On Fri, 14 Nov 1997 17:06:01 -0700 Eric Gustafson <n7cl@sparx.mmsi.com>
writes:
>
>
To: <topband@contesting.com>
>>Date: Fri, 14 Nov 1997 08:36:13 -0800
>>From: Ron Hill K6UR <k6ur@lightspeed.net>
>>
>>I am thinking of putting up an inverted L antenna using my 89 ft
>>tower however I have a packet node with uhf/vhf antennas in constant
>>use. Has anyone experienced problems with RF giving problems of
>>getting into uhf/vhf radio's?
>>
>>I don't want to go ahead with a 160m antenna project only to find
>that
>>I've blown up 5 packet radio's and my node computer. :(
>>
>>Ron, K6UR
>>
>
>Ron,
>
>You should not have any problems (we call 'em "insurmountable
>opportunities" around here) with safely driving that system as a
>160 merer antenna so long as:
>
>1. All of the feedlines (and control lines, etc.) associated
> with the tower are properly routed and bonded. This means:
>
> a) Feedlines (and other cable shields) bonded to tower at
> business end and again at the point where the line leaves
> the tower. This is proper lightning damage control
> protocol in any case. If the tower was much taller you
> might also want to bond the halfway point as well. But I
> wouldn't bother with that for 89 feet of tower.
>
> b) Feedlines and control lines routed down the inside of the
> tower structure. I'm assuming triangular open tower
> construction like Rohn 25, 45, 55, etc.
>
> c) Feedlines and control lines exit tower as close to the
> base (low to the ground) as possible. More than a foot or
> two up is too high.
>
> e) No unshielded control lines
>
>2. The ground at the base of the tower is good enough to support
> the vast majority of the 160 meter antenna current.
>
>3. (Possibly not necessary if condition (2) is well met) All the
> lines mentoned in (1) above decoupled so that the impedance
> (at 1.8 MHz) looking from the tower along these lines to the
> equipment is much higher than the impedance looking from the
> tower into the ground connection.
>
>4. (Also possibly not necessary but you might want to consider
> depending on how much 1.8 MHz RF power you intend to
> generate) RF bypass capacitors from individual control wires
> to shield at both ends of cable. And VHF RF feedlines
> connected to radio equipment through highpass filter(s)
>
>"Feedlines" includes the 160 meter feedline where appropriate.
>
>Hope this is useful.
>
>73, Eric N7CL
Hi Eric,
I had a somewhat similar tower setup at my old QTH including 3 VHF-UHF
antennas, a 10-15-20M stack on the mast and another 15M down on a
sidemount.
It was not possible to run the 3/4" and 1" CATV hardline down the inside
so they were all strapped to the outside of the legs.
Control cables for the rotators and relay switching of the 15M stack were
unshielded and taped to the outside. This was all done a few years before
I got interested in 160M and built the shunt feed.
No interaction problems were noted when on 160M but I did have an
excellent radial system. Also all cables were run in 4" conduit and
buried about 2' underground...I'm sure that helped the decoupling. DC
grounding was via three 8' rods right down into the water table which was
usually 3-4' down.
Also all antenna feedpoints had multiple large ferrite beads for
decoupling.
73 Carl KM1H
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