[Towertalk] ping-pong stacks

[Jim Lux]

As a first order approach... use the phased array concept of pattern multiplication ... Figure the pattern for two isotropic antennas fed in the way you want, then multiply that pattern by the pattern of one element.  This assumes two identical antennas, of course.. Given that one is probably at the top of the tower and one is "next to" the tower, this might not be a valid assumption.  You might try a simple model with just a pair of dipoles (so that you get the polarization and images taken care of).

The other issue to think about is how are you going to do the phasing?  You have to know the mutual impedances of the antenna, and then figure out the networks  (or lengths of transmission line..same thing essentially) to get the right drive currents.  I also suspect that the Z of a reasonably high gain yagi changes quite a bit over a fairly small frequency range, so the phasing scheme may not be particularly broad band.

On the other hand, you're probably not looking to steer the beam with milliradian precision, nor to keep all sidelobes 50 dB down, so a pretty crude phasing scheme might work fairly well in practice.

One of the big advantages of phased arrays (especially with limited numbers of elements) is not that they have particularly high gain or narrow beamwidth (compared to the component elements) but that they can put a null on something you don't want. You could, for instance, reject signals coming from a higher elevation angle (i.e. stations closer to you) to favor the DX signal.

For this kind of thing, a simple LC phasing box, a coax "T", and a couple of DPDT reversing switches might work pretty well.  You tune the phasing to enhance the interfering signal, then flip the phase of one element, which puts the null on the interferer. (There was something in one of the Antenna Compendia about this, using a pair of verticals).  Sure, the current balance isn't ideal, etc., but, if it works, who cares whether it matches the theoretical model.


jljarvis wrote:
> 
> Folks,
> 
> Anybody got any feedback they'd care to offer on the topic of
> feeding a 2 yagi stack OUT of phase?
> 
> Anybody experimented or modelled with less than 180 degree
> phase reversals?  i.e. 90, 60, 30...and what happens with
> vertical angle lobe steering?
> 
> I'm working up to an experiment preceeded by a model,
> and possibly only a model, if it doesn't look good...
> but thought I'd ask for inputs first.
> 
> Tnx.
> 
> Jim n2ea
> 
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