De-tuning an electric fence is something I have thought about (I have some
here).
Two possible ideas (these are not proven - so I won't say "solutions").
If the fence doesn't need to be active while "detuned" - you can break it
up into odd multiples of quarter wavelengths (mind your velocity factors).
If the fence does need to be active - you can do step #1 - but then put
some kind of choke in series of the places you break it up.
The one beverage I have near electric fences doesn't run too close to the
wires except at one point where they are nearly perpendicular.
Tree N6TR
On Thu, Aug 15, 2013 at 7:57 AM, Shoppa, Tim <tshoppa@wmata.com> wrote:
> 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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> Topband Reflector
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