Topband: detuning shunt fed tower

[Markus Hansen]

Hi Paul:  

I have a small rotating terminated loop receiving antenna which is located only 20 feet from a Inverted L - FCP (folded counter poise) transmit antenna.  

The amount of noise re-radiated from the inverted L to the terminating loop is very high, over S9. ( receiver sensitivity -128dB at BW of 500 Hz.) 

Experimenting with different ideas I found what worked for me was to short the transmitting antenna to ground right at the point the coax feed line enters the house.  The entry box where the coax connects to go through the house wall is connected by a one foot #6 wire to a 8 foot ground rod.  I installed a vacuum relay at that point which on receive, closes the relay and shorts the transmit antenna to ground.  I used the T (transmit signal line in my transmitter) to drive a transistor switch to close the vacuum relay on receive as the T line opens the relay 5 mS before the transmitter actually begins to transmit RF and the T line also keeps the vacuum relay open until the RF diminishes to zero.  

The result, zero re-radiated noise from my transmit antenna, the terminated loop acts like a terminated loop with a deep null in the back lobe.  

This may not work with your particular set up but easy to try.  You may have to short your transmitting antenna right at the antenna not at the entry to your radio shack?

Good luck.

Markus  VE7CA
Web  ve7ca.net


On 2012-11-29, at 5:42 AM, N1BUG <paul at n1bug.com> wrote:

> I have made numerous attempts to "detune" my vertical over the past 6 years. I admit at this point I have no idea what I did wrong or what to try next. I have what I believe to be evidence the vertical is interacting significantly with the several Beverages, which of necessity are close to it. I attempted to follow the advice in "Low Band DXing" and on W8JI's outstanding web site but I didn't get very far.
> 
> The vertical is 100 feet of Rohn 25 tower with a ~30 foot long 7 element 6 meter beam sitting at 103 feet. There are approximately 100 on-ground radials ranging from 60 to 200 feet in length. It is gamma matched by a 4.5 inch triangle of #6 wires spaced approximately 30 inches off one side of the tower. The short is at about 32 feet (from memory, may be off a couple of feet in either direction), BUT the 3 wire gamma "rod" continues to approximately the 60 foot level. Is this a problem? Should I get rid of the "excess" length of the gamma match?
> 
> Previous attempts to detune the vertical ended in frustration. First I attempted to use the gamma match as the loop since that would be very convenient. Using an MFJ-259B I was able to get the loop down to about 2 ohms. This occurred with about ~900 pf capacitance vs the few thousand pf I was expecting. It did not seem to have any noticeable affect on what I presume to be vertical / Beverage interaction. I tried the same thing with a similar sized loop placed half way up the vertical on the side opposite the gamma match, with similar results.
> 
> Perhaps I am wrong about the nature of the problem? Local noises (eg. plasma TV) don't change by the expected amount (F/B of Beverages) when I switch among Beverages. Often, while listening to such a noise on one Beverage, switching from the vertical to a different transmit antenna (thus leaving the ~250 feet of coax feeding the vertical open at the shack end), there will be a dramatic change in noise on the Beverage, depending on the particular noise and Beverage selected. I take this as evidence the vertical is re-radiating noise and that detuning it should help. Is that a valid assumption?
> 
> Is there any other method I can use to detune the vertical? How about listening to a signal coming from the back of a Beverage and tuning the vertical decoupling section for minimum signal on the Beverage? Would that be valid? Any other ideas?
> 
> Noise is getting out of hand around here. Every time I listen there seems to be a new one. Of course I will continue to track them down and attempt to mitigate at the source. But the need to get my station receive performance as good as it can be has never been more evident.
> 
> 73,
> Paul N1BUG
> 
> 
> _______________________________________________
> Topband reflector - topband at contesting.com