Topband: PHASING SHUNT FED TOWERS

[Herb Schoenbohm]

Phasing towers and even detuning structures in AM broadcast radio 
systems is not without a multitude of variables which as Tom points out 
require so much testing, a good calibrate FSM with reliable check 
points, cable measurement with expensive equipment, on and on.  not 
taking these steps may be a mistake to assume any benefit.  Just phasing 
two exsiting metal structures in your back yard may make you feel good 
but not produce the theoretical values by the book of model.   So I have 
suggestion of an old remedy that works well.  It is three phased 
verticals without any phasing coax and switches.  it is the classic 
Bobtailed Curtain which resembles a elongated letter M consisting of 
three 1/4 wave drop wires from a full wave flat top which is fed by a 
center drop wire at the ground with a matching network or Hi-Z to Low Z 
transformer or taped line section.  This antenna has a bi-directional  
broadside pattern with cancellation of high angle radiation from the 
flat top.  The predicted gain is about 6db. Wooden poles are recommended 
unless of course you could have two insulated steel towers properly 
spaced for the support. The base voltage at the bottom of the drop wires 
is very high and not many tower insulators unless kept very clean would 
be exempt from flashover with high power.

Years ago I had one of these hung from tress on 40 meters and it always 
out performed a two element commercial yagi at 75 feet on 40. You might 
want to try one on 10 Mhz from a couple of trees or wooden posts and see 
what the classic Bob-Tailed curtain will do for you.


Herb Schoenbohm, KV4FZ



On 5/31/2013 10:12 AM, Tom W8JI wrote:
> I have a system with two gamma matched towers that works pretty well.  I
> did all I could to make them appear to be similar.  The gamma dimensions
> and capacitance are identical - and I used a loading wire on top of 
> one of
> them  - making it longer and longer until the match was similar.>>>
>
> Results are heavily dependent on the particular gamma match system and 
> tower resonance, because the gamma acts not only as a transformer, but 
> as a transmission line. Sometimes, through luck, it works. Sometimes 
> it does not work at all.
>
> The key in all of this are current phase and current levels in the 
> towers.
> All good topband ops know how to put up a beverage at night.
> _________________
> Topband Reflector