Re: focal lenght and helix

```All,

Thanks for the info. Even us "old" hams can learn new tricks. hi.

URL for other's interested.

www.phillips-tech.com

Select Internet Grids from main page and you see two Parabolic Grid
Antennas. I have the SPG-36. The SPG-24 was the Dayton Special.

So just for prespective - the pod containing the "log periodic" starts at
14.5 inches. I measured where 15 inch point was but did not measure full
length. Next time I lower the tower.

Just waiting for the next transponder time. After some contacts, then I
will make some more improvements.

But this dish, with the UEK-3000 downconverter does hear something.

Jerry, W6IHG

>The description of the of W6IHG's existing feed is "Quasi Log Periodic End
>Fire Array Feedhorn".  That is not a dipole.  It apparently is a
>multi-element log periodic [like a miniature TV antenna] so we need to
>determine the phase center for the log periodic.  The antenna mfr.
>apparently used this feed for wide bandwidth.  W6IHG states it sits at a 15
>inch focal distance...the question is "what" sits there?  The farthest end
>of the feed or what?  How long is the feed itself?  How is it mounted?  I
>suspect it is on a boom that connects directly to the center of the dish
>with the feed line run through it or along it.  I have no experience or
>have I seen this dish so not being very helpful.
>
>You may have to go the basics of finding the feedpoint [focal point] by
>calculating the f/d.  You state the f/d=.46-.56, hmm.  OK, what they are
>saying is the phase center of the feed is changing with frequency and that
>makes sense since it is a log periodic, apparently the phase center moves
>as you change frequency.
>
>I think I would try f/d=.52 as a design number:
>the focal point = .52*diameter = .52*32 inches = 16.64 inches.
>Of course the spec sheet said 15 inches?  That would be f/d = 15/32 = .47
>So take your choice, or try it over the range from 15 to 16.64 inches.
>
>The 5-turn helix might be OK or a 4-turn helix might work better for this
>f/d.  Since making a helix is pretty easy, I'd just make one up for 4-turns
>and try it.  Then if you are adventure-some you could add a "cup" cavity as
>has been previously discussed on the list and that may narrow the bw and
>drop your antenna sidelobes for better receiving.
>
>Ed
>
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------------------------------------------
Dr. Jerry R. Pixton, PIXOS Designs LLC
http://www.pixos.com/designs/
jpixton@shentel.net
------------------------------------------

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