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Re: 435 ant question



Chad,
        The polarity switch combines the signals from each antenna
(perpendicular each to other) into a single 50ohm line, so each of the
signals must come to the pol.sw. with 100ohm impedance. Also it must let one
of the signals travel 1/4 wave length (90deg.) more than the other depending
on the switch position. To acomplish this without changing impedances the
piece of coax. used for the 90deg. difference should be close to 100ohm. You
could use 50 or 75ohm coax. if the necesary lenght were 1/2 wave l.
multiple, but it is not.
        The most common way to do all this is by means of a single-pole
double-throw coax. relay (SPDT). Its common terminal goes to the transceiver
(or preamp.) and each of the switched terminals connects to a T (so 2 T's
are needed). From one T to the other runs the 1/4 w.l. (90deg.) piece of
100ohm coax., and from the remaining conection of each T, each antenna is
fed. As the impedance on the T's is 100ohm, a 1/4 w.l. of 75ohm coax. may be
used to transform to 50ohm, and then any lenght of 50ohm coax. (same lenght
for both antennas) can be used to reach the feed point of each antenna.
         The 50ohm coax. that runs to the feed-point of the antenna that is
ahead can be slightly longer to compensate for signals reaching it before
the other antenna. Simply multiply the driven elements shift by the speed
factor of the coax. and add this amount (or shorten the other run).
        It is necesary to know the velocity factor of every type of coaxial
cable used (3 from the above). For example: 1/4 wave length at 435MHz is
172milimeters, if the speed factor of a coax. is 75% then a piece of 129mm
should be used for 90deg. shift.
        In general when 1/4 w.l. is needed, odd multiples can also be used.
Always keep multiples as low as posible or it decreases usable bandwidth of
the arrangement.

        I made my coaxial relays from double-sided fiberglass PCB material,
BNC connectors, 24VAC relay coils and 220VAC relay contacts. I think it is
too much work for the cost of coaxial relays in USA. I made them myself
because they are not easily found in my country: Uruguay. I recomend you buy
yours, it is best with N connectors but not absolutely necesary if you put
it under shelter.


At 08:57 16/04/98 -0500, you wrote:
>Im building a cp beam for 435 to use at demonstrations. I need help with the
>phase harness. I know you can use 50ohm and 75ohm coax. What I forget is the
>lengths you use to make it 90deg. Also did anyone make a polarization switch??
>How did you do it??
>
>Thanks in adcance,
>Chad Phillips
>kg0mw
>
>




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