Tom,
This is not encouraging news for those of us with towers already ground
and are either shunt fed or cage fed. Even though they are not
resonate, although top loaded and approaching resonance, it seems that
even with some elaborate decoupling arrangements, and I hope I am wrong,
not to much can be gained by even trying to get them "detuned". This is
crucial for me to know, and I would assume many others that have
Beverage terminations or feed points 100 feet away,
Thanks for the warning as this summer I was going to install a HV relay
on the shunt cage to ground....but as you point out it may not help as
the grounded shunt or cage fed tower is already a noise source for close
proximity RX antennas.
On 6/20/2012 2:09 PM, Tom W8JI wrote:
> There are two potential problems with this. As general rules:
>
> 1.) Grounding any antenna which is dependent on a ground system to be
> resonant will maximize reradiation.
>
> 2.) Resonant elevated radials, even without an antenna connected, are
> resonant and re-radiate.
>
> 3.) Things that are not resonant can still re-radiate.
>
> To minimize coupling from a resonant radial, the radial has to be
> disconnected from ground and from other radials.
>
> Different systems can be different, and in some cases re-radiation can
> actually cause a null that reduces noise, but the general rule is
> self-resonant antennas with a ground (Marconi), or nearly self-resonant
> antennas when grounded, should be floated. Other antennas can be terminated
> in an inductor, capacitor, or opened, depending on the system.
>
> My 220 ft insulated tower, in the center of a four square, is "opened" by
> shorting a specific length of coax to the "L" matching network. It has much
> more radiation when open or grounded. radiation is minimal when detuned by a
> proper impedance.
>
> 73 Tom
>
>
>
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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