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Re: Disparaging Helical Antennas for AO-40...some notes



Scott,

NX7U states:
> As another example, the oft-quoted 16 turn helix will probably really
> get you only 15.3dBi or so (again based on King and Wong's
> measurements).  That's 5dB less than the dish and a lot of margin to
> give up!

Well said.  You also referenced D.T. Emerson's work, whose formulae extrapolate a "gain optimized" 16-turn helix to a mere 14 dBi.  His work was based on NEC simulation, while King and Wong's was based on empirical testing.  While the absolute gain value is argued by the experts and academicians, the simple (but unpopular) truth is the gain curves for helical antennae fall off rapidly beyond 6 lambda.  I would say 7 lambda becomes pretty much the point of diminishing returns.  For an axial spacing of 12 degrees, that works out to be 32 turns, more or less, and only 2 dB more than the 16-turn version (regardless of whose absolute gain values you believe).  

> I should also point out that the King and Wong measurements used helixes
> that were backed not by a flat ground-plane, but a "cup" or cavity. 
> Other investigators (references not handy at the moment) have found that
> the "cup" gives about 2dB additional forward gain.  

I have wondered if the discrepency between King & Wong and Emerson was due to that.  Kraus discusses the semi-parabolic reflector.  I note my NEC4Win95 models report numbers much closer to Kraus, so I don't know who to believe!  NEC does confirm the cupped reflector reduces the 45 degree side lobes.  A similar design was used in RV3TH's helix I tested last year:
http://home.swbell.net/k5oe/rv3th_69.jpg

I have a similarly-designed reflector in test now on a 28-turn "Emerson gain optimized" helix, but have had trouble getting a decent feedpoint match.  Much work to do and little time to get to it :-(  Maybe next weekend...
73,
Jerry, K5OE

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