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!

A Wideband Receiving Loop

The antenna I’ve been using the most recently is a wideband receiving loop. The wideband nature of the antenna allows me to quickly find out what’s going on on different bands without having to re-tune the antenna.

The loop element is the one I built for the active tuned loop. The amplifier was designed by John Hawes; I built about two years ago in an enclosure designed for wire loops. The wire loops did not work well. But I found that the amplifier worked well with the aluminum tubing loop element, so I built a new enclosure that can be weather-proofed. This setup works very well.

The original enclosure, shown below on the left, was great for experimenting with different loop elements, but the connections where exposed to the elements when the amplifier was left outdoors. I used it not only with loops, but also with a short dipole (old telescopic TV rabbit ears); it worked, but not terribly well. The loop element works much better.

The antenna works well from at least 7MHz to 14Mhz, which is the range of my Softrock radio. In the map below you can see WSPR reception reports from one night, mostly at 7MHz. It also works at 18MHz (the Softrock received some WSPR signals there even though it’s input filter attenuates them significantly). I suppose that the antenna works well below 7MHz, but I can’t test this right now. An article by Chris Trask shows that the resistance of a 1 meter loop element is low up to about 15 MHz, where it starts raising very quickly (more on his article later). So I suspect that the amplifier, which has a balanced 50Ω input, is reasonably well matched to the loop only up to about 15MHz. I’ll test the loop at higher frequencies at some point.

The main problem with wideband preamplifiers, like the one I used, is that they are prone to overloading by large signals at frequencies far from the one you are interested in. I did not experience any obvious overloading problems. But it’s of course possible that if somebody starts transmitting close to me, the amplifier will overload. Chris Trask’s article, which I mentioned above, describes a wideband loop preamplifier that’s a lot more sophisticated than the one I built. It’s dynamic range is probably much higher. But since I did not experience any problems yet, I was not eager to build the more complex circuit. (His article, wideband loop antenna amplifier, is available on a Yahoo group; you need to register to access it.)

My build of John Hawes’ amplifier is almost an exact reproduction, except that I used a ready-made 1:1 output transformer from Coilcraft, a WB1010-PCL with a 780μH inductance, rather than a ferrite torodial transformer. It works well.

Failure to Use WSPR to Compare Antennas

Earlier this morning I decided to try to compare my the performance of my three loop antennas using WSPR. Because the WSPR protocol allows you to transmit and receive beacon transmissions and since it records the signal-to-noise ratio (SNR) of each received transmission, I thought it would be easy to compare antennas. I was wrong. It is not easy, even with WSPR.

The plan was to collect around an hour of spotting reports with one antenna, switch to another for an hour, then a third. By comparing the SNR of my signal at some specific remote stations, I would be able to compare the effectiveness of my transmitting loops. By comparing the SNR of some specific remote stations here, I would be able to compare the receive effectiveness.

I ran the experiment and started to look at the data. What I discovered very quickly was that the SNR reports varied considerably even with the same antenna. Each station beacons only every 10 minutes or so, so there is usually 10 or even 20 minute difference between signals of the same station. During this time, propagation conditions along the path change. Here is a graph of the SNR of several stations I received over a couple of hours (the graph only show the stations I received more than 5 times).

It’s easy to see that the SNR changes quickly and dramatically. DK8FT went up by about 10dB in 40 minutes. UA3ARC took a dive of about 10dB at the same time that the signal of OH2GAX was getting stronger. To make sure that this was not something caused by something going wrong at my end, I plotted the SNR of signals received by another station, UA3ARC, over several hours (covering the time in the graph above). The results are similar.

The signal of G4IHZ degraded by 15dB in less than an hour. The correlation between the SNR of the two English stations, G4IHZ and G4CUI, shows that the variations are caused by changes in the propagation along the path (the two transmitter are close so the paths to the receiver are similar).

Clearly, you can’t compare antennas using WSPR using the naive technique that I was using. The differences I saw between antennas were on the same scale as the variation of the reports from a single antenna, so it’s impossible to compare the antennas this way, at least not without filtering the huge noise in SNR that the changing propagation creates.

Other people who took the time to run similar experiments for long periods of time and to average the reports got more robust results. Patrick Destrem switched between two transmitting antenna and averaged the results. He used a computer-controlled antenna switch to switched between the two antennas, so he could use a different one in every WSPR transmission cycle. It is not exactly clear how he got the smooth curves that suggest that his vertical antenna is better than the dipole, but the curve does seem to show this. His plot of all the reports is very noisy, like mine, so the averaging is crucial. I think he fitted a low-degree (probably degree 4) polynomial to all the data points. This fitting is a bit strange, because it suggests reasonable propagation even between 2m and 6am, when in fact there was no propagation at all. Patrick used a similar methodology to compare two receiving antennas, although with a relatively small number of data points.

Stu Phillips compared two transmitting antennas by beaconing from each one for about a week and comparing the reports. This is not as good as Patrick’s rapid-switching method, because propagation in the two weeks is not necessarily the same. He did draw some interesting conclusions from the results, but they the differences were not as dramatic as he hoped they would be.

Charles Preston compared two transmitting antennas by beaconing from them simultaneously, using two separate transmitters. This is even better than Patrick’s rapid-switching method, but this requires more equipment, of course. Unfortunately, he did not collect a lot of data, so it’s hard to draw conclusions from his results.

In hindsight, using WSPR to compare the two transmitting loops was a lousy idea. What I wanted to know was which antenna was more efficient. That is, which one converts more of the input power to electromagnetic waves and less to heat. It probably makes more sense to figure this out with a field-strength meter than with SNR reports from thousands of kilometers away. The WSPR comparisons are probably useful for evaluating the usefulness of radiation patterns, but all my loops should have pretty much the same patterns. I’ll try this again with a field-strength meter to see if I can see a difference.

A Tuned Active Receiving Loop

Charles Wenzel presents on his web site a simple and clever tuned receiving loop antenna, which works very well. The antenna consists of essentially three parts: a large wire loop, a varactor diode (voltage-controlled variable capacitor) that forms a parallel resonant circuit with the loop, and an amplifier that transforms the high impedance of the parallel tuned circuit to a low impedance for driving the coax and the receiver.

There are two clever ideas Charles’ design. One is the use of parallel resonance, rather than the series resonance that many active loop circuits use. The use of parallel resonance allows the design to maintain balance without transformers, and I think that it also reduces the significance of Ohmic losses, although they are probably less important in receiving antennas than for transmitting antennas, at least on HF. The other clever idea is the use of a differential video IC, a TL592, as the amplifier. The chip has high-impedance input, high enough bandwidth for HF, and is very easy to use. The amplifier part of the antenna consists of the chip, 5 resistors, and two capacitor. No transformers at all. You can set the voltage gain of the TL592 to values between about 15 and 400; the amplifier sets it to 15.

Charles not only put the design on his web site, he even sent me a few TL592 to try it out! Thanks Charles.

I built the loop out of the same polyethylene-coated aluminum tube that I used for my transmitting loop. I filed away the polyethylene at the ends of a 90cm-diameter loop and drilled holes for screws.

The amplifier is built on a piece of unetched PCB in which I scored pads with a knife. Two large pads with holes are used to connect the amplifier to the loop. I didn’t have the varacator that Charles used, so I used a pair of MV1404’s that I had in parallel, for increased capacitance. The flat ribbon cable that exists on the right carries power and control voltage for the varactors.

Charles’s amplifier includes a relay that can connect an inductor in parallel with the varactor, to cancel out some of the capacitance so that the antenna can tune higher in frequency. I didn’t include one yet.

To tune the antenna, I attached a 10-turn potentiometer to a piece of board to which I also attached the control wires and a 12V jack. Because the voltage limit of my varactors is 12V, I added a 2.7k resistor in series with the 10k potentiometer, to limit the control voltage to a bit over 9V. A 2k resistor is probably more appropriate, allowing the varactors to be biased at up to 10V.

That’s pretty much it, as far as construction goes. I didn’t put the amplifier in a weather proof box, just screwed the loop to it. This is good enough for experimentation, but not for extended use.

How well does it work? It works fantastically. Just outside the balcony, the 90cm loop pulls in lots and lots of signals. One evening I monitored the 7MHz, 10MHz, and 14MHz PSK31 frequencies, and was able to receive many stations on each one of them. I left the software running overnight monitoring 14MHz. The pskreporter.info map below shows the stations I received on all three bands (14MHz in orange, 10 in green, and 7 in blue). I was able to receive numerous European stations, some in the middle east, two in Africa, and several on the east cost of North America. I also logged one Japanese station now shown on the map.

The following night, I left the software running monitoring 7MHz, but with an even simpler receiver, a Softrock Lite II, a $10 kit. The results are just as good, again reaching the east coast of North and South America. The two African stations are spurious reports; they appear to be incomplete call signs that pskreporter.info incorrectly assigned to these countries (nobody else reported them, as opposed to the 14MHz African stations that many monitors reported the night before).

As Charles writes, tuning is not critical. He placed a 4.7k resistor across the parallel tuned circuit, which reduces the circuits Q. I’m curious as to whether the signal-to-noise ratio would improve if I increase this resistor (at the expense of more difficult tuning), but I have not yet tried that. With the 4.7k resistor, my 10-turn potentiometer is more of a liability than an asset.

I was a bit nervous about tuning a receiving antenna. Charles writes that you need to tune for maximum received noise. This is a bit more challenging than tuning a transmitting antenna, where you can tune by looking for a dip in a meter or a LED, but it works. It’s easy when the band is open and you can receive signals and harder when the band is almost dead. You may wonder why one would want to tune an antenna to a band that’s dead, but the point is that you don’t know it’s dead until you tune the antenna. When the tuning is completely off, you receive essentially nothing even when the receiver is tuned to a strong station.

The simplicity of the amplifier seems to come at a certain cost. At some positions of the tuning potentiometer, strong AM station appeared where they don’t actually transmit (that is, in the middle of an amateur band). As I continued to turn the tuning knob, the station would disappear. I think that this happened mostly when the tuning was a bit off, but not completely off. Maybe these were shortwave broadcast stations in the bands above or below the amateur band I was tuned to, but I’m not sure. In any case, it indicates distortion in the amplifier. This is not completely surprising in an amplifier that uses a chip that is not at all designed to sit at the front end of a radio. When I wrote about this to Charles, he wrote that he also experienced some problems, with strong FM stations in his case. It’s a bit annoying, but the antenna is still a joy to use.

I should also add that I forgot to include the 10u capacitor in the circuit. This might have contributed to the overloading; I’ll add it and revisit the issue soon. (Update: I added decoupling capacitors and they didn’t improve the overloading at all.)

There are several other designs on the web for receiving-loop amplifiers. About two years ago I built an untuned loop amplifier designed by John Hawes, which Des Kostryca published on the web. I’ve used it, but I’m not too impressed with its performance. To be honest, I didn’t really compare it to Charles Wenzel’s amplifier when both were using the same loop element, so maybe the loop I was using with Hawes’ amplifier was not good enough. I’ll have to investigate this in the future.

Chris Trask published two designs for receiving loops. Both use series tuning, not parallel tuning, so his designs need to transform the very low impedance of the loop to 50Ω, whereas Wenzel’s design transforms a high impedance to 50Ω. Trask first receiving loop design, published in QEX in July/August and September/October 2003, is an active loop with a 3-transistor amplifier. The circuit is pretty complicated for one using only 3 transistors, using 4 transformers for impedance transformations and for feedback. Trask’s second receiving loop is passive, using varactors for tuning and transformers for impedance transformation, but no transistors or integrated circuits. Chris writes that the designs has very good signal-to-noise ratio that eliminates the need for amplification. I assume that he means that this design is essentially better than his QEX amplifier.

Another example of parallel high-impedance loop tuning is Daniel Wissell’s SLR receiver (in QST, October 1997). The high-impedance tuned antenna is connected directly to the 1.5kΩ-impedance input of an SA602A mixer.

Interestingly, Chris Trask also presents a balanced, tuned, high-impedance active-antenna amplifier, but not for loops but for short dipoles. A short dipole is capacitive, requiring inductance to tune. Therefore, the input network in this amplifier is not suitable for loops. But I assume that the design can be adapted to parallel-tuned loops but simply replacing the input network. Such a design might perform better than Wenzel’s TL592 amplifier, but it is also quite a bit more complex.

In summary, varactor-tuned loops are excellent receiving antennas, and Charles Wenzel’s design is simple and effective. It does appear to get overloaded sometimes.

An Active Whip Receiving Antenna

In my quest to find small antennas that work well I decided to try to build an active whip, a small vertical antenna connected to a preamplifier. The role of the preamplifier in this case is to transform the high impedance of the 50Ω of the receiver and the coax cable.

There are several designs for such amplifiers on the web. The one I built is Chris Trask‘s wideband complementary pull-push amplifier whose schematics are shown on the right. One JFET (the bottom one) functions as a current sink, another to transform the high input impedance to a lower one, and a pair of NPN and PNP transistors to drive the 50Ω load.

There are simpler designs for such amplifiers. Charles Wenzel’s design uses only one JFET and one NPN transistors, and is also wideband. Todd Gale presents several designs, both wideband and tuned narrowband (here and here). All of Todd’s designs use two transistors in a cascode configuration.

Chris’s web site actually presents not one, but two designs. In addition to the one I used, he also presents a design that uses two wideband transformers to eliminate the need for an PNP transistors. I decided to use the simpler design with the PNP transistors, since Chris wrote that the amplifier works reasonably even with the common 2N2907 transistor (but he also writes that it works better with more expensive RF transistors).

I thought that the two extra transistors in Chris Trask’s pull-push design were worth the effort and built that one. I built it into an aluminum box salvaged from some small surplus VHF amplifier, and used a 50cm telescopic antenna as the receiving element. A longer antenna would work better, but that’s what I had. If I find a longer one, I’ll replace it.  Power is supplied through the coax, using a bias T near the receiver and using an RF choke to separate the DC from the RF in the amplifier.

The amplifier does not seem to overload or generate intermodulation, even though it is very wide band. This correlates with Chris’s analysis and measurements. In my usual location, the antenna worked reasonably. I was able to receive numerous shortwave broadcast stations, as well as radio amateurs on 14MHz. On the lower bands, receiving amateurs was more difficult. I didn’t hear anything on the 10MHz PSK31 frequency. On 7MHz I was able to pick up PSK31 transmissions, but very few. Even on 14MHz, signal strengths were weaker than with a 1.25m tuned loop, but that’s not surprising.

I left the receiver and computer working overnight, sending PSK31 reception reports to pskreporter.info. The map shows most of what I received during one weekday evening and night. The data collection was done with a Softrock Ensemble hardware, sdr-radio.com software, and DM780‘s PSK31 decoder and collector. DM780 decodes only some fraction of the signals, not all of them (partially because I use a trial version of VAC to route audio), so these are not the only stations I received. But the map shows that the antenna performs reasonably. You can also see on the map the one 7Mhz station received. The receiver was tuned to 14MHz most of the time, so don’t read too much into this lone reception, but as I wrote, signals on 7MHz were very weak.

The final transistors get quite hot. The dissipate about 300mW each (50mA across 6V). They can dissipate up to 500mW at 25C without a heat sink, so the 300mW is not way too much, but it’s close. Chris wrote to me not to worry about this unless the amplifier is used in the sun in the tropics. Tel-Aviv in the summer probably qualifies as the tropics for this purpose, so I should probably put a heat sink on the transistors.

I can still receive signals when the bias T, and hence the amplifier, is not powered. Signals are weaker, but they are still there. Chris wrote to me that this is caused by the signals leaking through the unpowered transistors.

In summary, this is a reasonable antenna for receiving strong broadcast stations and even for amateur signals on the higher bands. The amplifier does not overload easily. It does not require tuning, so you can quickly jump from one frequency to another, which is nice. With a short receiving element, signals are weak, especially on the lower bands.

The Coax Loop

It turns out that the first transmitting loop I built, which I thought was not efficient enough, is actually working fine. It is a modular and portable coax loop. It’s very similar to Julian Moss’ Wonder Loop. The loop itself is made out of a piece of a surplus coax assembly consisting of 2.5m of LMR-400 coax with N connectors on both ends. I found it at a surplus store and also bought there two matching bulkhead connectors and an air variable capacitor. I mounted the bulkhead connectors on the sides of a small box first:

I then took them out, soldered a coax braid to each one of them (both inner and outer conductors), and screwed the small box to a larger plastic box that houses the variable capacitor. The capacitor itself is a 4-section unit, of which I used only two in a split stator configuration. (This configuration essentially uses a series connection of two capacitors, each formed by one stator section and one rotor section; the rotors are connected by the metallic shaft.) The variable capacitor is tuned using a reduction drive.

The coupling loop is a piece of house wiring. Here too I used a choke balun between the coupling loop and the BNC connector.

To test the loop, I secured the capacitor box and the coupling loop to a wooden pole using straps of webbing. Here is a picture of the antenna supported by a tripod inside the balcony. To make contacts, I hang it outside the balcony.

Hung outside the balcony, the loop enabled me to make contacts with Europe with 16W on 20m PSK, so it is radiating reasonably well. It is much smaller than the water-pipe loop (80cm diameter versus 125cm), so it is less efficient for a given value of Ohmic losses. It also has more Ohmic losses, because of the resistance in the N connectors and in the variable capacitor. So the fact that is it radiating is encouraging.

The main attractive aspects of this antenna (and of course of Julian’s Wonder Loop and earlier loops) is that it is modular, portable, and easy to tune:

  • It is modular because I can use longer coax assemblies for better efficiency and for lower frequencies. The modularity also allows me to exchange the coax I have for semi-rigid coax, which will probably have lower losses. The surplus store where I bought the coax has a range of coax assemblies with attached N connectors, so trying out better/longer coax is a real possibility, not a theoretical one.
  • It is portable because the coax detaches from the capacitor assembly and from the coupling loop, so the whole thing is easy to pack and transport.
  • The variable capacitor, especially with the reduction drive, is easy to tune. I use a resistive SWR bridge with an LED to indicate match. I just turn the knob quickly until the LED blinks (good match), then turn the knob back and forth slowly until I find the null again, and that’s basically it. My body affects the tuning  a bit (as do nearby structures), so sometimes it’s necessary to leave the capacitor just before or after the null to obtain a match when I’m no longer near the antenna. But it’s pretty easy.

The main problem with this antenna is the weight of the capacitor assembly. It is quite heavy. When the loop is mounted on a vertical pole, as in the picture above, this is not much of an issue, since the pole transfers the weight to whatever it stands on. But when I stuck the antenna out the balcony on a horizontal pole, the weight of the capacitor box at the far end of the pole makes it difficult to secure the antenna. I guess that I can use Julian’s strategy of putting the capacitor at the low or near end of the pole, to reduce the problem. The water-pipe loop is much lighter so it’s easier to support.

For testing and light use, manually tuning the capacitor worked well. It will obviously be easier to tune the antenna with a stepper or geared motor, but the mechanical construction required is not trivial. (Even connecting the reduction drive to the capacitor was not trivial; Ezra Shaked helped me with this by creating a coupling from a potentiometer shaft.)

An Unusual Transmitting Loop Antenna

I live on the first floor of a concrete apartment building, so radio antennas are a challenge for me. I decided a while ago that a magnetic loop would be the best option for me. I built one out of coax, but after some testing I decided it was not efficient enough (I later found out that this conclusion was based on wrong data, but nonetheless that’s what I thought.

A few days later, I found a few meters of discarded hot-water pipe that looked like it could be used for an antenna. It’s a weird kind of pipe that I did not see before. It’s made of three layers glued to each other: polyethylene, aluminum, and polyethylene. It’s basically an aluminum pipe with a thick polyethylene coating on both sides. It’s flexible enough to bend by hand, but also stiff enough to retain a shape.Here is a diagram from the manufacturer’s web site:

A magnetic loop antenna consists of a loop conductor, which acts as an inductor, a capacitor to tune the loop, and some feed mechanism. Because the radiation resistance of small antennas is small, their efficiency depends on their Ohmic losses. These losses depend on the conductivity of the conductor (copper is better than aluminum), the diameter of the conductor (larger is better), and the resistance of any connections. The connections were going to be a problem: I knew that I could not solder a capacitor to an aluminum tube, and the loop needs a capacitor.

What I did eventually was to form a capacitor out of the tube itself. I cut a 4m piece of tube. One side remained flush. On the other side, I split the tube in half lengthwise with a saw, and removed the top half. I compressed  the remaining half tube to give it a smaller diameter, and stuffed it into the other end of the loop. The surface of the half-open tube on one side of the loop forms a capacitor with the surface of the tube that it is stuffed into. By pulling out or pushing in the cut end I was able to vary the capacitance. The cut cut fits into the whole tube very snugly, which stiffens the entire structure and keeps the capacitance fixed. Here is a picture of what this capacitor looks like (you can also get a good idea for what this tube looks like from the picture):

I feed the loop with a coupling loop, which I made out of the same material. This was really not important; the coupling loop can be made of thin stiff wire, but the tube one looks good and it keeps its shape. The coax is connected to the coupling loop with lugs and screws. This is not a low-impedance point, so the resistance of the connection is not much of an issue. I used a ferrite choke on the coax just before the feed point to obtain a balanced feed. The following picture shows the whole loop (the big one), the coupling loop (the smaller one) and the feed point.

I tuned the loop in the balcony by pushing the cut tube in and out. I used a resistive SWR indicator to determine at what frequency it was tuned. When it was tuned, I pushed it on a wooden pole outside the balcony. The near end (the feed point) is about 0.5m out of the building, at a 45 degree angle (gravity decided on the angle). At this position the SWR was reasonable, less than 1:2. When it was closer to the building, the SWR was much higher. Here is the loop sticking out of the balcony. The boxes near the laptop inside are the radio.

The results were fantastic. In one afternoon, with a 16W on 20m PSK31, I was able to work stations from Azerbaijan to Britain, and PSK reporter shows that even people in the US heard my signal. The antenna is only about 4m above ground.

Summary:

  • This is basically a fixed-tune loop. It works well on one frequency, but it’s hard to tune it to another frequency.
  • No electrical connections means there is no losses in the connections to worry about; this point and the previous ones form the basic trade-off.
  • As in all magnetic loops, making them large improves the efficiency (up to a point). This one is made of aluminum, so this is even more important.
  • The polyethylene might cause some losses, but I really don’t know a thing about this; if somebody does, I’ll be happy to know.
  • The capacitor end of the loop is not water proof right now. Water probably won’t harm anything to the tube (it’s a water pipe, after all), but it would detune the loop and might cause the capacitor end to arc.
  • Arcing is an issue even when the loop is dry. I don’t know what kind of voltage this capacitor can withstand, but it didn’t arc. If you build such a loop, keep in mind that the voltage across the capacitor depends on the frequency, size of the loop, and the power level; you can use loopcalc to estimate this voltage.
  • A similar loop made of copper would be more efficient (you can try to isolate the capacitor plates with some kind of plastic tubing, coax jacket, etc.

The web is full of web sites on magnetic loops, and it’s worth the effort to read them if you build one. Good descriptions of home-made loops include Brian Levy’s, Alex Krist’s, Alexandre Grimberg’s, Julian Moss’, and Steve Yates’. Chris Trask’s web site contains an article on a small transmitting loop with a very clever matching network (and some complicated theory). His web site also contains a number of articles on receiving loops (he also wrote a pair of QEX articles on receiving loops a few years ago). Douglas Miron’s book, Small Antenna Design, contains a lot of theory, but also good intuitive explanations for why small loops tend to work better than small dipoles and small verticals. (His explanation is that the efficiency of all small antennas depends primarily on their Ohmic losses; loops need a capacitor to resonate while short dipoles/vecticals need an inductor, and capacitors are a lot less lossy than inductors.) The programs loopcalc and capcalc, available from Glenn KI6GD, are very useful for estimating the behaviors of magnetic loops and for computing the capacitance of simple plate capacitors.