Line Mismatch -- was -- Re: [BC] Problems that make you want to scream!

[Alan Alsobrook]

I started out mentioning a sooty connector, and of course everything
you've said is correct. So just to jump back to my point for a second
for a bit more explanation, Not only can it be as you describe with a
mismatch looking good. you can also have a bad line that is NOT
mismatched. I've seen this with water in connectors and with damaged
(lightning struck & bullet shot) lines. If things happen just right you
can make a dummy load out of the feed line.

The most surprising case of this I've seen was at WHIE. I got the call
that the station couldn't be heard more than a block away. Yet all
readings on the transmitter were normal. When I arrived sure enough the
gates 5P2 was happy as a clam yet the station was off the air. I went to
the tower base and switched in the base meter which read 0. I used a
grounding stick on the feed line coming into the ATU, and absolutely
nothing. Now I'm scratching my head back at the transmitter wondering if
I just have DC values up on the TX. So (this is NOT a recommended
trouble shooting procedure) I decided I would see for sure if I had RF
leaving the TX.  To do this I loosened up the clamp on the coax center
conductor and pulled it back, sure enough I got a nice modulated arc.
I put that back and with confidence started walking the transmission
line. 40' from the transmitter in a raceway running down the rear of the
building I found a VERY hot spot. (luckily it was a concrete block
building or I could have asked the fire department where the problem was
when I got there) Upon opening up the raceway I found a splice in the
RG-17 cable that had turned into a dummy load.

Cowboy wrote:

>  We're not changing the voltage standing wave ratio along the line,
>  but we are certainly changing the Z as seen looking into the line.
>  The CB trick was a who cares, as long as the transmitter sees 50 ohms ?
>  The line cares, as line losses are not reduced even though the tx can
>  couple more power into it.

-- 
Alan Alsobrook CSRE AMD CBNT
St. Augustine Fl. 32086 904-829-8885
aalso at Bellsouth.net