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Re: Need recommendations on 435 yagi feed



The Gamma Matched Cushcraft A432-20T (crossed dipoles) antenna sounds very
much like your antenna.  Ten elements in each plane.  The feed system has
no relay in it since it was built for a fixed circularity depending on how
you connected the harness.  The feed line from the transmitter is 52 ohm
line.  It connects to a UHF T connector feeding two 12 inch, RG-59 (75 ohm)
sections of coax.  This length does not include the length of the center
pin on the coax connector, just connector barrel end to barrel end.  An
additional 12 inch delay line section of RG-58 ohm coax is connected in
series with one of the RG-59 sections.  The end of this RG-58 section and
the end of the other RG-59 section are then connected to the antennas.  

The polarity can be switched by changing the RG-58 section from one antenna
to the other.  It would appear that you would need a DPDT coax or two SPDT
coax relays to do the switching.  I would not use less than PL-259 quality
connectors. Hope this helps.

Floyd Sense wrote:
> 
> What I'd like to achieve is switching between left-hand and right-hand
> circular polarization with an antenna I have on hand for the 435 MHz uplink.
> I've read everything I can find, including the latest Satellite Handbook,
> which illustrates some methods in Figure 10:11.  I also have on hand a very
> low loss relay with N connectors.  What I need is this: based on your vast
> experience with these things, what's the most cost effective way to
> accomplish the switching while keeping losses under control.   I forgot to
> mention: the antenna, of unknown manufacturer, has two crossed yagis of 10
> elements each on a 57 inch boom.  The feed points are gamma matches, with an
> SO-239 connector as part of the gamma match (can't easily be changed to N
> connector).  The two sets of elements are offset by only 1 inch along the
> boom - not 1/4 wavelength or 90 degrees as with some of the newer antennas.
> Now the details:
> 
> Technique "A" in figure 10.11 involves using a relay to switch a 90 degree
> phasing line of RG-133/U coax in series with one or the other antennas.  One
> of the satellite books mentioned that this coax is "hard to get".  I've not
> been able to locate a distributor at all.  I suppose one option would be to
> use RG-62 which is 93 ohms instead of the 95 ohms of the RG-133.  Of course,
> that brings with it some connector fastening problems.
> 
> This technique involves running a 1/4 wavelength of 75 ohm coax from each
> driven element (which I'll assume are adjusted for 50 ohms) to a pole of the
> relay and the 1/4 wavelength of RG-133 is also connected to those same relay
> points.  The feedline is then connected to the operating contact of the SPDT
> relay.  Operating the relay causes one yagi to be fed directly, and the
> other to be fed via the 1/4 wavelength of RG-133.  A rather simple
> configuration, but the only Tee connectors for N connections I found were
> $55 each and two would be needed.
> 
> If I were to use standard UHF Tee connectors (accommodating PL-259s) and the
> necessary PL-259s, that would put three PL-259s in the circuit to one yagi
> and two to the other.  Will that be an unreasonable amount of loss?  Is it
> worth the trouble to use RG-11 for the 75 Ohm 1/4 wave matching sections, or
> is RG-59 acceptable?  Along that line, perhaps it would make more sense to
> use F connectors on RG-6 and RG-62 instead of the larger cables?  Would the
> F connectors withstand 20 watts?  100 watts?  I assume that using one of the
> SO-239 to male N connector adapters would also introduce a loss at these
> frequencies.
> 
> So, give all this, how would you accomplish the objective?  Thanks for any
> info you can provide.
> 
> K8AC
> Floyd Sense - Angier, NC
> 
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-- 
73, Roy

Internet: w0sl@amsat.org
Home Page: http://home.swbell.net/rdwelch
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