I decided to take a day off work to see if I could fix my TB Tx before
leaving for a week away. I duly bought a length of acetal rod, as
suggested here, and had my friend turn it down to size for me.
Replacing the old insulator was quite a marathon, for reasons I won't go
into here.
While reassembling the choke I have wound in the control cable for my
SteppIR yagi, I saw what the problem had been all along. I found a
scorch mark on the hot side of the control cable where it had obviously
touched the cold side of the stub mast. So although the insulator had
bubbled quite badly, that was not in fact causing a problem at this
stage. So in the end it took me all day, and cost me $150 (I had to
replace the stub mast as I could not remove the old insulator) for
something that I could have fixed in less than an hour, for nothing! I
had been a bit careless when replacing the stub mast after storm damage
a couple of months ago.
One or two list members have made the assertion that I should abandon
this top feed arrangement, and revert to a standard gamma match. One
reply was quite emphatic. I am not going to do that, as I have proved
by comparitive ground wave tests at 1 km, that the top feed is a couple
of dB better. This has been verified over a long period with DX
contacts as well. I believe the reason to be that my vertical is quite
short, only 65 ft, and I cannot deploy a full set of radials. Feeding
the top of the mast reduces the current in the radials, resulting in
lower loss. The top feed idea came from a Rad Comm article by G3LNP in
2002.
Once again, thanks for all the posts on insulating materials. In spite
of the fact that the insulator was not causing my immediate problem the
bubbling in the old one cannot be healthy, and I am glad to have
replaced it with something better. What's more, I now hope to be on in
the Stew, as long as nothing else goes wrong in the next couple of weeks.
73, Greg, ZL3IX
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