# Re: RE: Helix Calculations

• Subject: Re: [amsat-bb] RE: Helix Calculations
• From: "Edward R. Cole" <al7eb@xxxxxxxxxxxxx>
• Date: Tue, 24 Jul 2001 20:35:31 -0800

```>From: "Howard Long" <howard@howardlong.com>
>I've added prime feed focus dish f/D figures if you happen to be using one
>(i.e., _not_ an offset fed dish) and know the f/D but not the illumination
>angle. If you're using an offset fed dish, stick with matching the 10dB
>beamwidth to the angle subtended by the dish at the focal point of the dish.
>
> Turns  G (dBi)  F/B (dB)  3dB BW  10dB BW    dBAR  Prime f/D
>  2.25    9.6      13.3     61.2     116      2.24     0.45
>  2.75   10.0      14.8     57.2     107      2.50     0.49
>  3.25   10.6      14.6     52.8      95      2.46     0.57
>  3.75   11.0      13.2     50.8      88      2.35     0.62
>  4.25   11.2      13.3     49.7      85      1.91     0.64
>  4.75   11.4      15.0     48.1      81      1.36     0.68
>  5.25   11.8      15.7     46.0      78      1.37     0.70
>  5.75   12.0      14.2     44.3      74      1.72     0.75
>  6.25   12.2      13.8     43.3      71      1.72     0.78
>  6.75   12.5      15.4     42.1      69      1.27     0.80
>  7.25   12.7      16.8     40.6      66      1.06     0.84
>  7.75   12.9      15.5     39.4      64      1.28     0.87
>
>I've also had a question about why I used odd numbers of quarter turns. This
>is for two reasons:
>
>a) Ed Krome on page 118 of Mode S - The Book (2001 edn) describes how the
>return losses minimize at odd numbers of quarter turns;
>
>b) Especially at turns < 5, the far field patterns look more symetrical and
>prettier with odd numbers of turns than when using even numbers of 1/4
>turns.
>
>FWIW, I've seen there's a school of thought which suggests that one should
>use, say, 15dB beamwidths for weak signal extra-terrestrial operation
>because of the additional earth noise you get on your antenna as the feed
>overspill points to the ground.
>
>By the way, I invite fellow modellers and experimenters to submit their
>results to either support or dispute any of these results.

Everything you say I have found correct.  It can be proved that the
improvement in S/N for under-illuminating a microwave receiving dish more
than offsets the loss of gain.  The S/N improvement from -10 dB to -15 dB
edge illumination is around 1 dB, I believe.

My 5-turn helix would probably be considered a 5-1/4 turn helix since I
have a 1/4 WL stripline added to the overall length.  When I was modeling
these helices [I ran the model from 2-1/2 to 5-1/2 turns] I found that the
resonant freq. and return loss varied cyclically around the design point as
the length increased... seems to verify the N+1/4 turn theory.

Finally, regarding beamwidths for offset dishes...a good rule is to use an
antenna for twice the f/d ratio of an equivalent center-fed dish.  Thus f/d
= 0.7 often being recommended.  The 5-turn helix as compared to a 3-turn
helix does this.  My 5-turn feed is only -10 dB at the required
illumination angle.  Later on I will try adding a cavity to the helix to
reduce sidelobe level.  I should be able to slide it on/off rapidly to make
a real-time comparison off the satellite [or the sun].

Ed
I am off to Dallas-Fort Worth, Tx early in the morning to attend the CSVHFS
conference...be back next Monday.

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