difficult to use with various pieces of equipment sloshing about on the dashboard. Initially, I’ve been using the short vertical antenna that came with the UV-3R radio. Both the radio/antenna and the GPS antenna were placed on the dashboard. This antenna did not work well inside the car, especially when it was lying horizontally with the radio on the dashboard. Then I switched to the small PCB magnetic loop antenna I built a while ago. This worked much better, especially when the antenna was standing up. But now I had three pieces of equipment (the radio, the loop antenna, and the GPS antenna) sloshing on the dashboard. This was only good enough for a bit of testing.

I was considering how to mount the antenna and the radio more securely on the dashboard. I didn’t feel like spending a lot of effort on building an enclosure. Then the idea of building the enclosure, or at least a prototype, from Lego. The box you see in the picture above is the result. It actually works very well. It’s a bit heavy, so it doesn’t move at all on the dashboard. It contains the antenna, the radio, and the GPS antenna. There are holes in the box for the audio cable to the microcontroller, for the GPS antenna cable, and for the display and buttons of the radio. Being able to turn the radio on and off is important, since it runs on its internal battery. The hole on the side is also large enough for the radio’s charging cable. The antenna and radio slide into specially-built compartments that hold them securely. It’s also easy to slide them out. Both antennas work well inside the plastic box on the dashboard (Lego bricks are usually made of ABS plastic). The box has enough space for a small microcontroller board and for a GPS module, but my current microcontroller board is a large development board, so for now it and the GPS module still lives in a cardboard box that I stick in the glove compartment.

As you can see in the snapshot from aprs.fi below, this Lego tracker actually works, even though it’s inside the car and even tough the UV-3R puts out only 2 Watts. Still, I plan to deploy a 1/4-wave vertical antenna on the car’s roof for even better coverage, and I’ll probably replace the Lego with some other box, mostly to avoid having the Lego bricks ruined by exposure to sunlight. But for now, I’m using the Lego tracker. It’s also very good to know that the 10cm magnetic loop antenna works well in APRS trackers; such antennas can be used in small trackers that are completely self contained.

Two more comments are in order. First, driving with a heavy tracker on the dashboard is possibly unsafe, since in an accident it might hit the driver or a passenger. I’m mostly using it around the neighborhood, but once I switch to the vertical antenna on the roof, I’ll move the whole thing off the dashboard. The other thing I wanted to mention is that I performed the two almost-necessary modifications of the UV-3R radio. One adds a capacitor to the VHF low-pass filter, to attenuate the second harmonic, and another adds a decoupling capacitor to the external transmit-receive control signal; without it, the radio often locks in transmit mode when used with a headset or an external modem.

Two Monoband Magnetic Loop Antennas

I recently built two more magnetic loop antennas, both for single bands. I built the first one mostly in order to find a good use for a piece of Heliax (coax with a solid copper shield) that I found discarded. Copper tubing is an excellent material for small loop antennas, so building a loop seemed like a good idea. The Heliax was only 1.60m long, so it only made sense to build a loop for one of the higher HF bands. The main difficulty in building a small loop is to find a suitable capacitor to resonate the loop. I decided to try to build the loop using two capacitors in parallel: a piece of coax that will form a high-voltage fixed capacitor and a small butterfly air-variable capacitor that I had. The butterfly capacitor had too little capacitance to resonate the loop alone. The coax would serve as a fixed capacitor, but I would be able to easily rough-tune it to the band by trimming it.

I decided to build the loop for the 21MHz band, because when I built it a couple of months ago, the higher bands were not open for significant periods of time (this changed later and I re-tuned the loop for the 28-29Mhz band). I soldered the ends of the Heliax to the butterfly capacitor and to a piece of RG-58 coax. This was not easy; I did not have a high-power soldering iron, and the excellent heat conduction of the Heliax’s shield made it difficult to heat it up enough to solder. I eventually managed to solder the capacitor, coax, and Heliax using short pieces of house wiring. It’s not pretty but it worked.

I trimmed the coax capacitor to the correct length using a borrowed antenna analyzer. I tuned earlier loops without one; it’s definitely easier with an analyzer. The coupling loop is the one I use with the coax loop. It’s larger than 1/5 the diameter of the Heliax loop, but it still resulted in impedance close to 50Ω. I set the air variable capacitor to maximum capacitance and trimmed the coax until the antenna resonated just below 21.000MHz. I could now resonate the antenna anywhere on the 21MHz band by tweaking the air variable. I made several contacts on 21MHz with the loop, but after a few days I re-trimmed the coax capacitor all the way to the 28MHz band, which was more active.

The loop with the RG-213 coax capacitor

I noticed something strange in JT65, a mode that transmits for 50s at a time. I would tune the antenna low SWR and transmit. At 10W or below, everything was fine. But at 20W and up, the SWR would rise from near perfect to horrible after a few seconds of transmission. The rise in SWR was rapid but not sudden; it would take several seconds. This was very consistent and it happened on both 21MHz and 28MHz. I realized that the power was doing something bad to the antenna, so I went out to check. There was a slight odor, and the end of the coax stub was burned (and warm). The high voltage across the capacitor was causing current to leak between the inner and outer conductors of the RG-58 coax, and that current was heating up the coax, burning it, and changing its capacitance (which is what caused the antenna to de-tune and the SWR to rise). I replaced the RG-58 with a much more robust RG-213. I tuned it to the band but kept the inner conductor longer by about 1cm than the outer shield, to separate the two conductors even more. The antenna can now withstand high power. I made many contacts with it.

I hoped that the antenna would be better than my coax loop, which is made with LMR-400 coax and N-type connectors, thanks to the solid-copper outer conductor and thanks to the soldered connections between the loop and the tuning capacitor. But on the air, there didn’t seem to be a significant difference between the two. The coax loop is a little larger (80cm diameter vs. 52cm), so maybe that compensates for its disadvantages. The coax loop is more flexible, since with a large air variable and a vernier dial I can tune it all the way from 7MHz to 29MHz, so I don’t use new Heliax loop much any more.

It was still fun to see that I can have reliable SSB and FM contacts (not to mention PSK31 and JT65) using such a small antenna.

An Update: I’ve finally soldered the Heliax directly to the tuning capacitor. I did well with this antenna during a recent contest, in which I made quite a few SSB contacts including to Australia, China, Japan, and quite a few other countries. I’m pretty happy with the antenna now.

The second small loop I built is a PCB loop for 144MHz. I envisioned it as a way to build APRS trackers that are really small: that is, have a small antenna, perhaps in the same plastic enclosure containing the transmitter and the GPS receiver and antenna. Furthermore, if an effective VHF loop can be built on a PCB, the antenna could be etched by a commercial PCB manufacturer, making is relatively easy to duplicate. PCB antennas are very common on UHF. They are also common in RFID tags, some of which work at 13.56MHz (where they are very inefficient but still good enough for RFID). For APRS, the antenna can be fixed-tuned to 144.800MHz, so the tuning capacitor can be fixed rather than variable.

I designed the loop to be a square with beveled edges. The square is 10cm-by-10cm, and the copper loop is 1.5cm wide, to make losses low. I am not actually sure that making the PCB conductor so wide would actually keep losses low, but it seems like a reasonable bet (if somebody can run simulations to determine this, I would be happy to read the results). I was hoping to tune the loop to resonance using a sliding capacitor that I would form from the two open ends of the loop itself and another piece of PCB material sitting on top of it, separated by either a small air gap or some polyethylene. I ended up, however, using a coax-stub capacitor, which you can see in the picture. I also used a gamma match to feed the loop, rather than a coupling loop.

The small piece of PCB was designed to form a tuning capacitor with the ends of the loop, when mouned on top of it

I had a lot of trouble tuning this loop. I started by installing the PCB air capacitor, which I fixed to the loop using nylon screws. My intention was to make slots in the PCB material that would allow me to slide the small piece of PCB up and down to change the capacitance, but initially I used simple holes that fixed the capacitor at maximum capacitance. I connected the loop to an antenna analyzer and tried to find the resonance point. I could not find any resonance point. I moved the gamma match up and down and tried again and again. I replaced the gamma match by a coupling loop. No resonance. I abandoned the project for a few days.

Prototyping the loop took quite a bit of work, to cut the PCB and to file away some of the copper using hand tools, so not being able to get it to work was frustrating. After a few days, I decided to use methods that worked for me on lower bands and to try again. I took apart the hand-made air variable capacitor and soldered a short RG-58 coax stub to the loop. And instead of the antenna analyzer, I used a receiver (in SSB mode) to search for a peak in the noise. I was able to find a weak peak below 144Mhz. I repeatedly shortened the stub and searched for the peak, which slowly rose toward 144MHz. Once it was near, I was able to transmit and to trim the coax to low SWR. The antenna is now tuned to 144.800MHz and gives acceptable SWR over a range of about 0.5Mhz.

The back of the loop, showing the SMA connector

I did some non-scientific comparisons of the loop and a short flexible handy-talkie whip, the 15cm-long Diamond SRH-815. There didn’t seem to be a lot of difference between the two antennas, which was a bit disappointing. So either I didn’t do the comparison properly (entirely possible), or this loop is not much more efficient than a short whip. For most uses a short whip is more practical, so right now I can’t really think of a good reason to use a small PCB loop on VHF. But I’m still happy I was able to get the antenna to work.

The Trombone Magnetic Loop for 50MHz

A while back I found another discarded piece of the orange polyethylene-aluminum-polyethylene tubing from which I already built several loop antennas. This piece was small, about 1.5m, and thicker than the tubing I’ve been using so far: the diameter of the aluminum tube is 16mm rather than 14mm. I decided to turn it into a 50MHz (6m) magnetic loop.

The main challenge when building magnetic loops is the tuning capacitor. After considering several designs, I decided to build a trombone capacitor using the loop itself as one plate, using 16mm polyethylene-aluminum-polyethylene tubing for the loop and the outer plate of the trombone, and a small piece of the 14mm as the inner plate. This is essentially a refined version of my unusual loop; in the unusual loop I squashed one end of the loop and stuffed it into the other end to form a capacitor. This time I would stuff a thin tubing into both ends of the loop, creating two capacitor that are connected in series by the thin inner tubing.

The thin tubing was too thick to slide into the thick tubing, so I had to file some of the outer polyethylene layer from the thin tubing. In the picture at the top, you can see scratches from this process and you can also see that the aluminum got exposed in some places. This is nothing to worry about, because the inner coating of the thick tube is about a millimeter thick, giving plenty of insulation. After filing away some of the polyethylene, the thin loop slid into the thick one.

Sizing the loop, sizing the capacitor section (the small piece of thin tubing that slides into the open ends of the thick-tubing loop), and tuning the loop were all difficult, because I don’t have an antenna analyzer. I could sometimes detect the frequency to which the loop was tuned by the peak in the noise, but not always, especially if the resonant frequency was way off. Also, the KI6GD loop calculator that I usually use to size loops and tuning capacitors refuses to go above 30Mhz. I initially started with a loop that was too large, so it resonated way above 50MHz even with no capacitor at all. I then decided to scale down the loop from a 25MHz design; this suggested a 38cm loop, which turned out to be a good size. I made some rough calculations as to how much capacitance each centimeter of the trombone would produce; I did not attempt to measure the capacitance, which perhaps I should have. The capacitor section too started much too long, but after I cut it to about 9cm I was able to tune the loop near 50.1Mhz, which was my goal. Tuning took a long time. Once the loop was tuned, I secured the trombone in place using a lot of electrical tape. It’s been several weeks since, during which I moved the loop many times, and it remained tuned. Even being in the sun does not seem to detune it significantly. The coax is coupled to the loop 1/5-diameter loop made from stiff copper wire. The coax connector and the coupling loop are secured to an ABS base using lots of epoxy glue; the base is secured to the loop using two cable ties.

A relatively large magnetic loop that requires very little capacitance to tune has a relatively wide bandwidth. That, together with the fact that the 50MHz allocation here is only 200kHz wide (of which the lower 100kHz is used mostly for beacons), means that a fixed-tune loop is good enough for me.

I don’t know how good the capacitor is (how high its Q) and I didn’t measure this, but in theory the Q should be high, because polyethylene is a good dielectric; Peter Rhodes used polyethylene bags (e.g. freezer bags) in his PIC-A-TUNE antenna tuner to build high-Q high voltage capacitors. In practice the coating of my tubes might have some additives that lower the Q, so I can’t say anything definite.

I have not found too many other designs for 50MHz loops. Ed Bosshard describes a couple on his web site (both larger than mine). Juan Antonio Siles Sanchez published two videos documenting (in Spanish) the construction of a square copper loop with a trombone capacitor; the loop itself forms the outer tube of the trombone, as in my antenna, and a threaded rod forms the inner part.

Higher in the VHF and certainly at UHF, magnetic loops do not make much sense, because the size of half-wavelength antennas and even full-wavelength loops is manageable, even if you do not have a lot of space. But the 3m that a 50MHz dipole would require may still be large enough to make a magnetic loop attractive.

So far I’ve made only local contacts with the loop, but there is no reason that it would not work for long-distance communication when conditions are good enough on the band. Relative to the wavelength, this loop is larger than my coax loop is at 14MHz and 21MHz, bands at which I’ve made many long distant contacts with the coax loop.

An update from early June 2011: Yes, the loop works fine for contacts with distant stations. So far I’ve made two SSB contacts with European stations and heard many more in several modes. The loop also seems to stay tuned, which is something I worried about a bit.

A Tripod Mount for the Satellite Yagi

A tripod mount made the Satellite Yagi a lot more usable. I found a few brass anchors that fit a tripod screw. I cut the wood beam of the Yagi so that it was just a bit longer than the antennas themselves. A small piece of the remaining lumber, drilled a hole that fits the anchor through it and forced the anchor through. There a bit of glue, but it’s mostly pressure that keeps the anchor in place. The anchor extended from the other side of the piece of wood, so I drilled the boom too to take the extra piece. Finally, I secured the piece of wood with the anchor to the bottom of the boom using both glue and screws.

The antenna is now easy to rotate by hand as the satellite moves overhead. I managed to receive many different satellites and to work AO-51 from my balcony, with the antenna essentially indoors. (I can only hear and work the satellites when they are east of me, because the balcony faces east.) The pictures below show a closeup of the tripod mount and the antenna mounted on the tripod.

Finishing Up the Satellite Yagi

Over the past week I finished the satellite Yagi. I added a VHF Yagi and a band splitter to the UHF Yagi that I constructed first.

Kent Britain, the designer of the antenna, wrote that you can obtain good results with a 2-element 145MHz antenna, but I decided to stay on the safe side with a 3-element antenna, like the one described by Richard Crow. I noticed that Crow’s 3-element antenna was a little different than Britain’s, and decided to build to Britain’s measurements. I first cut the elements out of the same stiff copper wire that I used for the 435MHz Yagi. I was not sure, however, how to combine the two antennas into a single structure. Britain suggested to mount the VHF antenna behind the UHF antenna, on the same boom and on the same plane. This makes it easy to transport and store the antenna, since it is flat, but the boom is pretty long. Crow mounts the antennas side by side; this is easy with his very lightweight construction, but would be harder with wood booms. Shamai Opfer suggested to mount them on separate booms connected by hinges, so I can store them together flat but flip them into a crossed configuration for operating.

I eventually decided to start by mounting them one behind the other on the same boom, and to saw them apart later if I want to try a different configuration. I initially mounted the VHF Yagi 6 inches behind the UHF one, but when I re-read Kent’s article I realized that h wrote that they must be spaced only 3″ apart. So I removed the VHF elements, drilled new holes in the boom, and mounted them in the correct places. I soldered a connector to the VHF Yagi and verified that it was a good match to the transceiver. I was; no tweaking was necessary. I was also able to receive stations communicating using Morse code through VO-52, a satellite with a linear transponder whose downlink is on 145MHz.

To communicate through satellites with the FT-857, I also needed a band splitter, a 3-port filter that would allow the single VHF/UHF connector of the radio to be connected to the two antennas (you don’t need it if you use separate UHF and VHF radios). Kent Britain’s article describes a simple splitter consisting of two 3-component filters, a high-pass for the UHF port and a low-pass for the VHF port. I did not have capacitors with the values in Britain’s article, so I designed my own splitter with Elsie, a filter design program.

Elsie has an option to design diplexers, which are band splitters that aim to keep the impedance at one port constant over a large frequency range. Diplexers are used following mixers, for example, where you want a proper termination for the image frequency and to various spurious frequencies that the mixer might output. You can use a diplexer as a band splitter, but if you don’t need constant-impedance termination you have much more freedom to design the low-pass and high-pass filters. I designed the two filters by telling Elsie to design a Chebychev filter and playing with the cutoff frequency and ripple settings until I got the capacitor value I wanted in combination with very little attenuation at in the 145 and 435MHz bands. My capacitors were one 12pF cap for the 145MHz low-pass filter and two 5pF units for the 435MHz high-pass. This generated two inductor values. I then used Elsie inductor calculator to come up with appropriate air inductor designs. I played around a bit to get a sense of what inductance different diameters and turn counts  gave. When I got close, I designed the actual filters by specifying the diameter and number of turns. For the UHF filter, I specified 3 turns and a 0.5cm diameter; a length of 0.9cm gave the proper inductance. For the VHF filter, I specified 6 turns and a 1cm diameter, which required a 1.9cm length. I wound the two VHF inductors from the same piece of wire and then bent it so that the inductors were at a 90° angle, to minimize coupling. The UHF inductor was mounted upright, also at a 90° angle from the VHF coils. The wire for all inductors came from a choke from a broken PC power supply.

I connected the antennas and the radio to the splitter and tested the match. I was almost perfect on both UHF and VHF. I could open repeaters on both 145MHz and 435Mhz.

I clamped the wooden boom to a camera tripod, as you can see in the picture at the top. The weight of the antenna is not balanced on the tripod. To relieve some of the force it applied to the tripod’s head, I tied the back of the boom to a leg of the tripod with a cord. It works, but I’ll need to find a better solution.

The next step was to try the antenna on an FM-repeater satellite. Yesterday two of them passed overhead at reasonable hours, allowing me to try satellite communication for the first time. The first was AO-27, a very old satellite (launched in 1993). It’s web site specifies a schedule, which suggested that only its telemetry beacon would be active when it was over me. I tried to receive the UHF signal nonetheless. When it came over the horizon, I could hear the quieting in the receiver, but then heard a voice station calling. I replied and was able to work that station and 3 more. The next repeater sat to pass over was AO-51, whose repeater is active all the time, and which I was able to hear well when I only had the UHF antenna. AO-51 was much more crowded, with stations transmitting at the same time (and causing severe interference), but I was still able to work a few stations. Tracking the Doppler shift on FM is not hard; I’ll explain the issues in another post. I tracked the location of the satellites by manually rotating the tripod a couple of times during each pass.

I am very pleased that the antenna allowed me to communicate through the satellites. The antenna is sitting in my balcony. As you can see in the picture, it is essentially indoors, facing a large window. There’s a ceiling above it; the walls around it and the ceiling are all reinforced concrete. From the balcony, about 120° of the sky is visible, towards the east and north. When satellites are to the west or south of me, I can’t see them at all. But in spite of all of these limitations, I was able to communicate through the sats.

Building the UHF Cheap Yagi for Satellite Communication

One of the reasons to buy the FT-857D transceiver was to experiment with satellite communications. There are many antennas that are suitable for satellite communications in UHF and VHF. After investigating a bit I discovered articles on so-called cheap Yagi antennas that seemed easy to build. I started with the 435MHz Yagi, which is a little easier to build than the 144MHz version.

The antennas were designed by Kent Britain starting in 1994. Kent’s web site contains one document with designs for the satellite bands and another with construction data for other bands. Kent suggest to build the boom from wood, which is what I did. For the elements I used stiff copper wire that I salvaged from a piece of power line that the local utility discarded when they replaced a power pole near my home. At least for the 435Mhz elements, which are about 35cm long, it is stiff enough. The boom is made of a 240cm piece of 18mm-by-36mm lumber. The 6-element Yagi I built does not use the entire 240cm length, only about 75cm. I drilled the holes for the elements with a hand drill.

Three articles by Richard Crow on the AMSAT web site shows how to build satellite UHF and VHF Yagis with a boom made of a lamination of foam board. For some people they might be easier to build than the wood-boom Yagis, and they are lighter. I found that constructing the wood boom with hand tools very easy; I think that cutting and laminating foam boards would have taken me more time.

The antenna was as easy to build as Kent claims. I just built the antenna according to the measurements in his article (Crow gives the same dimensions except for the separation in the driven element, which Kent wrote to me is not critical). I then soldered the coax and tested. The antenna had low SWR without any tuning.

On the right you can see the discarded power line I used for the elements and the connection of the feed line to the driven element.

After I finished building the antenna, I tried to receive a signal from a couple of satellites, but I heard nothing. The satellites I tried to receive were operational according to the AMSAT web site (click on Sat Status), they were overhead according to the satellite tracking module of Ham Radio Deluxe, but even when the receiver was tuned to the correct frequency and the antenna aimed in the general direction of the satellite, I could not detect any signal. I did not expect this to be easy, so I was not surprised at all.

The next morning I tried again. Again I could not detect the first satellite I tried to receive, but the second attempt was successful; I was able to hear the Morse beacon of CO-58 very clearly. I was very happy. Experiencing the Doppler shift for the first time was really interesting.

[An update from March 18: I was also able to hear people communicating through the FM repeater of AO-51; very cool]

To actually communicate using satellites, I’ll also need a 144Mhz antenna. Richard Crow’s construction articles show how to mount the two Yagis side by side on a camera tripod. The configuration is the same as when two Yagis are mounted on azimuth-elevation rotators. It works on a tripod because Crow’s Yagis are very light; I am not sure that this setup will work well with wooden booms. Kent Britain’s article shows a 5-element 435MHz Yagi and a 2-element 144MHz Yagi on the same boom, one behind the other. This is more convenient, but would be a bit awkward with longer antennas. The Arrow II antenna puts the two Yagis on the same length of boom, one vertically and the other horizontally. This is more compact than putting them one behind the other, but it makes it hard to stow and transport the antenna. This is not an important concern if you keep it mounted outdoors, or if you take it apart when you stow it, but the cheap Yagis are not easy to take apart. I’ll have to ponder this issue for a while.

Satellite communication with a single transceiver also requires a band splitter; fortunately, Kent Britain’s article also shows how to build a simple band splitter; I’ll probably build it after I build the 144MHz Yagi.

An Update on the Coax Transmitting Loop

I’ve been using my two transmitting loops for a while now, so I now have a better sense of how they perform and of their usability.

I don’t use the “unusual” loop much. According to the KI6GD Magnetic Loop Antenna Calculator, it should be about 50% efficient (this figure depends on the material, aluminum, the diameter of the conductor, the circumference, and the frequency, 14MHz). I can tune it, but pushing the gimmick capacitor in and out of the tube is not a convenient tuning method; this is why I don’t use it much.

It’s a little difficult to tell how efficient the coax loop is. The calculator program says it’s 32% efficient on 14MHz, but it is not made of solid copper but rather the coax braid and inner conductor. The two N connectors also add resistance, but I don’t know how much. The program estimates the resistance of the loop to be 0.078Ω, so if the coax and the connectors add something in this range (which is very little; if you use the coax and connectors as 50Ω transmission lines, 0.078Ω is almost nothing), the antenna could be a lot less efficient than the 32% that the program calculates. On 7MHz the theoretical efficiency is only 4%, 12% on 10MHz, 66% on 21MHz, and 84% on 28MHz.

I use mostly the coax loop because it is easy to tune. Its variable capacitor, which you can see in the picture, is driver by a vernier dial, so it’s fairly easy to tune. I can now do it by ear most of the time: I turn off the automatic gain control (AGC) in the receiver and tune to maximum noise. This often gives acceptable SWR. If not, I reduce power to about 1.25W (5W in AM mode) and tune by watching the SWR meter.

When I started using the loop with the FT-857D, I wanted to be able to transmit on 7MHz, to participate in the local weekend net. The tuning capacitor is a 4-section type with a common shaft. I initially soldered the loop to two sections in a split-stator configuration (the loop was connected to two stators and the rotors are connected by the shaft and floating; this puts the two sections in series without a moving contact). The antenna did not tune on 7MHz, so I connected the two remaining sections too. This allowed it to tune from below 7Mhz to about 24Mhz. In the picture above you can see one section-to-section jumper in place and one half connected; the jumpers are made of RG58 coax.

In this configuration, I was able to participate in the 7MHz net, but my signal was obviously weak (even in the best case that the loop is indeed 4% efficient, I was only emitting 4W peak). Since the higher bands began to open in the last few days and since my signal was very weak on 7MHz, I decided to modify the antenna to target higher bands. I unsoldered the jumbers to leave only 2 sections connected. The loop now tunes from the upper part of the 7MHz band to 30MHz or so.

Over about four weeks, I was able to make PSK31 contacts on 14, 18, and 21MHz, JT65A contacts on 14MHz (my serial interface injects noise on the 21Mhz JT65A frequency; I’ll investigate, but so far this prevented me from working there), and SSB contacts on 14Mhz and 21MHz. I did not manage to operate when the 28MHz band was open, but I expect that I can make contacts there too.

Working PSK31 and JT65A stations with this antenna on the higher bands is easy. Stations usually hear me and reply. In PSK31 I worked stations from all over Europe; on JT65A also in the Philippines and Sri Lanka. SSB is not so easy. In many cases I can hear the a station very well but it does not hear me. Working with such a small antenna requires patience. But I did manage to work European stations on SSB. I normally get weak signal reports. But when conditions are good, the loop permits clear and pleasant communication; I was able to have a long chat today with a German station 2800km away from me. A big antenna on the other side obviously helps too.

The antenna does not arc across the capacitor even with 100W SSB and 50W in digital modes. I was worried about this possibility but it did not happen.

Small loops have a narrow bandwidth, but I have not found this to be too annoying. Once I tune it in a given part of the band, say 21Mhz SSB, I can tune the transceiver up and down quite a bit without having to retune. Maybe this is an indication that the antenna is less efficient than the KI6GD Magnetic Loop Antenna Calculator estimates. But it’s convenient.

Finally, the experience with the loop so far validates my decision to buy a 100W transceiver. At some point I considered buying the FT-817, which outputs only 5W but is otherwise very similar to the FT-857D. An inefficient antenna and a low-power transceiver would not be a good combination; but with a relatively inefficient antenna and a high-power transceiver you can communicate.

Building Chavdar Levkov’s Active Wideband Loop

A little while ago I discovered on the web yet another design for an active wideband receiving loop antenna, by Chavdar Levkov. Chavdar clearly analyzed both the requirements that the amplifier needs to satisfy and the circuit that he built very carefully. Therefore, I hoped that this antenna would outperform my existing active wideband loop, which uses a different amplifier, designed by John Hawes. I use my wideband a lot; it is sensitive (much more than the active whip) and does not require tuning (unlike the tuned receiving loop and the transmitting loop). Therefore, I really welcomed the possibility of a more sensitive receiving antenna that would still not overload.

The basic difference between Chavdar’s amplifier and John Hawes’ is that Chavdar’s input stage is a grounded base amplifier with a very low input impedance, which matches the impedance of the loop better than Hawes’ common emitter amplifier. The grounded base amplifier drives a common emitter stage, so Chavdar’s amplifier is also a bit more complicated to build, but not by much. When I wrote to Chavdar with some questions on his design, he compared it to Hawes’ and wrote back that he thought that his own design has higher dynamic range and higher gain at lower frequencies, but perhaps also more noise at high frequencies. I decided to give it a try (particularly since I wanted another antenna of this type anyway, because the existing one is tied all the time to a WSPR spotting activity).

One interesting aspect of Chavdar’s design is the use of networking cable rather than a coax. The CAT5 or CAT5e cable carries the power to the amplifier and the signal back on two separate twisted pairs; in my earlier active antennas I used the conventional method of putting the DC and RF on the same coax and separating them with bias tees. I was a bit hesitant to use networking cables, mainly because I did not want to deal with the 8-pin sockets, which won’t fit into a 0.1″ breadboard hole pattern. Chavdar encouraged me to use networking cables and sockets, because they are cheap, deliver high performance, and they eliminate the need for bias tees. I eventually realized that if I mount the sockets upside down and wire them ugly style, I could probably use them. It worked. (I later discovered that this is exactly how Chavdar mounted the sockets; I didn’t study the pictures in his article carefully enough.)

Before I could put the sockets into the circuit, I had to figure out how to ground the cable shield on the receiver side but not on the antenna side. Until then, I never paid any attention to shielding in these cables. I looked at one plastic connector connector and couldn’t figure out where the shield connection was. It was not there at all, because some of these cables are unshielded (they are marked UTP) and have plastic unshielded connectors. Shielded cables, marked STP or FTP, use connectors with a metal shield that’s connected to the cable’s shield. After I figured this out, I mounted the two sockets; on the receiver side, I mounted a metal-shielded socket by soldering it to a scrap PCB; on the antenna side, I super-glued a plastic socket to the PCB.

The rest of the construction is pretty similar to the way I built Hawes’ amplifier, both in terms of circuit construction and in terms of the enclosure and loop connection. Chavdar’s design used a 10V regulator on the antenna side, powered by an 12V input supply. Chavdar used a 7810 fixed-voltage regulator which I did not have, so I used an adjustable LM317 regulator. This caused me a some problems. I initially put a REG1117 regulator in the circuit. It promptly died. I replaced it and the replacement also died almost immediately. At that point I read the data sheet more carefully, and realized that the regulator needs 2 protection diodes if it is used with large-value capacitors. I used a 10μ tantalum cap on the input terminal and 22μ tantalum caps on the output and adjustment terminals; these caps can easily provide enough current to destroy the regulator when you power the unit down, and they did. I then put in the diodes, replaced the dead REG1117, and put in a new regulator (this time an LM317, but I could have used a REG1117). Now the regulator section works fine.

On the receiver side, you need a unit with an RJ45 socket, BNC to connect the receiver, and DC power input (and some filtering for both, which I did not yet put in). I initially mounted all 3 sockets on a piece of PCB, for lack of an appropriate enclosure. A little later, I realized that since the amplifier unit contains a voltage regulator, there is no need to power the amplifier using a regulated 12V supply; an unregulated one would work just fine. I therefore added a little mains transformer, diode bridge, and filtering (a pi network with two electrolytic caps and an inductor on the receiver side. There’s still no enclosure, but it’s more convenient than before. It looks weird, but it works.

I did not compare the antenna carefully to the one with Hawes’ amplifier. The loop element I used in both antennas is exactly the same, so comparing the antennas should show any differences between the amplifier circuits. But I have not done that. In casual listening, the antenna performed very well. During the SSB context over the weekend, I was able to hear many US SSB stations; I never heard any with the Hawes loop at the same location (ouside 1st floor balcony in an urban area about 10,000km from the US east coast) . This does not say much, because the contest brought many strong stations to the air, and propagation conditions were good over the weekend. But this definitely made me happy. The antenna performed fine from 3.5MHz to 24MHz (28MHz was closed when I was listening). It also receives the airband (VHF AM) and 144MHz and 430MHz, but on these frequencies it’s a little less sensitive than a 144MHz dipole. It works on MF, but not as well as I hoped; I can receive BBC World on 1323kHz, but not as well as in my car.

I now agree with Chavdar that using networking cables and connectors for receive antennas is a good technique. I plan to replace the cabling in my tuned-loop amplifier, because it needs not only RF and power, but also tuning voltage for the varactor diodes; on a CAT5e cable, I can just use a third pair for the control voltage.

I am indebted to Chavdar for both putting the design and the detailed analysis on the web and answering all my questions via email; thanks!