Topband: Detuning transmit antenna in receive

[Bob K6UJ]

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