[TowerTalk] Some update on the AV640 proble

[K8RI]

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)