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Re: [TowerTalk] remote switching for balanced lines

Subject: Re: [TowerTalk] remote switching for balanced lines
From: Jim Lux <>
Date: Fri, 11 Apr 2008 20:47:21 -0700
List-post: <">>
Gary Schafer wrote:
>> -----Original Message-----
>> From: [mailto:towertalk-
>>] On Behalf Of Jim Lux
>> Sent: Friday, April 11, 2008 4:50 PM
>> To: Gary Slagel
>> Cc:
>> Subject: Re: [TowerTalk] remote switching for balanced lines
>> Gary Slagel wrote:
>>> Anyone have any ideas on how to remotely switch
>>> balanced lines.  The idea being to have 2
>>> dipoles/doublets on a tower with one balanced feedline
>>> running to the shack and be able to switch the
>>> feedline from one antenna to the other at the tower.
>> Assuming you want to fabricate something:
>> Double pole double throw relay, coil operated by DC carried on the
>> feedline, decoupled with a choke.
>> Use any of the relays that folks use in antenna switch boxes that are
>> available in DPDT configurations..
>> If you want something off the shelf, it's going to be a bit trickier.
>> You could modify something like an MFJ RCS-8V pretty easily by cutting a
>> couple traces.  The relays in that are basically SPST switches, so you'd
>> use 4 of the 5 to do your switching.
>> Jim, W6RMK
> You will want to have something better than the type of relay used to switch
> 50 ohm coax though. 1500 watts at 50 ohms is 273 volts. 1500 watts at 600
> ohms is 948 volts and that is providing the swr is 1:1. 

And those are RMS voltages... what you need to be thinking about here is 
peak voltage.

The relays used in most amateur market antenna switches are usually AC 
power relays of one sort or another.  Granted, I don't know that I'd 
want to run a kilovolt on one, but 500V is probably reasonable..

The ones designed for use in UL listed gear probably have hipot tests in 
the 2-5 kV range, although at RF, the HV breakdown behavior is going to 
be different. For instance, the Shrack/Potter-Brumfield (now Tyco) type 
RT, which is popular in switches and autotuners, has 10mm spacing and 
5kV dielectric breakdown coil to contacts and are rated to withstand a 
surge voltage of 10kV (DC, 1.2/50 microsecond waveform).  Dielectric 
strength between open contacts is 1kV for 1min, and between poles 2.5kV. 
  They're rated for 3/4hp motors at 480VAC, for instance.

more at  although I haven't 
checked to make sure the links all still work (the tyco one does) and 
updated with new info.

The limiting things are probably going to be stuff like corona discharge 
or arcs off the corners of the components.  Below 300V or so, of course, 
you can't get a flashover of any kind across an airgap, no matter how 
small the spacing or sharp the edges.

I think a lot of the "old time" sort of switchgear used with open wire 
lines was sized for use open in the outdoors with dirt and crud 
accumulating, or was repurposed from equipment actually designed for 
things like AC power panelboards (where hot switching might be in the 
list of requirements.. )  both of which drive towards physically larger 
sizes.  Think, if ham could have scrounged up a big old DP knifeswitch 
from a scrapped theater lighting or industrial panel board, especially 
if it was free, it would be a natural to use with open wire line.

Jim, W6RMK

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