Topband: Receive Antenna F/B

[Tom Rauch]

> Thomas KN4LF Says:
>   My experience is opposite. When my 3/8 wave L floats it degrades F/B on
> my pennant antenna's. When the L is grounded it is electrically invisible
> and everything is normal. My transmit and receive antenna's are separated
> by 2 wavelengths!

It depends WHERE you ground or float the antenna, and how long 
the antenna is. I'm sure Mauri was speaking of the specific case of 
a 1/4 wl inverted L and disconnecting at the antenna....not in the 
house or anywhere else but at the antenna.

With a 3/8 wl long antenna, you are better adding inductance from 
the antenna to earth at the antenna to detune it. Any capacitance 
will increase re-radiation.

You can add inductance by shorting the end of a short  
transmission line, depending on the matching network. Other 
lengths will require opening the line to detune the antenna.

I do that with my 200 foot tower by choosing the length of line to a 
switch that I open when I want to detune the tower. If I used a 
length of line exactly a 1/4 wl longer, I'd have to short the line to get 
the same effect.

I see changes in F/B on antennas hundreds of feet away, so I 
routinely detune all unused antennas.

73, Tom W8JI
w8ji@contesting.com


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