Topband: A 160 Meter antenne puzzle

[Michael Tope]

Glenn,

The impedance of the very short element at the point where it attaches 
to the 4x4 is very high (i.e. very high capacitive reactance).  When you 
stick your hand on the 4x4 you are adding the impedance of your 
hand/body in parallel with a very high capacitive reactance (i.e. the 
equivalent of a very small value capacitor). That is why you see such a 
large change in VSWR when you touch the wood. It also explains why you 
don't see a change in the 8MHz self-resonance of the element. At that 
frequency the element impedance where it attaches to the wood is very 
low (probably 15 to 30 ohms resistive). The impedance of your finger in 
parallel with that only makes a miniscule change, so hence there is no 
visible shift in element self-resonance.

Everything you are observing seems reasonable to me. I wouldn't be 
concerned.

73, Mike W4EF...............

On 1/7/2013 6:30 PM, Glenn Biggerstaff wrote:
> Hi everybody and happy new year .
> I have what I think is an interesting  antenna riddle . I have some receive  antenna verticals (4) that are selected by relays in pair and the pair is fed back to the shack to a phasing boxing .Each antenna is AL tubing about 30 feet tall, base loaded with an inductor and a resistor very much like ON4UN has for his 4 square receive  array .There is a 1 to 1 balun between the antenna and the feed line . The tubing is insulated  with an eight inch length of pvc pipe from  the 4x4 that supports it .The overlap between the tubing and the 4x4 is about eight inches as well . I was doing some testing today and discovered that touching the 4x4 at the top caused the swr to increase from about 1.2 to 1 to 1.7 to 1 . The lower on the 4x4 I touch the less the effect . I am only moving my finger a fraction of an inch and my hand not at all when this happens .The feed lines are disconnected when this is going on .There is no dc continuity between the antenna and any mounting hardware or th
>   e post .I disconnected the base loading and put  the analyzer on it and it is resonant at about 8 mhz and touching the post causes no change with this setup . There are eight 30 foot radials laying on the ground and a 2 inch wide copper strap about 3 foot long  in the hole that the 4x4 is in .
> What is going on here and should I make an effort to limit the coupling between the 4x4 and the antenna?
>
>
> 73 , Glenn WW4B
> _________________
> Topband Reflector
>
>