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Re: focal lenght and helix

>To: amsat-bb@AMSAT.Org
>From: Jerry Pixton <jpixton@shentel.net>
>My questions for the Holiday -
>Jerry wrote -
>The big question is where to place the focus--where is the phase center? If
>you are using the stock dipole feed, I would recommend locating the focus at
>half the distance between the dipole and the reflector (as a guess).
>Jerry, K5OE
>The spec sheet on my Philips Tech Parabolic Grid Antenna say -
>Focal length 15 inches
>Efficiency 50% Typical
>Beamwidth 10 - 13 degrees
>Dimensions 32 x 27 inches
>F/D Ratio 0.46 to 0.56
>The current dipole asssemby sits out about 15 inches but is several inches 
>long (have not looked inside yet, don't want to distroy an operational 
>station before I make my first AO-40 contact, hi). The feed is described as 
>"Quasi Log Periodic End Fire Array Feedhorn", Humidity Leak tested?????
>So I think that the "focus" and the "focal length" are the same 
>thing.  Does this mean that I should design a strut to place the helix some 
>30 inches away from the dish. This seems wrong to me. Can some one explain? 
>I was going to put the reflector behind the helix at 16 inches and was 
>going to start with a 5 turn helix.
>Jerry, W6IHG

OK Jerry(s):

Yeh "focus", "focal point", and "focal length" are interchangeable terms
for our purposes and is an attribute of the dish, not the feed.  The feed
has a phase center [sweet spot] that should be located at the focal point
for optimum performance.  Most correspondents on this list have agreed that
the phase center of a "short" helix is located at the first turn away from
the helix reflector plate.

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.


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