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Mode-S Helix Design



This is a very good dialog... According to my personal correspondence
with AF9Y, he believes that the tapering is important to the match.

Again, if the pattern and the gain is good with the antenna, the
losses with a bad match would be minimal compared to the gain
of the antenna.  The only reason I decided to do this experiment
is from comments by Ed Krome K9EK about circularity and F/B suffering.. so I
was curious.

I still think it'd be valuable to come up with an optimized and
reproducable design, as this could become the de-facto P3D antenna.

I'm considering in doing a direct and accurate scale of the AF9Y
design for 2.4 GHz (and especially 1269) but this would require
getting rid of the PVC pipe support, greatly adding complexity
to building of the design.

I find you comments of the reflector diameter to be critical interesting-- I
suspected that, but didn't have time to try too many
different sizes.  Is the 1.5" diameter you have posted the ultimate?

I too haven't as much time as I'd like to devote to this, but
I'll try to keep pushing as well..

Fred W0FMS




    From: K5OE@aol.com
    To: fspinner@hotmail.com, amsat-bb@amsat.org
    Subject: Re: [amsat-bb] Helix Match: Network Analysis
    Date: Sun, 30 Jan 2000 23:22:24 EST

    Fred,

    > So, sorry to Mike AF9Y and the group if my comments were misconstrued
    >  on this... In my opinion anyone building their own ham equipment 
should
    have
    >  resepect and encouragement.  AF9Y has done more than
    >  most of us, including me, to advance ham radio technology, and
    >  I feel bad that I (totally unintentionally) probably upset him.  And
    >  this goes for K5OE as well (in case I upset YOU!)... keep up the good
    >  work...

    I was not upset at all, only mystified by your results.  I do not have
    access
    to any sophisticated test equipment, hence my original desire to "play
    around" at 70 cm first.  I was able to duplicate the AF9Y design with
    excellent results at 70 cm, as I reported (excellent results being an 
easy
    low-impedance match).  That 3-ring double-helix with ring reflector on 
70
    cm
    was a terrific antenna for mode B uplink.  It handily outperformed my 
TPM
    II
    for both AO-10 and SO-35.  Unfortunately, it appeared way too 
broadbanded
    for
    my tastes as a downlink antenna.

    I then casually scaled that design to a 10-turn 2.4 GHz, but had no 
means
    of
    testing other than relative signal strength.  When I achieved the best
    signal
    I could determine on my "test bench,"  I put the antenna on the air and 
was
    immediately (literally 20 minutes later) able to copy the beacon on 
UO-11.
    Due to work and family, I have not been able to get back to that project 
to
    experiment more with the ring(s).

    I have built some models in  NEC4Win95 (virtual version) and found the
    position of the ring reflector to not be very critical, but the
    circumference
    is very touchy with regard to feedpoint impedance.  I keep trying to 
find
    an
    hour or two and attempt to model the multiple-ring version proposed by 
W8JK
    more than a decade ago.  Dave, ZL2AMD, is also doing some modeling on
    tapered
    versions and has some exciting results for improved pattern.  He may 
wish
    to
    comment as well.
    73,
    Jerry, K5OE
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