[50mhz] Pre-amp question follow up

[Bill VanAlstyne]

Mike,

Solid conductor coaxial cable is a bad idea between the rotating antenna(s) and
the stationary tower (regardless of where the rotator is inside the tower -- 
irrelevant) because it is rather inflexible. When the antenna(s) turn, the cable
needs to be able to flex easily without putting a strain on itself or on the
rotator. You want to run a stranded-center-conductor coax between the antennas
and your switch box or pre-amps or whatever. I use Belden 9913F7, a
high-quality, low-loss VHF/UHF coax with foam dielectric and a seven-strand
center conductor. It is quite flexible. There are other, cheaper "no-name"
brands of coax that are basically 9913F7 knock-offs. Some of these seem to be
OK, according to reports, but I always go with the Belden cable just to play it
safe.

You will find, once you really get into VHF work, that you will want to monitor
several bands simultaneously. You may even find that you want to monitor the
same band with more than one kind of antenna simultaneously -- for example, a
stacked omnidirectional horizontal loop system in addition to your yagi.  :-)
So operationally, separate feedlines are the way to go on VHF/UHF.

In addition, many relay-based remote coax switchboxes that work fine at 50 and
144 MHz start to look really bad above 200 MHz, and are completely unusable at
430 MHz due to high losses and high VSWR. You should definitely avoid ordinary
"frame" relays (found in most remote antenna-switching boxes) at UHF. Remote
switches that do work well at this frequency use constant-impedance switching
mechanics, and tend to be rather expensive.

Bill / W5WVO


Michael A. Urich wrote:
> Original comment
>
> My vision is to run a single LMR-600 feedline to the top of the tower
> and then switch it to masthead pre-amps with LMR-400 runs to the
> antennas.
>
> * * *
>
> A point of clarification + a follow up question.  I haven't ruled out
> running a single feedline per band and the pre-amps do not have to be
> installed at the time the tower goes up, and due to budget
> limitations may have to come later but I want to plan for them when
> the tower goes up.
>
> One individual indicated that the LRM400 from the top of tower around
> the rotor is a bad idea because of its solid center conductor.  My
> intention is to have the rotor down inside the tower actually a few
> feet lower than my thoughts about where the weatherproof box for the
> pre-amps will go.
>
> My question(s) is Why would running a solid center conductor near the
> rotor be a bad idea?  And a follow up, If a solid center conductor is
> the improper approach then would running an even larger feedline past
> the rotor be incorrect as well?
>
> Mike Urich, KA5CVH
> www.ka5cvh.com
>
> Moderator: Ray Brown, KB0STN
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