Topband: Shunt feed short tower

[George (K8GG) & Marijke Guerin]

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 at adelphia.net>
To: "Kelly Jones" <kjones at virtualcohesion.com>; <topband at 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