> To: btippett@CTC.Net, amps@contesting.com
To: <amps@contesting.com>
> Date: Wed, 30 Jul 97 01:11:20 +0000
> Although it obviously radiates Bill, I question the efficiency of a #12
> wire 4' away from a large tower cross section.
> Do you have anything to show that this is a correct d1/d2 ratio and
> spacing? It just does not seem proper IMO and I would think that it
> could be improved.
Hi Carl,
While a larger conductor would have less loss, the loss is so low to
begin with you could never notice the change.
Loss is only due to resistive losses, reactances do not cause loss.
There would be very little loss difference if the wire was #16 or
four inch copper pipe, since the current is so low and resistance so
low even in the copper wire.
It does make the antenna much broader however, since the thicker
gamma adds less inductive reactance so the series tuned feed system
has lower Q.
> An accurate way to grid dip a tower was explained in my 12/86 CQ article.
> Simply drop a #12 or so wire from the very top of the tower and couple
> loosely to the gdo at the base via a one turn loop and the antenna
> appears as a folded unipole. This required a minimum of 4 radials at the
> tower to function properly for me. .
That does not give an accurate measurement of self-resonant
frequency, because the gamma wire forms a stub with the
tower, and acts like a transmission line.
I have an insulated 135 ft tower with a top hat that used to be
grounded. I just modified it two weeks ago. I shunt fed that tower
as part of a two element phased array, and I have the drop wires
still hanging down the side.
I measured the resonant frequency by using one drop wire and a GDO.
With the tower base grounded, I measured about 1300 KHz.
When I tied the drop wire to the tower and measured the resonant
frequency with the GDO coupled to a link between the tower base and
ground, it measures about 1600 KHz.
But who cares? All you really need to know is the impedance of the
feedpoint. If the tower is resonant below 160, all it does is reduce
the bandwidth slightly. The only horror story would be if the tower
approached or went below 900-1000 KHz.
In that case an electrically fat skirt or cage surrounding the tower
could be used to raise the self-resonant frequency anyway.
73, Tom W8JI
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