A few years ago I acquired a beat up old Cushcraft A4S tri-band (20-15-10) HF Beam antenna with the A744 add-on that give me 30m band as well. It was almost complete, but it needed a bunch of maintenance. I had to buy a couple of aluminum elements and I replaced pretty much all the hose clamps and hardware. Once all the parts were cleaned and replaced I popped the beam up on my 20′ push up pole and tested the SWR.

Well, No Joy! The antenna was way off resonance in every band other than 20m. In 20m I had a decent SWR curve with a minimum 1:1.26 at 14.360Mhz. There was a resonance below the 15m band (1:1.2 @ 20.750Mhz), another 1:14 resonance around 24.599Mhz, and finally a 1:1.5 resonance at 8.340Mhz. That’s lots of numbers, but I’m thinking it was each element just way out of tune.

Here’s the way I understand this antenna works on each band: (There is a lot of black magic in antenna theory. I don’t pretend to understand it all. Very much of what you are about to read is likely childishly stupid or completely wrong or both)

• 30m Band: The A744 add on places a trap on the driven element and extends the driven element out to resonate on 30m as a rotatable dipole. At 30m wavelenght, all the traps on the antenna are well below resonance and just function as loading coils, electrically lengthening the antenna. The reflector and director elements are somehow factored into the mix on the 30m band. So at this frequency, the whole antenna is just essentially ignored/compensated for except for the driven element, which presents itself as just a regular old dipole with some other random “near-by” metallic parts that are compensated for through the use of arcane wizardry.
• 20m Band: The entire antenna minus the 30m trap and extensions functions as traditional 3-element yagi. The traps 10m, 15m, and 20m traps are below resonance and are functioning as loading coils, the 30m coils which are near resonance and act as an open-circuit, effectively cut off the remaining element length.
• 15m Band: Pretty much the same thing as 20m. The antenna is functioning as a traditional 3 element yagi, with the 10m and 15m traps below resonance and functioning as loading coils, electrically lengthening the antenna. the 20m traps are near resonanace and is isolating the outer sections of the driven element, the reflector, and the director. The 30m trap is irrelevant because the 20m coil is isolating it.
• 10m Band: You guessed it! With a twist! The antenna is again a traditional 3 element yagi. the 10m trap is below resonance and is acting as a loading coil. the 15m coil is near resonance and is isolating the rest of the antenna elements and traps. The twist is the 10m band uses a separate director element without any traps for “Reasons” that I don’t understand.

So here I am with my kinda busted antenna. Instead of being resonant on 30m, 20m, 15m, and 10m, it is resonant just high of 20m, somewhere between 40m and 30m, just below 15m, and kinda sorta near 12m instead of 10m.

So the good news? There are indeed four resonances and they are sort-of nearish to where they should be.

The bad news? Well they are not where they need to be. Cushcraft is no longer selling parts for these antennas. There is not a whole lot of information out there on repairing these things.

SO I START THINKING:

What can I learn from all this… Well…

20m band has a pretty nice normal looking SWR curve that is just a little bit too long. I think I can assume the whole structure of the antenna is close enough to “Okay” to function as a 20m yagi when all the traps are just functioning as loading coils. I can assume the 20m traps are working to electrically lengthen the antenna elements to near correct, and I think I can asssume the 30m traps are working correctly since the additional length on the driven element would surely throw the SWR way off if the 30m traps were not isolating them. This excludes a lot of things. I don’t have open circuits or missing parts, it just needs tuning.

15m and 10m are both too short, but they are both present. I’m assuming this is due to the 10m and/or 15m traps not doing their jobs. I’m guessing one or more of them is way off spec and is either not loading the element or going high impedence too early.

The 30m I think depends on the other antenna elements to be there and be correct in order for the resonance to be correct. At 10mhz (30m) every trap on the antenna is well below resonance and should be loading the antenna down. I can imagine a bad coil somewhere causing the resonance shift.

Now I know MFJ has closed its doors recently, but I figured maybe there was still a few parts laying around. I called the Tech support number and literally on the 2nd ring a knowlegeable guy answered the phone. I didn’t have to press 1 for english, I didn’t have to explain my problem to an AI receptionist, I didn’t have to listen to hold music. I just called tech support and they answered. Remarkable!

So while I was talking to the guy, I explained what I was seeing and my thought process. We discussed the readings and what I had done to troubleshoot (more on that next month) and he agreed, I likely have at least one trap that is defective.

Then he dropped an answer on me that we should all take to heart.

“Just take the traps apart and adjust them. They’re just aluminum tubing and wire. There’s nothing magical in there.”

THE PLAN:

• I currently have all the traps taken off the antenna and sitting on my workbench.
• I have run each trap through my Spectrum analyzer/tracking generator setup and verified that there are indeed a couple of bad traps.
• I’m working my way through the traps, cleaning, testing, and adjusting them all.
• Once they are all up to spec I’m going to take some extra time to make sure they are weatherproof and clean up the whole antenna with some scothbrite and dielectric grease.
• Then I’m going to test and adjust it before getting it up the tower. I ain’t scared of just some aluminum tubing and wire. I will beat this thing.