NOTE: My AV640 is mounted on a 24' 2" mast that is mounted on the side
of a 10' tall, guy anchor post. There is a winch mounted about 4' off
the ground on the back side of the post. The 6/16" double braided poly
rope goes from the winch to a pulley about 10" above the top of the post
on a stand off. From there it goes to a pulley mounted at the same
level on the 24' mast and then back to the top of the post where it is
anchored.
Once the AIM 4170 was set up, calibrated and I had passed through the
*basic* learning curve I ran curves on all the bands 40 through 06 with
the results being quite surprising. Other than the minimum SWR being a
little higher than when I first set the antenna up on the 32' Aluminum
tower all bands were good except 6 which was showing 4 dips in SWR with
the lowest being 1.3:1 at 50.100 and less than 100 KHz wide. The other
dips were above 2:1. Originally there was only one dip at 50.125 which
was 1.0:1 and quite broad at well over 300 KHz wide but I don't have a
trace of that. 40 was looked almost normal at 1.17: 1 and over 270 KHz
wide centered on 7.240. Originally It was centered around 7.150 at
1.0:1 and about 2:1 or a bit less at the band edges.
These traces are great.
Sooo... I decided to let the antenna down and add 6" to the 40 meter
radiator which should lower it to about 7.175. I lowered the antenna
and mast down onto 2 tall sawhorses, extended the radiator, and raised
the whole works back up.
Low and behold, when I checked 40 the SWR was off scale for most of the
band with the top 50 KHz being at the top of the graph. Six was a tiny
bit more screwed up with the SWR dips being a bit sharper. HOWEVER, Now
none of the other band were resonant at all.
As I have two matching networks, I swapped them out, but still showing
about the same results. (The one I installed has the compensating cap
replaced with a 50 pf transmitting doorknob which I think is very close
to the original value) I brought the new one inside, pulled the cover
off and found the inside of the box covered with a lot of white powder.
Then I noticed bluish shadows on the cores for the 4:1 transformer and
the insulation melted in a few spots. The current balun, or choke balun
had been so hot it had melted the cable ties on it and the cores are
discolored with a dull, mottled finish. They must have been really hot
some time or other. I've found no burn marks on the PC board..."yet".
My guess is lightning.
No power was run to the antenna during these tests other than the tiny
amount from the AIM 4170 and I'm pretty sure that doesn't have the
capability to melt anything.
Right now by best guess is that some where along the line the antenna
was struck by lightning, heating the cores and melting some insulation.
Lowering the antenna and putting it back up must have moved something
causing a short some where as the current matching network should be good.
This is about the point where I ended up getting whacked in the back of
the hand with the winch handle and lost all interest in the project for
a few hours.
BTW although still a bit swollen along with a big ACE bandage and
technicolor fingers I can now type.
I'm going to replace the cores for the choke balun with the #32
material. I need to find some good silver wire with teflon insulation
for the 4:1 Balun and I don't know what material they used for that.
I'll give Hy-Gain a call Monday and hope they'll tell me.
I'm pretty sure something else is shot in the antenna, but it's
certainly not obvious.
73
Roger (K8RI)
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