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Re: Dielectric Loading of Helical Antennas REPLY



HI Gunther, Peter

Interesting subject :
To add some more words ,   I made a 20 turn helix that never worked .    
The construction tried to use # 10 copper wire.   The helix was wound on
a poly tube which had 1/2 inch wide and 1/8 inch thick poly strips glued
to the tube to raise the helical wire above the tube .    These strips
were place at 120 degrees around the tube diameter.       The thought was
to reduce the dialectic loading since Kraus stated that poly tube loading
lowered the center frequency .     There was no mention of reducing the
band width of the helix

An example of the thin wall plastic cylinder axial mode helix on page 282
of Kraus indicates that he used a strip width which was 0.03 wavelengths
( equivalent to 0.15 wavelengths diameter of a round conductor ) and a
pitch of 12.8 degrees .   In this case the strip was bonded to the
plastic cylinder.
In my case I used calipers to precisely set the helical spacing and then
bonded the wire .

The 12.8 degree pitch is standard for a maximum gain monofilar helix
where the diameter of the helix is approximetely 1 wave length .

I had no problem using a return loss bridge and getting the matching down
to 50 Ohms on my 20 turn helix .    Your statement about all side lodes
Gunther may have been what I wound up with also .   Not sure.    I know a
free standing monofilar, axial mode prime feed helix will begin producing
many many side lobes if the number of turns in the helix is less than
three unless there is a cavity to hold things together .
Kraus throughout always used three turns.

There was one other difference in my 20 turn helix.    The reflector had
a hole in the center large enough for the poly tube but not so large that
the diameter of the helix did not have a reflector directly beneath the
helical wire.    I made the judgement that since Kraus accepted the one
turn reflector that my hole in the center of the reflector  ( for
mounting purposes ) would not disturb the launching function of the
helix.

Ok that's enough ramblings

Joe K0VTY



On Thu, 1 Aug 2002 14:17:16 -0400 "Gunther Meisse" <gjmouse@neo.rr.com>
writes:
> Peter,
> Just for what is worth. I built a 30 turn helix on PCV with 3m 
> copper tape
> and had very poor results. See my web page: www.wmfd.com/w8gsm  I 
> think that
> the problem is the thickness of the tape. It is a known fact that 
> the dia of
> the radiating source effects the bandwidth of the antenna. I am 
> afraid that
> the very thin tape (1 mil) just does not radiate in the forward 
> direction as
> you would hope. The field test I did on mine resulted in a swr of 
> about
> 1.05:1, however the radiation was all in the side lobes.... I also 
> built a
> dual helix feed for my portable which you will see on the site. It 
> also was
> worthless. I went to the store and came back with # 10 wire and 
> gently
> soldered it over the tape and solved the problem. It now works 
> great.
> Keep me posted if do the project. I would love to hear how you do.
> 73
> Gunther Meisse
> W8GSM
> 
> 
> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of Pieter Ibelings
> Sent: Thursday, August 01, 2002 10:40 AM
> To: amsat-bb@AMSAT.Org
> Subject: [amsat-bb] DIelectric Loading of Helical Antennas
> 
> 
> Has anyone tried dielectrically loading a helical feed antenna. I 
> have an
> application where I would like to reduce the 2.4 GHz helical feed in 
> my
> portable setup to about half the size. I was thinking of using a 
> fiberglass
> tube to wind the copper tape helix on. I would then use the next 
> fiberglass
> tube in size to cover the outside of the helix. This way the helix 
> would
> have fiberglass on the inside and outside. Does anyone know if I can 
> just
> reduce all of the dimensions by the squareroot of (Er)? Any ideas?
> 
> Pieter N4IP
> 
> 
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