Short of expensive fast vacuum relays, what sort of readily available T/R (not
QSK) relays handle QRO power at non-50-ohm points?
If it was purely 50 ohms, I wouldn't feel bad using a big ice cube relay. But I
suspect that on many bands the voltages at the base of the antenna would easily
jump the air gap in that kind of relay.
I'm thinking not just of detuning the transmit antenna but also general band
change activities that right now I go over to the L network at the base of the
antenna to do. That's the wall of my shack so not a huge distance but it would
be cool to do band change activities all from the operating seat rather than go
over to the wall and move banana plugs like I do right now.
Tim N3QE
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Charlie
Cunningham
Sent: Thursday, August 15, 2013 10:49 AM
To: 'Tom W8JI'; 'Tomas Magyla'; topband@contesting.com
Subject: Re: Topband: How to detune a wire Inverted L? How to
'detune'electrical fence
Sounds like a robust relay, just above the feed-point would de-tune the L
rather nicely. At least that was my thought. Probably simpler and more
straightforward than trying to detune it in the shack.
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Thursday, August 15, 2013 10:39 AM
To: Tomas Magyla; topband@contesting.com
Subject: Re: Topband: How to detune a wire Inverted L? How to
'detune'electrical fence
> Q1) Would a simple relay disconnecting the vertical wire at the TX
> antenna feedpoint be good for detuning TX antenna? (I am using a piece
> of wire attached to 18m Spidermast for the vertical section) and
> another piece of horizontal on the top.
>
> Q2) Are there any practical solutions for detuning TX antenna by using
> some circuitry at the shack end instead?
>
> Q3) What in practice could be done about the impact of the electrical
> fence? (other then moving RX antenna as far away as possible/ and
> considering using a shielded coax loop / or Waller Flag to 'null' the
> fence
> out?)
>
>
Assuming it is a 1/4 wave L, anything that electrically opens the base of the
antenna at the feedpoint with maximum possible impedance detunes the L.
Depending on any networks or transmission lines between the detuning switch and
the element, the optimum termination at the switch might be almost anything. It
is quite possible to detune the antenna from the shack, although it will
generally require a certain value of inductance or capacitance to be switched
in. I detune my 200 ft tower, fed through an L network, about 50 feet from the
element and L network with a short.
Tom
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