Topband: How to detune a wire Inverted L? How to 'detune'electrical fence

[Shoppa, Tim]

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 at contesting.com] On Behalf Of Charlie Cunningham
Sent: Thursday, August 15, 2013 10:49 AM
To: 'Tom W8JI'; 'Tomas Magyla'; topband at 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 at contesting.com] On Behalf Of Tom W8JI
Sent: Thursday, August 15, 2013 10:39 AM
To: Tomas Magyla; topband at 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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Topband Reflector

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Topband Reflector