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Author Topic: Cushcraft A3 Questions  (Read 5381 times)
K3AN
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Posts: 787




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« on: August 01, 2010, 07:32:01 AM »

Our FD group acquired an old Cushcraft A3 tribander that we used for the first time this past June. The antenna did not come with its instruction manual so I downloaded the A3S manual from the Cushcraft (MFJ) site. When we dimensioned it, something wasn't right. We set each element's A, B, and C dimensions for mid-band and double-checked them, but when we measured the overall length of each element, it wasn't the same as the D dimension shown in the manual. Also, the point of lowest SWR was at or near the bottom of each band instead of mid-band.

So, does anyone out there have the A3 manual (NOT the A3S) and could you provide us a copy of the dimensioning pages? Will pay reasonable expenses.

Also, the lowest SWR on 20M was around 2.2:1, whereas the lowest SWR on the other bands was 1.6:1 or less. Is this normal behavior for the A3/A3S? I wouldn't be surprised, as 3 elements on a 14 foot boom is pretty narrow spacing for 20M, which would result in a low radiation resistance.

Many thanks,
K3AN
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WB2WIK
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« Reply #1 on: August 01, 2010, 08:39:35 PM »

The only difference between the A3 and the A3S is the A3S came with stainless steel hardware (the "S").

I have installed several A3's and A3S's over the years and the 20m SWR was always just fine, could place it at about 1:1 anywhere in the band I wanted it to be, by adjusting the element tips beyond the 15m traps.

If you haven't checked the traps visually/mechanically by sliding the end caps off and looking inside it might be good to do that.
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AA4PB
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« Reply #2 on: August 02, 2010, 05:46:47 AM »

I've installed a couple of A3s with the factory dimensions and not had a problem. You've got to get it up in the air, in the clear however. The SWR will be higher if you try to mount it close to the ground (less that about 20 feet).

Since it is used, checking the traps is a good idea.
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WW5AA
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« Reply #3 on: August 02, 2010, 06:19:20 AM »

I have the A3S with 40 meter add on and have since modified it. If you set the antenna on a non-conductive surface and point it straight up, it will be very easy to tune. Don't forget the co-ax choke on each end!

Lindy
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K3AN
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« Reply #4 on: August 02, 2010, 08:39:49 AM »

Thanks for the replies. When the measurements were taken, the antenna was on a wooden platform structure, about 35 feet off the ground. There was a ferrite bead (W2DU-style) choke at the feedpoint.

We had originally made the measurements with the antenna temporarily installed on the platform in advance of FD, so we were able to adjust the element dimensions to put the SWR low point closer to mid-band. The SWR curves are quite "normal" but as mentioned the minimum on 20M is above 2:1.

The antenna had been lying on the ground for some time when we bought it, so we had to flush out a lot of debris from many of the traps and then let them dry out before we reinstalled/replaced the end caps. We did a continuity check of each trap, but not a resonance check, so that will be the next step.

What's puzzling is that the D dimension (the overall element length) is shorter by over two inches than the manual says it should be when the A (10M), B (15M) and C(20M) dimensions are precisely set.
This was true on all the elements, which leads me to wonder if perhaps the A3 trap housings are a bit shorter than the A3S trap housings.
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K3GM
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« Reply #5 on: August 02, 2010, 01:31:44 PM »

How far did you dismantle the antenna?  Did you identify the traps before dismantling?  I ask this because the 15M director traps resonate at a slightly higher frequency the the ones used in the driven and reflector elements.  They look the same, but there is no permanent identifying marking on them.  If they're mixed up, this could contribute to what you're seeing.
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WB2WIK
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« Reply #6 on: August 02, 2010, 04:45:16 PM »



This was true on all the elements, which leads me to wonder if perhaps the A3 trap housings are a bit shorter than the A3S trap housings.


I don't think so, unless they made a very recent change or there's a misprint in the current instructions.

They went from A3 to A3S many years ago and I installed both versions many times.  At the time, there was no difference in the design at all and the only difference was the SS hardware on the S model.
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W5CBO
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« Reply #7 on: August 03, 2010, 10:24:10 AM »

I had an A4S and I installed the traps in the wrong place and although it tuned perfectly on 20 meters it was way off on 10 and 15 so I believe your issue is the traps.
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K3AN
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« Reply #8 on: August 03, 2010, 11:06:27 AM »

"They look the same, but there is no permanent identifying marking on them.  If they're mixed up, this could contribute to what you're seeing."

I noticed the lack of markings once we started cleaning the antenna. The guy who removed it from the tower did bundle each element's components together, and we were very careful to keep them separated as we worked on them, but still a mix-up could have occurred.

Six physically identical 10M traps and six more physically identical 15M traps with no permanent identifying markings. Wow. This must have been the very first multi-band antenna Cushcraft ever designed, and the designers obviously had no real-world experience at the time. I can't imagine any other explanation, and they did fix the problem with the A3S.

I'm hoping the resonant points are sufficiently different that the difference will show up using an antenna analyzer.
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K3GM
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« Reply #9 on: August 03, 2010, 04:06:30 PM »

Years ago, Cushcraft offered a service to identify the traps once the user had mixed them up.  I guess it was too much to engrave to trap ID on them!  It happened to me.  I found a grid dip meter was too coarse to identify which was which, but fortunately I had the use of my company's HP network analyzer and with that I was easily able to sort them out.
« Last Edit: August 03, 2010, 04:32:37 PM by Tom Hybiske » Logged
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