Using a NanoVNA to measure the resonant frequency of a trap from a hf antenna is easy.
To determine what tuners will work with a 43 footer the impedances need to be known for each band so tuners can be utilized that can handle those impedances.
For an interesting read on radials for vertical antennas see an extensive article by Al Christman, K3LC. http://ncjweb.com/bonus-content/k3lcmaxgainradials.pdf
The vertical tested here is a DX Engineering DXE-MBVE-5A 43 foot vertical. Radials are four pieces of welded wire fencing each 25 feet long laid flat and terminated on a DX Engineering DXE-RADP-3 radial plate. The fencing is 48 inches wide. A RigExpert model AA-55 was used to make the measurements. Each band was tested, 160 meters through 10 meters, except 12 meters. Here are the results including the (poor) snapshots of the AA-55 screens.
|Z| = 506.9 ohms (notice the R component is only 11.8 ohms)
SWR = infinity
|Z| = 216.6
SWR = infinity
|Z| = 58.6 ohms
SWR = 3.5
The 60 meter frequency of 5357 kHz is very close to the resonance of a 43 foot vertical. A dip at 5957 confirms the expectation.
A quarter wave vertical with a perfect ground system should have an impedance of 36 ohms. For curiosity the AA-55 was adjusted to the antenna’s resonance at 5957. Here is what this antenna measures:
|Z| = 45.7 ohms
SWR = 1.10
This reading of 45.7 ohms indicates a ground loss of 9.7 ohms (45.7 – 36 = 9.7) or approximately 10 ohms. This value agrees with the amateur literature for a typical ground system. One example is Phil Salas, AD5X’s presentation on The 43-Foot Vertical : “Assume 10 ohms of ground loss — Probably a much better ground than most hams have”. The efficiency calculation in the AD5X presentation should match the vertical in today’s test very closely. AD5X calculates 78%. For every 100 watts delivered to the antenna 78 watts is radiated.
An idea for improving this blog post would be to test a 43 Footer over a better radial system for comparison.
|Z| = 131.9 ohms
SWR = 4.8
|Z| = 636 ohms
SWR = 12.77
|Z| = 227.7 ohms
SWR = 17.03
|Z| = 102.7 ohms
SWR = 2.93
Notice another dip. This one at 17180 is the third harmonic of the fundamental frequency of 5957 kHz.
|Z| = 385.3 ohms
SWR = 7.8
|Z| = 61.2 ohms
SWR = 1.23
Matching 30 meters should be the most difficult at 636 ohms but that’s well within the range of most automatic tuners. An additional challenge should be 160 and 80 meters with their infinite swr’s. One of many good tuners to use as an example is the MFJ 998RT. It is specified to handle impedances from 12 to 1600 ohms and swr’s up to 32:1. In practice with this model of tuner installed on this 43 foot vertical it matches beautifully on 80 thru 10 but not on 160, maybe because the R component is only 11.8 ohms on 160. Optional coil and relay kits are available to add 160 meters. No matching problems have been noticed on 30 or 80.
A note of caution. Just because an antenna matches does not guarantee it is getting out possibly due to objects nearby or due to radiation patterns on each band. It may match perfectly at 10 meters but all of the energy is straight up to the clouds, with only a little radiation at low angles.
On the other hand antennas with a poor match still can make contacts with even a small amount of power being radiated, although inefficiently.
A very short antenna had about the same power gain and radiation pattern as a full half-wave antenna.
“My first interest in the short vertical antenna began when my thesis advisor at Harvard, Professor R.W.P. King, developed the full theory of the short antenna. In it he disclosed that a very short antenna had about the same power gain and radiation pattern as a full half-wave antenna. The main difference was that the resistive component, the radiation resistance, was very small in comparison. In turn, the short antenna has a very high capacitive reactance, which has to be cancelled by various loading techniques.
The theoretical results show that the power gain (when compared to an isotropic radiator) for a very short antenna, even one that is less than 1 foot high on 40 meters, is 1.5. This increases slowly to 1.513 for an 11-foot vertical. These gains are to be compared to about 1.62 for a resonant 1/4-wave vertical. This difference amounts to less than 0.4 dB or 0.07 S unit, based on 6 dB per S unit.
Base loading,[ as shown in Figure 2-3 above ], yields the lowest value. Top hat loading yielded the largest value of radiation resistance for a particular height. A top hat is also considered the most low-Q loading element. The reduction in height due to top hat loading (with a conducting wire around the perimeter) is approximately equal to twice the diameter of the top hat. A four-spoked wheel approximates, to a good degree, a solid disk. Doubling to eight spokes only improves the loading by about 9 per cent. Thus, a few radials on the top of a vertical, which are electrically connected by a perimeter conductor, are very effective.
The data were obtained by essentially cancelling out the reactance and measuring the resistive value with a simple impedance bridge. Because I used an extensive ground system together with base loading coils with Qs approaching 900, the resistance measured was actually that of the antenna itself. At the radial point the input resistance approaches the theoretical value of 35 ohms which strongly indicates low earth loss and reliable data.”
Summarized from a book by Dr. Jerry Sevick, The Short Vertical Antenna and Ground Radial, CQ Communications, 2003.
Fact checking Dr. Sevick there is a book published by ARRL written by Robert J. Zavrel, Jr. called Antenna Physics: An Introduction. From pp B.2: “A short vertical’s value will be about 6 ohms for a 1/8 wave physical height. For a perfectly top loaded 1/8 wave vertical, value becomes about 24 ohms…a fourfold increase in radiation resistance when compared to an unloaded simple vertical of the same physical height” These values agree very closely with those in Figure 2-3 above.
The above excerpts have been field tested in a 160 meter vertical antenna constructed in 2018. This vertical has been very successful and is still in use today. It has eight radials, each 100 ft long made of 4-ft wide welded wire fencing laid flat on the ground. The vertical element is made of 43 ft tall aluminum tubing tapered to average 1.5 inches in diameter. The top hat is made up from 6 102″ CB whip antennas with hookup wire connecting the tips.
Inside the box is a matching coil which is tapped with a coax feedline at the 50 ohm point. This setup is good up to about 300 watts. It needs to be hardened before it will get up the legal limit.
Project 7 of 7 for October, 2020 – projects to keep sane during Covid-19 Lockdown
Status: All work is completed.
A new Yaesu G-450ADC rotator was ordered from GigaParts. Still needed are the rotor plate, the cable, and the interface to make it remote controlled. Cable and interface are on order from DXEngineering and HRO. This project has hit a snag. Rotor plate is special order and has a November ship date. It has not even been ordered and by the time it gets here the weather could be too wintry to install it.
Universal Towers saved the day. An order was placed directly with them today and they promised a much earlier delivery date.
“The rest of the story” is this tower is decrepit. It was used 10 years to hold up wireless Internet antennas on a windy hill. It has two blow outs from being overloaded. Even though the blowouts have been repaired the concern now is it might not be able to keep the A3S in the air. Torque is the tower killer when wind makes a beam twist. An overloaded tower can fail from that twisting motion. When this tower is down for the rotor installation a torque arm will be attached in an attempt to reduce the twisting motion.
Remote Controlling The Rotator
All parts have arrived. The Yeasu G-450 rotator is the new DC version which makes absolutely no detectable difference in the operation but it makes lightning protection easier. Relays had to be used on the AC rotator because the AC voltage was too high for the 26vac/31vdc MOV’s on hand. Perfect for the 20 volts DC the new version uses. Modifying the controller for access over the Internet looks like this:
Actually the controller is not modified. A few wires are tacked onto existing terminals inside. Wires are brought out through an existing hole. The controller could be easily restored to original. In the picture above, the green thing at the bottom is the remote interface from RemoteRig.com model RCU-1216:
The interface talks to the RemoteRig 1216H Webswitch and will get mounted inside the controller. A Webswitch already exists at the remote site to provide remote access of the first rotator on the taller tower. That missed getting written up. That’s why this is being written up now. The unit has the capability of two rotators so all that was needed was this interface to the Webswitch. The installation just needs some hookup wire and a solder iron.
There is no brake release button on Yaesu control boxes so a brake release connection is not needed. Only the Pot potentiometer connection and the two motor activation buttons are needed. A data pair and a power pair connect back to the Webswitch.
For this rotator the two jumpers, P5 and P6, are opened up to accomodate the voltage on the Yaesu rotator for direction indication potentiometer. Voltage maxes at 1.3 volts on the pot. Next attach a little Blue Tack or 3M gray stickem to hold the interface in place.
Stick it to the inside of the cabinet and you’re done. It should look something like this:
Coming out of an existing hole in the back are two pairs of wires. One is for 12 volts DC. The other is the 1-wire data pair (1-wire really means 1-wire and ground). Next step is to install and test at the site.
Today the rotor plate was mounted and the rotator is mounted to the plate. A short piece of aluminum tubing was cut to go from the rotator to the mast. A hole was drilled for a bolt to keep the mast from twisting and slipping.
The tower was raised a few feet to see if it is too heavy with the rotator. It is noticeably heavier and harder to lift but not impossible. None of the gin components complained. The rotator only weighs 7 pounds and the wire is probably 2 more pounds. Another 9 pounds is apparently not overdoing it. Next the cables will be extended where needed for slack and they will be dressed. The bolt will be installed. The rotator connector will have a waterproof boot installed. The balun will be reworked to provide enough slack for turning the antenna (the balun is near the center of the picture with cable ties holding it to the tower leg). The rotator cable will be run through the cable entrance at the shed and MOV lightning protection will be provided. Inside the shed the controller will be connected to the RemoteRig Webswitch and all will be tested. It will be really nice to be able to turn this beam in the direction of the signals as they change.
Today, the balun was rebuilt by replacing the RG-58 windings with LMR-400. It is still 5 turns through a stack of 4 Mix 52 ferrite toroid cores. It looks like it can handle a lot more power now. Only the common mode current is flowing through the toroid.
Proving the balun is working is a matter of observing the signal pattern on pskreporter. In this case it is a nice flashlight beam shape in Europe indicating the balun is doing it’s job. It’s job is to keep common mode currents from generating stray radiation which distorts the pattern. No pattern distortion, the balun is working.
The rotator mounting is completed and the cables are dressed. Ready to raise the tower.
Back up in the air the rotator turns the beam perfectly with no issues. As for remote control, a relay is being used to switch between the two rotators. The other rotor turns the 203BA 20 meter beam on the big tower. One rotor at a time is accessible over the Internet.
This project is completed.
Project 6 of 7 for October, 2020 – projects to keep sane during Covid-19 Lockdown
For the virtual Dayton Hamvention this year Contest University had some wonderful online presentations. One was especially interesting to someone who still has countries needed on 160, W3LPL – “Effective Low Band Receiving Antennas”. A video is still available on YouTube. Frank Donovan, W3LPL, listed receive antennas from the smallest to the largest along with ranking their effectiveness. The best of size, cost, and effectiveness looks like the 4 square high impedance active antenna. DXEngineering offers one in a bundle with all possible parts needed, DXE-RFS-SYS-4S. One is on order.
In Frank’s excellent presentation only one antenna performs better and that is twice as big and only has 1.5 dB Receiving Directivity Factor (RDF) improvement. The Hi-Z 4square appears to be the best bang for buck.
As of today grounds rods to mount the antennas have been made. A 1000′ roll of direct-bury CAT-5 cable is on hand to run power and control signals. A location has been selected about 500 feet north of the transmit antennas. A scheme to reduce the number of control wires so direction can be switched remotely has been invented. This receive antenna should coordinate nicely with the omega-matched tower transmit antenna upgrade this year.
One concern is how far apart to put the verticals for the best performance. The DXEngineering user guide says 135′ or one quarter-wave is optimum for 160. Other references says 80′ or 88′. An email was sent to Frank, W3LPL, to get the word from the master himself.
Optimum 4-square spacing for 160M receive is 67 feet. Wider spacing produces higher signal levels but also higher side lobe levels.
Much more important is maximizing the spacing from resonant 160 metertransmitting verticals, towers more than 90 feet tall power lines andhomes or buildings that may contain RFI sources. I would try for at least one thousand foot spacing.
Five hundred feet is do-able but 1000? Hmmm. We’ll see with a site visit tomorrow.
Cable was pulled from the shack to the 4-square site today which is 1000′ away. One thousand feet seems like no problem after today’s site visit. The site is very close to the Field Day site for 2019.
One cable is quad-shielded RG-6 and the other is direct burial CAT-5. One of the CAT-5 pairs will be used for direction control. The other three pairs will be grouped to provide power. Resistance of a single 24 gauge wire 1000′ long is 26 ohms at room temperature. Three wires grouped together will cut the resistance to about 9 ohms (8.77 to be exact). With 1 amp of current the voltage drop would be 9 volts (E=I X R). To provide 12 volts to a device the supply voltage will need to be 21 volts. Fortunately the battery voltage for the new inverter will be 24 volts. A linear buck converter (LM-317 voltage regulator) can be installed at the 4-square end and provide a regulated 12 volts. Per the instruction manual:
“The DXE-RFS-3 phasing unit uses and distributes the voltage to power the active antenna elements.
For all four active elements, a nominal +12-15 Vdc at 250 mA current is required.“
At only 1/4 of an amp load the voltage drop would only be 1/4 as much or 2.25 volts. If a 13.35 supply was used the voltage at the 4-square would be 11 volts. That might work but might be unreliable. Using a 24 volt supply and a buck converter is a more reliable choice.
Today the ground rods were driven for mounting the verticals. They were measured out to be within 1″ accuracy, using the 67′ side lengths recommended by W3LPL. The sides are precisely east- west and north- south. That makes Europe at 45 degrees the default direction.
Status: 4-square arrived today (some assembly required). The Quad Shield RG-6 cable has been measured out as accurately as possible and cut to length at home for the delay lines.
Next the excellent quality Snap-N-Seal connectors provided by DXEngineering were crimped on.
Tomorrow the cables will be taken to the remote site and connected. The verticals will be installed on the ground rods.
Installed, here is one of the verticals:
This is the hub at the center of the four verticals. The three delay lines are coiled up and strapped to the post. The gray box on the far side contains the voltage regulator (buck converter). The black box in the foreground is the DXEngineering control box.
Inside the gray box is a LM317 voltage regulator or buck converter. It converts the 24 volts input to a 12 volt output. A LM317 is used instead of a LM2596 because the LM317 is linear technology (analog) and does not generate digital hash like the LM2596. This method was chosen to carry 12 volts over a 1000 feet of CAT5 to overcome it’s inherent cable loss. In the picture below the blue/white pair carries the BCD data to switch the relays that control the direction.
Testing to see if the design works in the field shows good results. The gold thing is a 50 ohm resistor which simulates the load of the relay controller. (E = I X R, E = .25A X 50 ohm, E = 12.5 volts).
With the weeds looking very much like vertical antennas a wide shot of the 4 square doesn’t show much. It blends in quite well.
Performance is extremely directional as expected. A station to the northeast that comes in S9 completely disappears when the relays are switched to null it out. Come on winter and 160 season. It will be fun to use this.
Next, FT8 signals on this antenna were compared to the same signals on the 160 meter transmit verticals. Signals are stronger on the transmit antennas but there is no way to null out unwanted signals. A preamp is under consideration to bring up the gain where it will be even with the transmit verticals. The loss in 1000 feet of RG6 may need to be compensated for.
Status: completed (but always looking for improvements). To try to get a little more gain on 160 the jumpers were put in place to peak the response on that band. Gain is still low but it is working as designed.
January, 2021 update: Performance is amazing. A YouTube demonstration should be produced. A preamp was added at the 4 square to overcome the loss in the 1000’ of coax.
An omega-matching system was added to the tower when it was on the ground in the hopes the tower could be made into a 160 meter vertical antenna. The concept was suggested by Bill, N0CU and illustrated in ON4UN’s Low Band-DXing, Fifth Edition on p 9-68.
PVC pipes were attached to the tower with stainless steel clamps and 8 ga aluminum wire was connected at the top.
Today was the first chance to see if could be tuned. Two large variable capacitors were mounted temporarily, one in parallel with the tower and the other in series with the feedline. It is held up by Velcro strips and connected with clip-leads.
The results were amazing with the capacitors set at half way on the first try. This is what it looks like on the RigExpert AA-55 antenna analyzer.
With a little tweaking the match could probably be made perfect. It is quite useable right now with no other adjustment. Upon re-reading some of the technical articles on gamma matching a tower some new (or missed) information came to light. The main point is the resistance of the match is determined solely by the gamma apparatus connection height and distance from the tower. Resistance cannot be changed by the matching unit. If this proves out we are stuck with 66.5 ohms.
Next the clip-leads were carefully removed without touching the capacitor settings. A Banggood tester was connected to the capacitors and readings were taken to determine a ballpark figure for required capacitance.
The Banggood shows one capacitor is 82pF and the other is 116pF. Capacitors to achieve these values but with higher voltage ratings will be obtained. Next step is to mount them more permanently in a NEMA box and attach the box to the tower. Getting ready for winter and the fabulous upcoming 160 meter DX season will be fun this year.
A new box with two variable capacitors is ready to be installed and adjusted. The capacitors are capable of handling a peak voltage of 3KV. That should be right on the edge of being ok.
Update 9/6/2020: New box with room for two 3KV variable capacitors has been installed and tuned up.
The results are below 2:1 swr but not down to 1:1 which was the goal, of course. Moving the gamma tap point location would probably be necessary but too much work to accomplish. One article mentioned moving the gamma wire closer or farther from the tower down at the matching unit level. That will be given a try later this month. Here is an analyzer view of the match. This was taken at the shack end of the coax.
The SWR of 1.27 is lower than we measured at the base of the tower due to the coax loss. At the tower the swr was slightly higher at 1.45 and the resistance was up to 72 ohms.
Next update will show some on-the-air results.
An attempt to use a tri-band yagi on the 17m WARC band instead of the 20 meter band for which it was designed is successful. This project proved that an A3S will work on 17 meters without any modifications to the traps and only needs changing the length of the tip pieces. Today the balun was installed and the antenna went up after final tuning. It works quite well upon initial testing based on one contact.
The antenna farm as it stands today buried in a lot of smoke from a distant fire. The modified A3S is on the right.
First contact was with Estonia on 17 meters. Yay. Here are the analyzer screenshots for each band giving a first look at how well the modified A3S measures.
Ten meters was not modified so one would expect this band to look good and it does. The Cushcraft dimensions were used unaltered. Below 2:1 SWR for the entire band. That’s an admirable accomplishment for such a large band. SWR for FT8 is 1.21. Great. Ready for Sunspot Cycle 25.
Likewise, 15 meters was kept according to the original Cushcraft instruction manual dimensions. It works well as expected, SWR of 1.37 at the FT8 frequency and below 2:1 for the entire CW portion. SSB will be a problem with this antenna. Fortunately most SSB will be stateside rather than DX and that should be ok.
The 17 meter band is the nut that was sought after and it has paid off. SWR below 1.2 for the entire band and 1.14 at the FT8 frequency. Ahhhhhh. Good feeling. It is now proven that a A3S can be modified for 17, 15, and 10 successfully.
The tips were removed on the driven element and reflector and replaced with much shorter pieces of about 12″ each. The director had the tips removed and no replacement tips were needed. On the driven element and on the reflector tuning for resonance on 17m with the replacement pieces was all that was required. Note that the reflector was tuned 5% lower than the driven element per accepted yagi design. The standard trap resonance of 20 MHz is such that 17m does not interfere or cause any interactions and works perfectly. This is a winner.
Currently the antenna bearing is fixed on Europe because there is no rotator installed. Next upgrade project will be to add the rotator.
1:1 Balun Made Using Four Mix 52 Cores With 5 Windings of RG-58
Baluns, it turns out, are not all that simple. An important goal of a balun is to prevent common mode current on the feedline coax. Incidentally it is supposed to convert from the balanced antenna to the unbalanced coax. In the words of Bill Leonard, N0CU, “Only recently has the importance of Zcm been understood.” Mr. Leonard presented talks on baluns to the 285Tech Club, Conifer Colorado. His slides are here: http://www.na0tc.org/lib/exe/fetch.php?media=technical:balun_p2_website.pdf
Mr. Leonard stated in the presentation that coax is a better choice for conductor than solid wire. The reason concerns the impedance of the conductor. With coax the impedance stays at 50 ohms from beginning to end. With solid wire the impedance could be something other than 50 ohms and may influence the effectiveness of the balun.
If common mode currents radiate from the feedline coax it will act like a vertical antenna and that radiation will alter the yagi antenna’s pattern. The balanced to unbalanced conversion is apparently very easy to accomplish and almost any style or type of balun can do that. The common mode rejection part is not so easy. After lots of reading on the subject from the ARRL Antenna Book to articles on the web and YouTube, this is what has been decided upon. Five turns of coax thru a stack of 4 cores of mix 52 toroids. Here is how that combination was determined. First the requirements. The balun must function well from 17 meters to 10 meters. “Function well” is defined as having a high common mode rejection ratio and be non-reactive in the frequency range being used. It must have only resistance and no reactance. A typical problem in many baluns is they function well up through 20 meters then fall off above that. Commerical baluns are not a solution. Why not a commercial balun? No manufacturer publishes specifications of either common mode rejection or reactance. One does not know what one is getting from a commercial manufacturer other than glowing product claims. That problem could be easily overcome if there was a way to measure a balun. Measuring HF Balun Performance by Ron Skelton, QEX, Nov-Dec, 2010 explains the issues involved.
Mr. Skelton discusses that a balun is a 3 port device and thus needs a three port VNA to adequately perform measurements. A nanoVNA like at W0QL is only two ports. Some examples of using a two port VNA have been shot down as only testing the differential mode. Switching the leads around and repeating the tests will work but the results must be run through a complicated algebraic process to get an answer. Depending on the measurements taken by others is the obvious next step. ARRL Antenna Book 24th Edition contains graphs and pictures of practical transmitting chokes on page 24.50. (Earlier editions have the same information).
Antenna Book 24th Edition
In the chart above notice that all baluns fall off above about 14 MHz except one. That one is 3 turns on a stack of 7 cores. Not ready to build a stack of 7 cores the search continued. Another deal killer is the lack of mention of reactance. Next a chart was found by G3TXQ, Steve Hunt (SK). His charts include reactance as well as common mode rejection ratio.
http://www.karinya.net/g3txq/baluns/ (this link works intermittently)
In this chart we are looking for a balun that will work in the 18 to 30 MHz range and have resistance higher than reactance. Two meet that bill. First is 12 turns on one core. Second is a more manageable 5 turns on 4 cores. That is our choice and is the balun pictured at the top this posting. Mr. Hunt explains in his articles why the mix is 52. Mix 31 is for the lowest bands like 80 meters. Mix 43 is ok for the mid bands. Mix 61 is primarily used for the upper bands and vhf. Mix 52 is rather rare but a perfect match for 17 meters through 10 meters. These charts visualize baluns are not broadband as some literature states. They are optimized for certain bands.
The one big problem remaining is how to measure the performance of baluns . Please post a response if you have a solution for measuring balun common mode rejection ratio, Zcm, or any other parameter important to common mode current.
This has been my path of determining how to construction a balun for a modified Cushcraft A3S. Next will be a follow up on how well it works.
Followup 8/19/2020: The balun is installed and the antenna is up in the air. Contacts are being made and the antenna analyzer measurements look good. As for the performance of the balun the pattern of spots of our station is the only indicator available. If the pattern is what would be expected from a 3-element yagi then the balun is doing it’s job of keeping current off the outside of the coax.
A derecho (pronounced dare-h-o) is a strong wind not accompanied by a thunderstorm. Apparently they occur in the Midwest regularly but none had ever been recorded in Colorado before last week. One went through the state and took out two elements on the big 203BA 20 meter beam. It also folded over one of the 43 foot antennas. Reports were the wind was 87 mph.
Things were going great when this picture was taken, except the elements seemed to sag a little more than expected. Once the tower was lowered and the new antenna was attached the sag became a real issue.
A week later a New-Old-Stock Cushcraft A3S was found on eBay, made before the company was bought by MFJ, and it was purchased quickly. Assembling the genuine Cushcraft felt like better qualtiy from the get-go. What’s more the elements don’t sag nearly as much. Expected reliability is much higher with this replacement antenna.
Up in the air now.
dB = log10(1-x) where x is the loss in per centage. A rule of thumb for 2.0 SWR is 10 per cent loss. Therefore log10(1-.1) or log10(.9) is .45 dB. Loss of output from a 2.0 SWR is .45 dB. That’s a loss that can be lived with.
In testing the antenna many, many FT8 qso’s were made easily. This is the best performing 15 meter antenna at this station. (Yay!) Ten meters is not as rosy.
Post script, 8/19/2020: A bad trap was found and repaired. One winding had shorted. Next, the A3S was modified to change the 20 meter band to 17 meters. It was then mounted on the small 30 ft. tower and works well. On the big 60 ft. tower the original Hygain 203BA was repaired and reinstalled. It works well, too, and all bands 20 through 10 now have a good yagi antenna. The A3S turned out to be more of an adventure than expected.
Reprint of an article four years ago that might still be an interesting read.