Re: [TowerTalk] elevated short vertical dipole orquarterwave monopole?

[Jim Lux <jimlux@earthlink.net>]

At 11:10 AM 12/3/2004 -0500, Tom Rauch wrote:
> > > The basic sleeve principle is proven at VHF, but the
> practicalities at
> > > HF overwhelm it, and one is left with all of Tom's
> objections.
>
> Actually it is pretty well known among antenna engineers
> that the decoupling isn't very good.
>
> The fact decoupling isn't good is what gives rise to the
> coaxial skirt collinears.
>
> Cheap low-gain VHF and UHF systems more or less just live
> with the flaws in systems. Take for example a 1/4 wl
> groundplane with four 1/4 wl radials. The common mode on the
> feedline causes substantial pattern distortion. End users
> live with it largely because what they don't know isn't a
> problem. The feedline is long enough there is no RF problem
> back "in the shack".
>
> Now think about this....
> Popular rumor that popped up a few years ago was that two
> radials were enough on a groundplane, yet if you actually
> look at the system 4 isn't enough for a clean pattern
> without using some additional forced decoupling!
>
> The pattern effect is there, we just don't have the RFI
> problems.
>
> > Of course, one could just let the coupling happen, and
> deal with its effect
> > . Say you're building a phased array, and perhaps (?) one
> could change the
> > phasing to compensate.
>
> The problem with allowing the system to have common mode at
> HF are the unpredictable results.


Unpredictable in the design sense, but probably reasonably stable, once 
installed.  With a sufficiently sophisticated phasing system, one can 
probably cancel the effects sufficiently.  However, I'm not sure how you'd 
go about actually doing the adjustments.  The usual "current probe on the 
element" strategy wouldn't work.  One could adjust on the basis of incoming 
signals using any of a variety of adaptive array techniques, but that might 
leave you with huge peaks or nulls in places that you couldn't predict.





> I really think the number one shortfall in amateur antenna
> design is not understanding or dealing with feedlines and
> common mode currents properly. The problem is there over a
> broad range of products, from log periodics that tell users
> to route feedlines along a hot balanced boom to no or poor
> counterpoise verticals. I'm sure it includes most vertical
> dipoles.

I think it's mostly the whole thing of any antenna design sold in a "mass 
market" sort of way (i.e. at retail) is going to embody some amount of 
compromise, both in terms of installation tolerance, expected performance 
(which itself can be defined a lot of ways), and so forth.


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