Cushcraft A3S Modified For 17, 15, 10 Meters

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.

Balun for Modified A3S

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:

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.

G3TXQ (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.

The only spots from DX are from Europe. That’s a good indication that the beam is working because it’s pointing toward Europe. Also that’s a good indication the balun is working.

Colorado’s First Derecho Wipes Out Two Antennas

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.

Elements came loose from the reflector. Damage is limited to the small diameter tip tubing.
The reflector shouldn’t be so short. Good excuse to replace the single band antenna with a tri-band beam since the antenna has to come down for repair anyway.
The 43 foot vertical is on the right hand side of this security camera image. This is the second antenna damaged. Good excuse to replace it with a better quality antenna from DXEngineering. The other antennas survived.
A Cushcraft A3S tri-band beam with a 40 meter add on kit is being put together to replace the 203BA.
Assembly is completed and ready for swapping out when the tilt-over tower is lowered this week.

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.

What was causing this? Opening a trap revealed a very loose screw. Upon trying to tighten the screw it became clear that it had been stripped out. Opening a second trap revealed the same thing. Searching the web and YouTube turned up many similar experiences. Each trap could be rebuilt with larger screws but there are 14 traps. That’s a lot of work just to be able to use a brand new beam. The antenna was returned as defective.
The dip was not where it should be which indicated something is wrong in the antenna–a loose connection or a short somewhere. Dimensions were verified. Loose screws in the traps are a good bet. Despite the terrible image quality the dip can be made out above the band where it should have been below when it’s on the ground. The frequency of the dip is at 14420 kHz.

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.

The elements don’t sag!
Now the dip is where it should be when it’s still on the ground, about 200 kHz lower than it will be when up in the air. Much improved. Ready to be raised.

Up in the air now.

The dip sure didn’t move as much as expected, from 13.830 to 13.914 mHz. Two hundred kHz was expected, which would have put the dip at 14.030. Although the dip is outside of the band the SWR at 14.074 is quite respectable at 1.48. Next time the tower is down the antenna will be adjusted to move the dip up into the band. Meanwhile here’s a look at 15 and 10 meters.
Dip is way outside the band and the SWR at the FT8 frequency is 2.0. How bad is 2.0? It is mainly a concern to the transmitter which likes to see SWR below 2.0. How bad is it for effective radiated power?

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.

Dipping outside the band above the band is a surprise. SWR at the FT8 frequency is 2.65. The transceiver we are using begins to reduce output at 2.0 but does not totally shutdown until SWR reaches 3.0. Several FT8 qso’s were accomplished during the testing but probably would have been more if the dip was closer to 28.074 MHz. Using the transceiver’s internal tuner was not tried. Again, next time the tower is down for maintenance the antenna will be adjusted. It sure is pretty to look at. The 160 meter gamma match support poles can be seen on the side of the tower. They are the four white things sticking out.

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.

630 Meters


Joy came today when the first contact was made on 630m.  In fact five contacts were made on JT9.  On WSPR, spots were posted from all over the United States.


Last month a vertical antenna tuned up on 474 kHz.  The matching network had taken several tries but today it seemed to come together. The ground screen is made up of four radials, two 100 feet long and two 50 feet long rolls of welded wire fencing laid flat. The vertical radiator is 36 feet in height.  Two horizontal 50 ft wires connected to the top of the radiator make up  a “tee” top hat.  The loading coil enclosure and one of the radials look like this:


The matching network (loading coil) is inside the enclosure.  It consists of 100 turns of 16 gauge wire wound on a piece of plastic pipe.  The top of the coil connects to the vertical radiator and the bottom connects to the ground screen.  Upon close examination one can see a ferrite rod attached inside the coil form.  The rod is the secret to easy tuning.  By moving it up and down the resonant frequency can be dipped, a lot like tuning an i.f. transformer on a receiver.IMG_0041

The coax is tapped onto the coil at the 50 ohm location.  Once it proves to be stable the clip will be removed and the wire soldered right to the coil. Holes in the coil form make it easy to clip onto the coil windings.


A snapshot of the antenna analyzer shows the resonant frequency and swr, both excellent at 474 kHz and 1.1.


Extrapolating from the above screen it can be seen that the 2:1 bandwidth is approximately 3.7 kHz.  The antenna is theoretically usable from 472.15 kHz to 475.85 kHz.   Extremely narrow bandwidth which could be problematic with shifts in temperature or wind.

Hitting the button to show the rest of the numbers reveals an almost perfect match of 53 ohms and almost zero reactance.


The antenna is ready to attach to a 630 meter transceiver.

Retuning on October 30 produced the following analyzer pattern:   (Very nice) Notice the new frequency of 475 kHz which is up one kilohertz from before.  The reasoning is when a 1 kHz tone is transmitted with the dial at 474.2 we are actually transmitting at 475.2.



Within the last few months information has been released on how to modify the Icom IC-7300 to transmit on 630 meters.  The IC-7300 already receives on 630 out of the box.  The “wide band transmit” modification consists of removing two surface mount diodes.  HRO will perform the mod at a charge of $40 and that is the method we chose.  This solution eliminates the need for a downconverter.  Output is 30 watts.

Stay tuned for the first tests.  See top of page.

The saga continues.  After a day of use the IC-7300 failed.  It refuses to put out power due to infinite swr activating the protection circuit.  At first a rig failure was suspected but swapping out the 7300 with a new one produced the exact same result.  A failure in the antenna was suspected.   Meanwhile, Googling the problem produced a possible explanation.  The IC-7300 was not designed to transmit on such a low frequency.  It has no low pass filters below it’s design frequencies therefore the output has harmonics.   The harmonics are reflecting back from the antenna and causing the swr meter to read high, which in turn triggers the protection circuit and shuts down the output.   Why did it work the first day before failing?   Hard to say.  Solution is to insert an external low pass filter.  A search is underway for one that can handle the 30 watts output of the IC-7300.  On the bright side neither the radio nor the antenna seems to be faulty.

The only filters that could be found are for qrp.  A higher power design was discovered at this web site:

Using junk box parts resulted in homebrewing a filter that could handle 30 watts and perhaps much more, shown below. The toroids are T200-2  powdered iron (red). The cores are wrapped with white Scotch fiberglass tape. The calculator available at the njd site above yielded 33 turns for this toroid.  One turn was removed after it was wound to obtain the 14.5 uH needed.  The capacitors are 1000V mica.


Pushing a few watts through the filter to a dummy load and varying the frequency resulted in a roll off around 700 kHz.   Ideal.  The filter was connected between the IC-7300 and the antenna coax.  SWR now reads 1.7:1 at 475 kHz.  Why not 1.0:1?   Hmmm.

A possibility has been offered by Larry, K0NA:  We need a diplexer.  The harmonics which are filtered need someplace to go and this filter does not provide it.  Otherwise the harmonics are reflected back to the transmitter, much attenuated, but still are registering on the swr meter.  Inserting a diplexer would route the filtered harmonics to ground.  As long as the swr is below the protection cutoff level we won’t worry about it.

Meanwhile the antenna was upgraded by replacing the vertical wire with aluminum tubing.  It still has a 100 ft “tee” top wire horizontally even though it’s invisible in the picture.


Interestingly, bandwidth has gone up to 66 kHz.  This will eliminate worries about mechanical changes shifting the resonance and producing an unusable swr.  Without retuning after adding the tubing the system resonants at 500kHz with a swr of 1.0:1 (note: if we added length the resonance should have gone down, not up).  Tuning the resonance down to 475 kHz does not result in the swr dipping to 1.0:1, however.  Why?  The swr at 10 watts output is 1.3:1 but at 25 watts out the swr jumps to 3.0:1.  Work remains to be done.

The coil has been stabilized with silicon caulk, the clip soldered for permanence, and a porcelain insulator has been installed for the antenna pass through.  Still up in the air on the next step to correct the swr. The basic question still looms; is it the antenna or is it the transceiver and the low pass filter?  Is this a faulty antenna or is the rig responding erratically on this non-standard band?

Results are still irratic.  Next we’ll try the good ole variometer wound on a 5 gallon plastic bucket.  Maybe.

2019 Follow Up:  For the 2019 season the original downconverter was revisited using the IC-7300 for a 80 meter signal.  The downconverter puts out about 10 watts.  The antenna seems to be stable this season with no changes other than dipping it to resonance with the ferrite rod.

The confirmed state count as of Spring 2020 is up to 19 now.   WAS is a long haul project on 630 meters.


Six Meters – The Magic Band

Six meters, the “magic band”, has been researched and some of our results are given for an optimum antenna system for our little pistol station.  We currently have a M2 HO-LOOP Halo antenna at 22′.  Below is the first step of putting up a tower and raising an antenna to 50′.  We will explain how we came to the choice of that height. Our goal is to get it completed before the end of the 6 meter season and that’s getting very close.


Wood instead of concrete for tower base.  This is a half way done picture.  The posts are 4 feet long and are shown buried half way.  Dirt will be filled in and only 6″ of cedar post will be above ground.  Seventy five dollars for wood versus the $1000 charge for concrete in the country.  The tower is one we at the old Parker remote which is a 30′ aluminum Universal Tower.  We plan to extend another 20 feet with aluminum tubing and put a 6 meter antenna at the top.  Work in progress.


This is what the finished base looks like with wooden posts.  Ten year life expectancy in the arid Colorado plains climate.  By the way the wood posts insulate the tower from ground lending itself to being used as a vertical antenna as well.


It’s up!  Complete with a Halo antenna and it seems to work well on the preliminary testing.  Stations came in with readings of minus 17 and minus 18 (FT8)  that KC0RF said he wasn’t decoding.  It totals 50 feet — 20 feet of mast above a 30 foot aluminum tower.  It was designed as a self-supported tower but it’s so old and weak it needs guy ropes so I can sleep at night.



Adding a stacked halo should provide more gain and keep the radiation angle almost as low as it is with one antenna.  Per the manufacturer, M2, the additional gain is 4 db and the radiation angle is 6 degrees at the heights of 50 feet and 38 feet.  The phasing line, or power divider, is 75 ohms coax cut to electrical quarter wave length  and three-quarter wavelength.  Times Microwave makes a 75 ohm version of LMR400 for the cable tv industry and that is what is used here.  A 75 ohm bridge must be used to measure the lengths, not a MFJ-259 because it is 50 ohms only.



No Climbing, By The Way


No tower climbing these days.  The base is hinged and a winch is mounted on a pole. Steel aircraft cable is attached near the tower’s apex.  It is now a tilt over tower.  It takes a lot of cranking but cranking beats climbing any day.  The hand winch could be easily replaced by a power winch someday.  (Apologies for the terrible quality of the snapshot.)

Both 65 Foot Masts Go Down In Storm

Both the north and the south 65′ aluminum masts went down in the snow storm Sunday.  They both failed in almost the same place about two-thirds of the way up.  What a mess.  First the south mast, then the north.



Plans for what to do next are not worked out.  KC0RF has made several suggestions:  slopers from the tower, a 43 foot vertical, a HyGain Hytower, and replacing the masts with 65′ Telrex tubular crankup masts.   We’ll use EZNEC to help us evaluate these suggestions and maybe come up with some of our own.  One of my ideas would be to replace the .058 tubing with .12 inch.

A Plan Emerges Finally

Consider this: they both failed at about the two thirds point and the highest level of guys had been moved up to the top of the mast against the DX Engineering instructions.  The top would not have been supported per the instructions and surely would have failed with  antenna ropes pulling horizontally.  That’s why the guys were moved up.  Unfortunately it left too large a section unguyed.  Armed with this analysis the next attempt will be to guy exactly per the instructions and then add a fifth set of guy ropes to the very top.  Replacements for the bent sections will be ordered.  Only one mast will be done this way as a test.

January 27, we salvaged the north mast and re-purposed the tubing that wasn’t damaged.  Taking one of John’s suggestions we made it into a 43 foot vertical.  The North mast now:


So far performance has been excellent, working quite a bit of dx.  Next upgrade will have to be a higher power tuner (MFJ 998RT) so we can run a few more watts to make up for the inefficiencies.  Frankly, I’m surprised at how well this antenna performs.

South mast:  A week later we cut off the bent top sections.    South mast now:


A road grader in the background is building an access road in the easement next to our property.  Progress I’d be happy to forego.

With John’s help we tried to raise the repaired 65 foot south mast complete with 5 guy levels.  Unfortunately it collapsed when it was just a few feet from being all the way up.  This makes the fourth time one of these aluminum masts has collapsed.  You know what they say.  Four strikes and you’re out.  This isn’t baseball.  No more 65′ masts at W0QL.  Next we’ll look for some suitable solutions closer to the ground.

Taller Vertical Antenna – “North Mast”

The main 56 foot low band vertical has been extended to 65 feet.  It took a full day to do it because of it’s unwieldiness and a constant 16 mph wind.  Thanks to John, KC0RF for giving up a full day to help.  Here’s what it looks like with the additional height and a small top hat to resonate to 3.573 kHz  (the FT-8 freq). This is both a vertical antenna and the north mast which is planned to hold up two dipoles.  The south mast (below) is planned to have the other end of one of the dipoles attached.


The 30″ top hat is very hard to see when it’s 65 feet up in air.  A better picture is coming soon.

IMG_2315 (1)

The antenna is designed as a full quarter wave on 80 meters.  Using a MFJ 259B  the impedance at 3.573 kHz reads 55 -j0.  SWR is 1.1.   Apparently we hit the resonance perfectly but the ground screen needs to be improved to bring the resistance down to 36 ohms.  The match was planned to be a direct match for a 50 ohm coax.  As an experiment we left the existing tuner installed and tried it on other bands.  It works on all bands, 160 through 10 meters even though the take off angles may be awful and the tuner losses may be especially high at the even harmonic frequencies.  We’ll leave the tuner in for a a few weeks to see where it works and where it doesn’t.


South Mast Erected


Sixty-five foot mast from DXEngineering completed today and looks amazingly vertical.  Getting it up was like raising a wet noodle.  Here’s what it looks like on the ground.

IMG_2293 (1)

The secret is to never let the attachment point get below the bow point.  Lift the upper sections first and keep them under upward pressure so a downward facing bow never forms.


When it’s all over it looks like the top picture. This will be the south support for a 40 meter dipole and possible for a 630 meter antenna.