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)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|