J-Pole Antennas for GPS

I normally use commercial antennas for GPS (e.g., patch antennas with a magnetic mount), but I needed more antennas than I had so I built a couple of J-Pole antennas for 1575 MHz.

I started with dimensions from an on-line calculator and built an antenna using a 1mm enameled copper wire. The return loss (SWR), measured with a portable VNA, was not good. I started again with a slightly longer antenna, which resonated about 100 MHz too low, and started cutting it until it resonated on 1575. The return loss is about -15dB; not great, but good enough. Given that there are 3 different dimensions to worry about (the length of the radiating half-wave section, the length of the quarter-wave matching section, and the location of the tap on the matching section), I decided not to try to tweak it to perfection.

Here is the result:

The antenna is soldered directly to two terminals of an SMA connector. This is a through-hole PCB connector, which has 5 terminals (5 for ground and 1 for the center conductor). I chopped 3 of the ground terminals so only two were left protruding, and I soldered them to the antenna.

I then made a second antenna with the same dimensions; it produced a -15dB return loss on the first try.

The length of the λ/4 stub in my antennas is 45mm, the lenght of the λ/2 radiator is 102.5mm, separation between the arms of the sub is about 3mm, and the feed point about 8mm from the shorted end of the stub.

In one of the antennas I inserted a 1000pF 0603 ceramic capacitor between the center conductor of the connector and the antenna, so that it does not short at DC. This allowed it to be used with GPS receivers that provide bias voltage to an LNA. Without the capacitor, the antenna shorts the GPS receiver, since the bias voltage is usually not current limited.

With this antenna connected to a NavSpark Mini GPS receiver outdoors, I was able to easily get a fix.

Building a CobWebb Antenna

Yesterday I finished building a CobWebb antenna, together with Itamar Melamed. The CobWebb is an HF antenna for the 14, 18, 21, 24, and 28MHz bands. It was designed by Steve Webb, who used to sell it (his web site seems to be inactive). The antenna is basically a fan dipole (five dipoles fed in parallel) in which the dipoles have been folded into squares. The folding makes the antenna compact; it fits into a square with a side of less than 3m. The folding also reduces the feed-point impedance down to something like 12.5 Ohm. The radiation pattern is omnidirectinoal.

I started the construction from the frame. I wanted the antenna to be rigid and to survive winds, so I built it out of fairly thick fiberglass poles, 25.4mm diameter with 2.85mm wall. The poles are very stiff. I could not find such poles at local distributors, so I drove to the factory that makes them (Pas-Gon), bought two 6m poles and had them cut into two 4m segments, one 1.3m for the feedpoint, and some leftovers. Driving back in a small car with the 4m poles strapped to the outside of the car was interesting (it’s a 2h drive). I decided to use 4m poles in order to achieve maximum strength; you can also make the antenna from four 2m arms.

The center support is a heavy aluminum bracket. I wanted to have the support made at a workshop, but it turned out that they had something almost suitable that somebody ordered but never picked up. They modified it for me by drilling holes and by cutting part of the aluminum to allow the lower poles to pass through. This part too is very strong and stiff.

There are two ways to feed the CobWebb. Webb’s original design is very clever. It used a balanced T-match to match the 50Ω coax to low impedance at the center of the dipoles. He constructed the antenna from twin-lead multi-strand copper wires, such as the ones used as speaker cables or mains zip-cord. One wire in each dipole is left whole; the other is broken in the middle and connected to the coax. The two leads are shorted some distance away from the center, in each side of the dipole; these are the actual feed points of the dipole. By moving the short towards the center or the ends, the resistance of the antenna at resonance changes. The use of the zip-cord T match makes mechanical sense and it feeds the antenna without any ferrite transformer. The design is well documented in a PDF document on the web, which includes measurements of the elements and the short points (I think the document was put together by Steve Topping, but it is not signed so I am not 100% sure; there is one error in it, reported by Alan Reeves: the sides of the 18MHz dipole should be 4040mm long, not 4400mm).

One builder, Steve Hunt, disliked the T-match arrangement and came up with an alternative matching mechanism: a pair of 1:1 current baluns arranged to create a 1:4 balun. Transforming 50Ω to 12.5 effectively requires a low-impedance transmission line, which Hunt achieved by paralleling two 50Ω coax cables. One aspect of the T-match that worried Hunt was the difficulty of moving the short point to acheive a 50Ω impedance. This is indeed not easy, and I, like many other builders, decided to try out the transformer feed. I was also worried that the optimal short points would depend on the specific properties of the zip-cord and that the measurements of the original design would not work with the zip-cord I decided to use. I built the transformer using RG-142 coax, which can handle a lot of power, and large ferrite toroids. However, the ferrites I used did not function well at 14-28MHz (the transformer did perform well below 1Mhz, but this is not useful for this antenna).

I decided not to try the original design rather than try other ferrite. Itamar and I started by preparing just the 18MHz wire assembly with temporary short points (not soldered yet) and connected it temporarily to a BNC socket. We hoisted the antenna and measured the impedance with a VNA (vector network analyzer) that we calibrated to the end of the coax. The results seemed good; the resistive part was close to 50Ω and the resonance was close enough to the target frequency that it seemed that by modifying the dipole’s length would tune it.

We lowered the antenna, built the rest of the dipoles, soldered the short points, built a more permanent feed-point box and connector, and hoisted it back up. Now it was time to tune the dipole’s length. The design uses dipole ends that are folded back, and Reeves advised to make the folded back sections longer, to make it possible to lengthen the dipoles; we did leave them longer. We measured the return-loss (SWR) dips to make sure the short-points were effective, and measured the resonance frequency (really the return-loss dip) at all bands. We calculated roughly by how much we need to shorten or lengthen the dipoles on each band, and tried to do this by changing the fold-back length, but without changing the physical length of the wires. This did not work, so I assume that Alan Reeves is right that the foldback essentially lengthens the dipoles. Next, we cut the elements whose resonance was too low and soldered pieces of wire to elements with resonance too high. This seemed to do the trick. Alan Reeves reported matching problems on 14MHz; our version works fine there too. We lowered the antenna one last time (out of 6 or 7), weather proofed it (I hope), added a coiled-coax choke, and fixed it in place.

The bandwidth of the antenna is less than spectacular. On 14MHz, the SWR≤3 bandwidth is close to 500kHz, so at the edges of the 14, 21, and 28MHz bands, the antenna might need a tuner.

Getting Started with the VNWA

I just got a vector network analyzer (VNA) from SDR Kits and after performing the calibration procedure, I decided to try it out quickly on a few VHF and UHF antennas. The version I got is called VNWA 3E. The antennas I tried were the 144-148MHz rubber ducky that came with a Kenwood TR-2500 (some 30 years ago), an 434MHz rubber ducky that came with the Texas Instruments (SPWH24433TI made by Pulse Larsen), an 144/430/1200MHz Diamond antenna (SRH815), short VHF and UHF antennas that came with a Baofeng UV3R, and a 145MHz PCB magnetic loop that I built a while ago. You can see the VNA and the antennas (ordered from left to right) in the picture on the right.

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The Kenwood antenna performs reasonably well in its design frequencies, but at least when mounted directly on the VNA (as opposed to being mounted on the transceiver it was designed for), it resonates a little higher than its design frequencies. When I placed my hand on the VNA, the resonance frequency dropped significantly, to well below 144MHz. Placing my hand on the VNA puts the antenna in a fairly similar environment to the one it was designed for, namely being on top of a handheld transceiver. Either with my hand or without it, the SWR between 144 and 148MHz was not much worse than 3:1 (return loss of -6dB).

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Staying on the same band, the Baofeng antenna performed a little worse. Without my hand the return loss was as low as -4dB.  With my hand on the VNA, resonance moved into the band and return loss improved.

 

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The Diamond performed poorly with or without my hand on the VNA. This is a pretty expensive antenna (about $30), so this is disappointing.

 

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The Diamond performs better at UHF, where its length is about λ/4 at 430MHz and about 3λ/4 near 1300MHz.

 

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The Pulse UHF antenna, which is also a  λ/4 whip, is the best performer of the lot. The SWR is less than 1.4:1 from 430 to 450MHz.

 

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The short UHF antenna that came with the UV-3R is okay at 430MHz, but pretty bad at 450MHz. Not too bad.

 

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Finally, the loop antenna. It is very narrow band, as expected from a small loop. The useful bandwidth is about 1MHz.

 

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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.