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Re: Helix vs. loopers



Hi all:

At 07:24 PM 5/9/2007, Clare Fowler wrote:
>----- Original Message -----
>From: "kc6uqh" <kc6uqh@cox.net>
> >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.

I have use a card of 1/2-inch plywood to support #10 copper windings 
(1296-MHz dish feed).  I have used a 5/16-inch diam bolt with PVC 
tubing slipped over to insulate copper winding (2400-MHz dish feed 
helix).  I used a 1-1/4 inch diam. wooden closet pole with wooden 
dowel posts for a 23-turn helix on 437-MHz (the painted pole broke 
after a couple years in the wx - not recommended).

>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.

Not that hard but all coax lines must be equal length.  Stacking four 
will net about 5.5 dB over one as about 0.5 dB is lost in the 4-way 
divider and lines.
Stacking more than four is probably counterproductive (use a dish 
instead) above 432-MHz.

>However the gain starts to fall off over about 30 turns.

I have seen that 25-turns is about max length.  Reason is that RF 
current is dissipated along the length of the helix as you go further 
from the feed point.  This is due to radiation and ohmic losses in 
the helix.  I was never happy with my Rx performance on 437 compared 
to a 21-element x-yagi  in circular pol.
What I use currently is the M2-436CP42 (16.5 dBdc) 18-foot boom.


73,
Ed - KL7UW
======================================
  BP40IQ   50-MHz - 10-GHz   www.kl7uw.com
144-EME: FT-847, mgf-1801, 4x-xpol-20, 185w
DUBUS Magazine USA Rep dubususa@hotmail.com
====================================== 

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