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[TowerTalk] The AV640 Saga Continues

Subject: [TowerTalk] The AV640 Saga Continues
From: "Roger (K8RI)" <K8RI-on-TowerTalk@tm.net>
Date: Sat, 25 Apr 2009 16:37:58 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>

I finally was able (got around to it) to lower the 40' tower with the 
AV640. The new arrangement makes this an easy task and with the 
sawhorses placed properly it makes the antenna easy to work on.

The matching network consists of a 9 turn current choke/balun at the 
input, followed by a 4:1 balun (bout 11 turns) to match the impedance. 
Both consist of two fairly large cores.  There is a coupling cap to the 
radiator, a tap to the counterpoise, and a choke from the radiator to 
ground for protection. My guess as to the reason it's derated on 
6-meters is due to core heating.

"This time" I found the choke across the output of the matching network 
fried. (I say fried, but there were no visible marks). I clipped it out 
at Hy-gain's suggestion.  Removing it brought resonance for all of the 
bands back to the proper location and the SWR was near perfect.  
Soooo...  I raised the tower back up to do a test starting with 40 
meters. It was working great at 100 watts so I kicked the amp on and 
started bringing up the power. Each key down was just long enough to get 
a steady reading or about one second each.  At around 500 watts out, the 
power and SWR started jumping around which appeared to be an arc...which 
is where I called it quits for the night.  Sad too as that evening I was 
hearing a lot of European stations quite strong. One Italian station was 
between 10 and 20 over 9.  The next day (Yesterday) the antenna analyzer 
showed a high SWR, but the resonant frequencies were OK and no jumping 
around for output power or SWR.  I think that's a pretty good indication 
of the coupling cap to the active element being shot although it looks 
OK on a ohm meter.
Yes, I checked everything between the rig and antenna by substituting 
with a single run of coax from the antenna to the rig. I have blown out 
N connectors before, or at least nature has.

No work on antennas today with high winds and severe thunderstorms. I 
can see a big batch of them headed this way at 50 - 60 MPH. A small 
batch went through early this morning at roughly 100 MPH and a bunch of 
hail.

So, other than waiting for a new cap, does any one know what value they 
might use.  I'm guessing it's probably just a standard 0.01 that could 
stand to have  higher current and voltage ratings.


73

Roger (K8RI)
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