[TowerTalk] 75 ohm CATV ?? DIP or PRUNE ??

[K0FF]

Hello Art,
I see in the Belden catalog that the RG-214 / Belden 8268 has
1.5 dB loss per 100' at 50 MHz. In terms of loss, that means you are
 losing 30 % of your power in that piece of line alone. Losses can't be
eliminated, but they should indeed be recognized and controlled. You are on
the right track here.
A realistic goal is 1dB (20%) or less per run, regardless of frequency.
I don't have a chart on the 3/4 CATV cable
 (can someone point me to one, for my records?..tnx), but it's pretty
good stuff, especially for long runs. It would no doubt come in at way
less than 1 dB/100.
I'm afraid that the introduced negative concerns aren't warranted for such
a short run however. These include impedance management, connectors etc.
The cost trade off just isn't there. If you had a really long run
 ( like my 450' to the 11 el 6M beam......) it would be different.
On the subject of using tuners at 50 MHz, don't forget that their
purpose is to flatten out slight coax cable mismatches to make the
 finals happy only. They introduce loss in the process. I can't say what
 every one across the board does, but in my IC 756 for example I get
1 dB LESS power into a dummy load with the tuner in line.
My suggestion (for what it's worth) : Buy a brand new piece of Times
 LMR600 with the proper connectors already installed.
Second choice: pick up some good used 1/2" Heliax.

Next: add another 6M antenna way lower on the same tower, and a select
relay for the feedline! This would be for the Es skip (low ant = high
angle).

Hope this helps..

NOTE: When I use 75 Ohm cable for a particular antenna, that antenna
is either designed for, or tuned for 75 Ohm feed. No big deal, and that
eliminates any mismatch at the load. In the shack, the feedline goes to
a tube amp with either a PI or a PI-L output network, which matches the
75 Ohm straight away. If the amp is bypassed, I run directly into the cable,
and flatten it out with an onboard tuner
( coax line flattener actually...wish they would not call them
"tuners"..they aren't).
This applies to HF. On VHF I shy away from the 75 Ohm and use 50 Ohm
commercial
stuff, up to 1-5/8", and use relays (or microwave switches) at the far end
to direct it to
different antennas.

Happy Building, Geo>KØFF
http://homepages.dstream.net/K0FF


----- Original Message -----

Subject: [TowerTalk] 75 ohm CATV ?? DIP or PRUNE ??


> The subject says it all, but here is the details.
>
> My 6 meter antenna is mounted at 75 feet on a Rohn 25 tower. The
> feed line from the antenna to just past the rotor is RG 213, then
> down the tower 70 feet of 1/2 inch 50 ohm heliax, from the base of
> the tower to the shack 110 feet of RG 214.
>
> I would like to replace the 110 feet of RG 214 with CATV 3/4 inch.
>
> Now for the question, should I dip it to correct length or prune it
> for best VSWR at the shack.  Also why do you feel this way.  By the
> way the VSWR is now about 1.2:1 at the shack.
>
> Thanks guys
>
> Art
> KB7WW
> CN85
>
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