At 3:02 AM -0800 3/22/98, Tom Rauch wrote:
To: <towertalk@contesting.com>
>> Date: Sat, 21 Mar 1998 12:51:52 -0800
>> From: Dick Flanagan <dick@libelle.com>
>
>> Grounding seems like such a logical choice, I am surprised they are shipped
>> floating. Am I missing something? Is floating unselected antennas better
>> than grounding them?!
>>
>> 73, Dick
>
>Grounding doesn't seem a logical choice at all Dick.
>
>Grounding the antennas provides no improvement in lightning
>protection when using that particular switch box.
>
>The reason no improvement is offered is simple. The relays pull a
>shorting bar between the antenna port and the centrally located input
>port. When the relay is de-energized, the shorting bar is grounded.
>
>Because of that, the only lightning path is broken and blocked by a
>grounded relay armature. It's exactly like you disconnected the
>center conductor and stuck a well grounded metal plate between it and
>the rig port. That is true no matter how you configure the
>unselected contacts.
>
>The only real concerns are RF effects.
>
>In the case of RF, without careful planning it becomes a matter of
>luck that either opening or shorting unused feeders would minimize RF
>interaction in most installations, because feedline and antenna
>lengths are so random.
>
>For example, if the unused feeder is around 50 ft long and connected
>to a 1/4 wl 80 meter vertical, you would want the feeder (when
>unused) shorted to minimize interaction with a close by 40 meter
>vertical. If you had a 160 meter antenna near the same vertical, you
>would want the vertical's feeder left open!
>
>
>
>73, Tom W8JI
>w8ji.tom@MCIONE.com
Thanks for the comments, Tom.
This is to be a top-of-tower-mounted switch, so feedline lengths from
switch to antennas will be minimal. Three of the antennas will be yagis
and the other will be an inverted V or some other wire antenna.
My primary reason for thinking it made sense to ground the unused antennas
was for static discharge. If they were grounded, any static buildup would
be automatically and continuously drained off to ground. The negative
scenario I envisioned was to have a floating antenna building up a
significant static charge and then switching my radio onto that "hot"
antenna.
Thanks, again, for your help.
73, Dick
--
Dick Flanagan W6OLD CFII Minden, Nevada DM09db (South of Reno)
Visit http://www.qsl.net/w7di/
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