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Re: focal lenght and helix


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

URL for other's interested.


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.
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

Dr. Jerry R. Pixton, PIXOS Designs LLC

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