Topbanders:
I just read the message from Larry, W6UB, and would like to toss in my 2
cents worth, as I think there might be a modelling error: See below with
double star (**):
----- Original Message -----
From: "Larry Rockfield" <w6ub@adelphia.net>
To: "Kelly Jones" <kjones@virtualcohesion.com>; <topband@contesting.com>
Sent: Monday, June 05, 2006 11:37 AM
Subject: Re: Topband: Shunt feed short tower
>
>> Summary: 42' tower w 4'mast, XM240, (12) 64' radials on ground, 2 shunts
>> of 1/2" copper tubing spaced 1 foot from the tower face> Kelly - N0VD
>
> Kelly, a rough model shows feedpoint Z=2+j330 for the lower end of 160M.
**I would expect a model to give more like
Z ant = 2 - j330 (that is capacitive reactance) since the overall
arrangement is less than 0.25 wl on 160 meters.
> Expect ground losses to be high, and matching network losses alone to run
> 16% or greater, most likely greater. .... snip...
** unless there are at least 30 radials at 1/8 wavelength, the losses will
be very high. Adding radials will help. I guess the ground losses would
drop 4 db with 18 more radials. ON4UN points out that radials for 160
meters do not need to be longer unless one is planning on putting down more
than 30 to 40 radials.
My guess is the antenna will resonate near 80 meters with a shunt feed. See
grid dip suggestion below, but leave out the top loading wire.
To get to 160 meters I would run the shunt feed up as high as possible, such
as 40 feet, and then tie a loading wire to the top of the shunt feed where
it connects to the tower and run out nearly horizontal as far as necessary
to get resonance with a dip meter with a link between the bottom of the
shunt feed rod junction and ground. It will probably be in the 80 feet
range, maybe shorter or longer depending on the effects of the XM240 antenna
above. If the elements are grounded to the boom (I recall they are not)
then the loading wire could be much shorter. Try some loading wire and find
the dip. That should help get you on track.
Good luck & 73, George K8GG
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