Topband: Spam:****, Re: Shunt-fed tower vs. T-vertical

[Julius Fazekas]

What distance from the tower is a good rule of thumb for the cage wires?

Three is the minimum, would four be "better", as in easier to match, provide better isolation?

Pondering this for a 48' mast for 80M, as I don't think I have enough on top to make it useful on 160...

Julius Fazekas
N2WN

Tennessee Contest Group
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Elecraft K2     #4455
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Elecraft K3/100 #1875


--- On Tue, 7/13/10, Herb Schoenbohm <herbs at vitelcom.net> wrote:

> From: Herb Schoenbohm <herbs at vitelcom.net>
> Subject: Re: Topband: Spam:****, Re:  Shunt-fed tower vs. T-vertical
> To: r_bakalov at yahoo.com
> Cc: topband at contesting.com
> Date: Tuesday, July 13, 2010, 11:11 AM
> 
> 
> Rudy Bakalov wrote:
> > If the cage does the radiation, does that mean that
> the cage has to be the entire length of the tower? That
> would be a no-go for me as there will be 2 additional
> rotating yagis on the tower, at 68' and 31'.
> >   
> 
> No the cage can be shorter and actually end at the 30 foot
> level...but 
> you will have to check the base impedance to design the
> proper match.
> > I have LOTS of space. How far from the tower should
> one end of the T be to avoid the interaction?  
> >   
> At least more than a half wave to reduce interaction
> especially if the 
> tower is at or near resonance the way it is...or less if
> you can 
> decouple it at RF.
> 
> > Also, if I remember correctly from my physics classes,
> perpendicular wires (top of the T and the tower) do not
> interact; what kind of interaction did you experience that I
> should be aware of?
> >
> >   
> > The horizontal wires of the "T" are not doing much of
> the radiation 
> > due to the cancelation effect. That is why a T is a
> great antenna as 
> > the horizontal radiation is minimized.  The
> imblance that is sure to 
> > occur is by the close spaced support metal tower in
> terms of a 
> > wavelength or less.You could conceivably detune the
> tower at 1.8 but 
> > this gets complicated.  There are lots of papers,
> i.e. NAB and IEEE on 
> > detuning large metal structures such as power line
> supports to 
> > minimize the interaction.  But IMHO the best
> thing for you is to go 
> > with your tower as you should be able to deal with the
> match presented 
> > even with a shorter cage below the bottom beam.
> 
> Herb, KV4FZ
> _______________________________________________
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