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Re: [TowerTalk] Beam problems A3S

To: Jack Brindle <>
Subject: Re: [TowerTalk] Beam problems A3S
From: Bill Coleman <>
Date: Sat, 29 Nov 2008 10:59:47 -0500
List-post: <">>
On Aug 3, 2008, at 12:21 PM, Jack Brindle wrote:

> Over the last several years I have spent quite a bit of time trying
> to get a 40m kit going on my A3 beam. I'm still unsuccessful, but
> about to give it another go. What I found is very similar to your
> results. My observations are that with the A743 the antenna is very
> difficult to tune, the SWR is way off for the given dimensions, and
> adjusting the driven element lengths to make the SWR look "right" are
> not fruitful. There just isn't enough aluminum...

I've written to Jack about his problems, which may be the result of  
trying to mix the older A3 with the A743. I'm sorry to hear that he  
hasn't gotten it working.

I've been running an A3S/A743 for about three years. I've owned the  
A3S for about 20 years, but it spent about 5 on the ground, and 2  
installed at a friend's who was waiting for Bencher Skyhawk. I rebuilt  
the A3S seven years ago when I finally got a tower up again.

For my A3S, I set the dimensions half-way between the CW and MID  
settings. This appears to be a good compromise to cover the entire  
band with reasonable SWR. Before I mounted the A3S, I had it on a  
short mast lying on a deck, pointing skyward with the reflector about  
3 feet off the ground. This is a good orientation for spot-checking a  
beam before mounting on the tower.

When I upgraded with the A743, I left the beam installed on the tower  
and removed the driven element. I then changed out the parts for the  
A743 and re-installed on the boom. This is not the easiest way to  
upgrade -- it is much easier to install everything with the whole  
antenna near the ground. I had a lot of trouble getting the element  
support tube clamped to the boom.

Once installed, it worked like a champ. There was some SWR change on  
the other bands, and the 20m SWR bandwidth was reduced, but the  
antenna works well, even on 40m. At my QTH, my 15m tower is the  
highest point in my yard, and the A743 works better on 40m than any  
doublet or sloper I've tried.

My suggestions would be as follows:

1) Take off the A743 traps and put the standard element ends on.  
Adjust the antenna to the factory dimensions. Double check all the  
dimensions, especially on the driven element.

2) Place the antenna pointing skyward, and check the SWR curves. If  
you can't orient it skyward, then mount 5-10 feet off the ground and  
measure. Mounting close to the ground will cause the SWR curves to be  
about 100 kHz low compared to on the tower.

3) If the SWR curves don't look reasonable, you most likely have a  
trap problem. Either you've gotten the wrong trap on the element or  
you have oriented it the wrong way. All of the A3S traps should have  
the outer shell side toward the boom. You can remove the end caps to  
determine which end is connected to the outer shell.

4) Another trap problem may be the trap connections. Cushcraft just  
uses a sheet metal screw to make the terminations on the traps to the  
elements. These can corrode and cause a poor connection. Make sure  
each screw is tight.

5) You can try checking the traps with a dip meter to see if they are  
on the same frequency. It's difficult to couple to these kinds of  
traps, since the coil is enclosed in the outer shell. Try coupling  
from the open end of the shell. Measure the two 15m traps on the DE  
first, and see if the dip anywhere near the same frequency.

Bill Coleman, AA4LR, PP-ASEL        Mail:
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901


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