The easy way to do that is with a relay between the coax center conductor
and the L wire at the feedpoint. When the relay is open, that makes the L a
1/4 wave on 160 which is not self-resonant. The quarter wave is only
resonant in conjunction with whatever counterpoise you are using for the L.
73, Guy.
On Sun, Jun 17, 2012 at 4:37 PM, Bob K6UJ <k6uj@pacbell.net> wrote:
> I am very interested in this subject, I am trying to figure out how to
> detune my 160 inverted L. I am using a flag antenna for receive and
> unfortunately
> it is only about 50 feet from the base of the L. By itself the flag
> works very well at reducing the local electrical noise, (which luckily it
> is coming from only one direction)
> When the L is strung up it the flag is useless, it reradiates the noise
> like mad. I'm hoping to learn more solutions on detuning xmit antennas
> during receive from the group.
> Wouldn't it be nice if all we had to do was ground the feedline to the
> xmit antenna during receive ? sigh................ I guess the idea
> is to make the xmit antenna non resonant anywhere near
> the operating freq during receive.
>
> Bob
> K6UJ
>
> On Jun 17, 2012, at 8:03 AM, W2XJ wrote:
>
> > There is no single solution for detuning an antenna it depends on
> > electrical length. For verticals a quarter wavelength or less it is
> > common to have a contactor between the network and the radiating element
> > and open the connection to float that element. This is the practice in
> > commercial directional arrays where some tower are not used in a day or
> > night mode. This does not work well on taller towers and I do not know
> > offhand how it would work with the T. It is easy enough to model in
> EZNEC.
> >
> > On 6/16/12 3:02 PM, Shoppa, Tim wrote:
> >> OK, several folks came back and recommended detuning my transmit
> antenna in receive.
> >>
> >> Certainly any receive antenna I have is going to be in the shadow of my
> transmit antenna, a 130-foot flat-top about 85 feet up, fed against ground.
> (Really it's an 80 meter doublet fed with ladder line. I tie the ladder
> line together at the bottom and feed against ground.)
> >>
> >> Trying to figure out what "detune" is.
> >>
> >> I match the transmit antenna to 50 ohm coax with a L network at the
> bottom. If I want to detune, then I can.... open up the connection between
> the antenna and the L network at the bottom? Unhook the L or C in the L
> network? Short out the antenna where it comes into the L network? I could
> rig up a relay contact to do any of those.
> >>
> >> Tim N3QE
> >> _______________________________________________
> >> UR RST IS ... ... ..9 QSB QSB - hw? BK
> >>
> > _______________________________________________
> > UR RST IS ... ... ..9 QSB QSB - hw? BK
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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