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Re: Helix vs. loopers




----- Original Message ----- 
From: "kc6uqh" <kc6uqh@cox.net>
To: "Clare Fowler" <lcfowler@magma.ca>; "amsat-bb" <amsat-bb@amsat.org>
Sent: Wednesday, May 09, 2007 1:51 AM
Subject: [amsat-bb] Re: Helix vs. loopers


>I have found that the support material has an effect on heilxes over 10T. A
> 40 turn helix can be a challenge to support, and winding it on solid PVC
> pipe will have a considerable effect on the gain of long Helixes. Plastic
> charaterstics at microwave frequencies must be considered and if it slows
> the wave the helix will loose its focus.
> Support rods help, but make for a flimsy antenna. Best to phase shorter
> ones
> for a robust antenna.

My direct comparison measurements showed no difference in gain or
pattern between a 13 turn helix of 1 wavelength in circumference and
a pitch angle of 12.5 degrees using either a 1/2 x 3/16 in fibre glass
stake, a 1/2 inch aluminum tube or 25 mm PVC pipe boom at 2.4 GHz.
However I never tested longer ones using PVC where it could well
start to have an effect.

I personnaly would not use PVC due to its weight.  I like to keep things
as light as possible and use aluminum wire, and fiber glass stakes.
These 5 foot long stakes can be coupled together to make a a 10 foot
boom.  It seems flimsy and does blow around a bit in the wind but they
are very strong and I have never had one break, in over 10 years of use,
even in the ice storm.

An aluminum tube down the center has little or no effect so you could
use a round or square one for a long boom.

The merrits of a long boom or 2 or 4 short ones has some trade
offs.  You get increased losses with connectors and cables and power
combiner for an array. You have to ensure that the signals from
each antenna combine exactly in phase.
However the gain starts to fall off over about 30 turns.
So about 40 turns is the trade off point.
You have less mounting hardware for longer ants but it is less robust.

Clare VE3NPC

> Art, KC6UQH
> ----- Original Message ----- 
> From: "Clare Fowler" <lcfowler@magma.ca>
> To: "amsat-bb" <amsat-bb@amsat.org>
> Sent: Tuesday, May 08, 2007 5:52 PM
> Subject: [amsat-bb] Re: Helix vs. loopers
>
>
>> Helix antennas can be constructed to provide usefull gain with up to 40
>> turns if designed with a circumference of one wavelength and a pitch
>> angle
>> of 12.5 degrees.
>> In my direct experimental comparison measurements outlined
>> in the May/June 2006 AMSAT Journal the max gain formulas used on
>> various web sites and amateur publications, for as yet some undetermined
>> reason, do not provide the predicted gain, but had less gain and
>> increased
>> side lobes. The longer the antenna the greater the deviation from that
>> predicted.
>>
>> As stated previously feed line losses are a big problem for many hams
>> at 1.2 GHz. My array (VE3NPC) is fed with 5 ft of RG/8U in the shack, 35
>> feet of 1/2 in hard line and another 5 feet of flexible RG/8U at the
>> antenna.
>>
>> I have 10 watts from my FT-736 and have never measured what it is at the
>> antenna.  It perfomed very well on AO-40 where I logged 832 SSB QSO's in
>> mode L/S.
>>
>> For helix design dimensions go to http://www.rac.ca
>> and select calculators, helix antennas and then Kraus.
>> (not Emerson as those max gain dimensions result in a
>> poorly performing antenna).
>>
>> Clare VE3NPC


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