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Re: Dielectric Loading of Helical Antennas



More fuel for the fire :-)
1. It would be good to remember that to a certain extent, impedance and
pattern are different pieces of the pie.  Just because some pertubation
changes, or doesn't change, the measured impedance doesn't lend to any
conclusions regarding the resultant pattern.
2. Jerry K5OE is right on mark, I think, with his comment regarding scaling
the helix down in physical size to compensate for the dielectric loading of
a former.  Johnson and Jasik (2nd ed) give the dielectric constant and loss
tangent (eps/tan.d) of PVC to be 3.0/500 at 100MHz.  Of note is that PVC is
not a "well-behaved" dielectric; it's eps varies downwards from around 6.5
at 100Hz.  Contrasted to PTFE which is 2.1 from DC to nearly 10GHz.  Also,
tan.d of PTFE is only 2 at 100MHz, so I'm not so convinced of the
"microwave oven" test.
2a. The scaling I think should be in both circumference/diameter and in
length/turn spacing.  That would imply a constant pitch angle.
3. What happens if you don't compensate for the dielectric?  Well, the
antenna frequency range is shifted downward somewhat (equivalent to saying,
to get equivalent performance to air-cored you make it smaller).  Although
it is true that the helix is "broadband", it all depends upon what you mean
by broadband.  The impedance may well be unaffected.  But certainly the
loaded helix is operating more near the "upper end" of its passband.
Inspection of the King and Wong paper shows that, for a fixed helical
geometry, as frequency increases, (a) beamwidth narrows and (b) sidelobes
increase.  K&W don't show that for operation above the "passband", the beam
splits (tends towards a null along the axis).  Short helices aren't exempt:
the published N=5 example at the lower end of the frequency range (I recall
735MHz) would give -10dB edge for a f/D=0.45.  If the loading scaled by as
much as sqrt(eps)=1.73, that would put the "equivalent" operating frequency
as 1270MHz.  The highest pattern shown is about 1120MHz or so, and it has
already narrowed enough to put more than -22dB edge taper on that 0.45
reflector, plus it has obviously higher sidelobes forming. 
4. Again, Pieter's helices are wound on polyethylene, which is nearly as
well-behaved as PTFE, and it's very thin as well.

I have wanted to measure all this myself for quite awhile, as I do have the
equipment to do so, but not the time; plus it's quite hot outside here in
Phoenix until about Thanksgiving :-(

My own personal experience:  all but my current feed for AO-40 I have wound
directly onto PVC.  Anecdotally, I always felt my RX was kind of "in the
mud"; but then my FT-726R isn't the hottest RX in the barn.  I did measure
the beacon several times with a spectrum analyzer, and consistently got 9dB
less signal than the AO-40 link spreadsheet said I should, and I never have
figured out where that went.  My feed right now is kind of like Ed Cole's,
wound around a smaller piece of PVC so there's about a 1/4" or more gap
between the #10 wire and the pipe.  But I haven't measured the beacon
signal strength with this setup (although the S-meter on the '726R isn't
showing any different).
------------
Scott Townley		
nx7u@arrl.net
Gilbert, AZ  DM43
     
HF/VHF/UHF/Satellite MOBILE station
--> NEW ADDRESS  http://www.mindspring.com/~nx7u

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