----- Original Message -----
From: "Jim Lux" <email@example.com>
Cc: <firstname.lastname@example.org>; "'Gary Slagel'" <email@example.com>
Sent: Friday, April 11, 2008 22:47 PM
Subject: Re: [TowerTalk] remote switching for balanced lines
> Gary Schafer wrote:
>>> -----Original Message-----
>>> From: firstname.lastname@example.org [mailto:towertalk-
>>> email@example.com] On Behalf Of Jim Lux
>>> Sent: Friday, April 11, 2008 4:50 PM
>>> To: Gary Slagel
>>> Cc: firstname.lastname@example.org
>>> 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
>> 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 http://home.earthlink.net/~w6rmk/relay.htm 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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