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Re: IC-910 / 3731



Hi All

My 2 cents:   I see folks only mentioning number of turns in a monofilar
axial mode helical feed
when discussing the feed antennas for various f/d dishes for receive
systems.
A better understanding of the above helical feed according to J. Holland
, Microwave J, 82-85, October 1981 will provide the basis of adjusting
the pitch of the helix to fit almost any  f/d dish .
Choose the beam width that provides the illumination angle for any
selected edge level and calculate 
the pitch angle needed.
Following this further it can be stated that the above helix acts as a
helix only when the spacing is the same for all turns.    The phase shift
between turns should be equal for the above case.
Most of the time the calculations for a helix is done for maximum gain .
Ever see someone suggest something else besides a maximum gain
calculation.
For the above helical  feed,  gain is not the driving factor .    Control
of the beam width is .
The number of turn in the above helix should never have less turns then
what provides a single main side lobe .
In preferrence to side lobes that look like hair on a dogs back.
I have seen only cavity backed helix feeds that can logically have less
that three turns and exhibit one main side lobe. 
Illuminating only the face of a dish ( any dish ) properly or having the
side lobe not hitting the face of the dish looking at the sky is proper .
   Anything else does a number on the G/T.
The above reference to J. Holland can also be found in "Antennas 2nd
Edition " by Kraus page 328

Regards

Joe  K0VTY
===================================



 
On Thu, 13 Nov 2003 22:18:14 -0500 Anthony Monteiro <aa2tx@comcast.net>
writes:
> Hi Drew,
> 
> Whooops! I was thinking of an 18" dish - not 60cm -
> I apologize for the misleading comments.
> 
> To make up for it, I did a quick model to generate
> some comparisons and here is what I get:
> 
> 1. For a 60cm prime focus, .35 f/d dish with a G3RUH
>     patch feed, the blockage is around .7 - .8 dB,
>     not really that bad. The overall gain of this dish
>     ought to be around 21.5 dB which is pretty close to
>     the 21 dB that I read was measured on a G3RUH dish.
> 
> 2. For a 60cm, .6 f/d offset fed dish with a -10dB taper
>     feed, I get around 22.5 dB gain. This is because there
>     is no feed blockage and you can illuminate the dish
>     a little more without increasing the noise (since the
>     spillover is into space.)
> 
> So, to compare, a (properly fed) 60 cm offset dish ought to be
> somewhere around 1 dB (maybe 1.5 dB) better than a 60 cm prime
> focus dish. I agree with you though, the prime focus dish is
> much easier to feed and set up.
> 
> Cheers,
> Tony AA2TX
> 
> 
> >I'd say it depends on the size of your feed. The G3RUH 60 cm 
> dish/patch
> >combo seems to work well. I think if you modeled it the blockage is 
> so small
> >a portion of the total area the difference would be negligible.
> >Empirically, I've worked a lot of guys using 60 cm dishes (them). 
> Most of
> >them seem to hear fine. Look at the fine results with W9AE's 
> DXpedition
> >setup with just a AIDC D/C, no preamp......
> >
> >I think I'd take noise from the sidelobes of a helix (which you can 
> reduce
> >with a cupped reflector) over the noise from overilluminating a 
> offset dish
> >with a patch feed.
> >
> > From my experience, having used a BBQ (K5GNA), Primestars (both 
> normal size
> >and 1.2 m), DSS 18" dish (as VP2EAG), and now a 3ft prime focus 
> (pansat Ku
> >tv dish), the prime focus is by far the easiest to set up and use. 
> Easy to
> >point, easy to not overilluminate with the right F/D, and makes a 
> good
> >birdbath after Florida thunderstorms....
> >
> >73, Drew KO4MA
> >
> >P.S. I enjoyed the heck out of the symposium, and can't wait to see 
> what you
> >dream up for next year...
> >
> >
> >
> >
> >---
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> 
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