archiv~1.txt: Re: Argus: Re: SETI New starter antenna info required

Re: Argus: Re: SETI New starter antenna info required

MarcusJohn@aol.com
Tue, 23 Mar 1999 23:16:13 EST

In a message dated 3/23/99 2:18:21 PM Eastern Standard Time,
al7eb@ptialaska.net writes:

> Gentlemen,
> I don't have access to my notes as I write this, but from memory helical
> antennas of 25 turns will provide approx. 15dbi gain.

The actual gain is hard to quantify, but this is about right, or a little low,
as per NEC.

> the standard
> SETI-Argus dish of 12 foot diam has a nomimal gain of 30 dbi . Lets say
> you built a long helical antenna of 35 turns and achieved 18 dbi.
> You would need an array of 16 of these to equal the 12 ft dish. The
spacing
> between antennas is approx half the boom length.

You may not need to space them so far. Anyway, if you think of the capture
area of a 12 foot dish, and the capture area of 16 helices, you would think
that they capture about the same signal area. Therefore they should have about
the same gain, assuming perfect phasing without loss. ( probably impossible).

> The other comments re:
> feeding apply. I'll dig out my notes tonite and give a more detailed
> reply. If you have thoughts of building extremely long helices, you'll be
> on your own, as I don't believe theory covers beyond about 20 turns.

An NEC simulation is fairly easy, as helices are input in one line. Of course,
learning how to use NEC is not trivial, hehe.

> I
> built a 17 turn helix for 432 Mhz a couple years ago and was pleased with
> it, however I have now gone to a 42 element crossed yagi for this freq..
>
> Another approach might be an array of loop yagis. They can be obtained
> commercially for 1296 Mhz from Downeast Microwave. I have a 12 foot, 45
> element yagi for ham use, which developes 20 dbi. You would only need 8 of
> these (@ arround $120/each). You could build these yourself for a fraction
> of the cost.

The ARRL operators manual states that the tolerances are very tight for a home
built loop yagi at 1296. You would need to get everything just right to make
it match its theoretical gain. This would probably be hard to do for a bunch
of them.

I think that the helix offers much wider frequency bandwidth. The half power
beamwidth would be undesirably larger. The helix would be less intolerant of
construction errors.

> I have seen even longer versions of these so maybe you could
> only need 4! Personally, I think the dishes are the best approach if you
> can do it!
>
> 73, Ed
>

If anyone wants to take a crack at simulating phased helices in NEC, let us
know. I might do it one day, if my other projects get somewhere near done.

John Marcus.