[TowerTalk] Grounding Unselected Tower Coax Switch

[Tom Rauch]

> 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

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