Jim and List Members,
A sidenote to Jim's comment: at least one vendor makes a SHIELDED rotator
cable. Yes, the usual 8 conductors, with 2 of them over-sized, all
enclosed with a shield braid and UV-resistant jacket. If I recall
correctly, the vendor is RadioWorks, in Portsmouth, VA, USA. They have a
website. I use some of their non-shielded rotator cable to control an
Ameritron remote antenna switch at the base of my tower. I intend to use
their shielded material when I eventually get a rotator on that tower. (I
presently have only fixed, wire antennas hung from the tower.) Either way,
they seem to be among the few sources of rotator cable made with
UV-resistant plastic jacketing. One would think that all such cable would
automatically have that characteristic, given the application.
Of course, one can always place ferrites over the rotator cables, but you
must find clamp on beads that are uable at HF. So many of them on the
market only start to become useful around 30 MHz. If you get the shielded
rotator cable, you would want to bond the braid to the tower at both the
tower base area and as close to the rotator as possible. Be certain to
seal the cable at the bonding points so that moisture does not get in and
corrode the braid.
Regarding Ian's comment below ... he is right on. Actually, any metallic
object will present an "extended grounding point" at some frequency
(typically, about 1/2 wave from actual ground); it will be Ian's "common
connection point" at all other frequencies. Case in point that I used as
an example in a class I used to teach: an aluminum flag pole, 40 feet (or
about 12 meters) high, could be considered a grounded object. (We'll
assume that the base is metal and set in concrete with mesh and a grounding
conductor for lightning current conduction.) Yet, one could place a
matching network either inside the flag pole (for you stealth fans) or
outside, locate a tap-on point, and use that same "grounded object" as a
fairly decent vertical antenna. So, is the flag pole a grounded object or
an antenna? Extend the analogy to a tower and you have the same condition.
73, Dale
WA9ENA
Jim Smith
<jimsmith@shaw.ca To: rfi@contesting.com
> cc:
Sent by: Subject: Re: [RFI] high noise
level...
rfi-bounces@conte
sting.com
09/11/2003 01:45
PM
In all the stuff I've read about current chokes, feedline radiation due
to asymmetry, etc. I don't think I've ever seen this possibility pointed
out (or thought of it). If coax can pick up RF and reradiate it, why
not the rotator cable? (A disturbing thought, why not the tower too?
Pretty difficult to wind it in a multiturn coil.)
Looks like another good reason to mount the rotator near the base.
73 de Jim Smith VE7FO
Ian White, G3SEK wrote:
>>
>>
> Another possibility is that it might be coming up the rotator cable,
> and crossing over to the antenna at the top of the tower. Remember,
> there is actually no "ground" at the top of the tower - there are only
> "common" connections.
>
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