Hey, while we are on the subject, how about gamma matched
yagis. I have a friend who swears by using a gamma match
on his yagis with no feedline choke. Seems to me that this
is a great way to add some stray vertical polarization to a
horizontal yagi via feedline radiation. Not a good thing if
you are in an urban environment with lots of ground wave
noise sources. The 10 meter monobander at W6UE is fed
like this and we are definitely immersed in a sea of ground
wave noise sources. I was thinking of taking it down and
changing it to a balanced feed with a choke, but its a major
pain in the butt.
Any thoughts on this?
Mike, W4EF......................
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; "Guy Olinger, K2AV"
<olinger@bellsouth.net>; "TowerTalk" <towertalk@contesting.com>
Sent: Friday, December 03, 2004 8:10 AM
Subject: Re: [TowerTalk] elevated short vertical dipole orquarterwave
monopole?
> > > 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.
>
> 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.
>
> 73 Tom
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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