[amsat-bb] Re: Polarity switching question
domenico.i8cvs at tin.it
Mon Jul 13 18:23:28 PDT 2009
Hi John, W6ZKH
The 2 meters KLM antenna uses two crossed folded dipoles wich are spaced 1/4
wavelength in air over the boom so that to get circular polarization no coax
delay lines are necessary.
Each folded dipole has a wounded coax balun with ratio 4/1 wich is made with
1/2 electrical wavelength of 50 ohm coax cable RG 303 to transform the
balanced impedance of 200 ohm of the folded dipole into a 50 ohm unbalanced
impedance at the output of balun.
The phasing harness coming out from the plastic barrel is made with two
maching lines of 75 ohm coax cable type RG 302 and they are both 1/4
electrical wavelenght long.
Each maching line of 75 ohm transforms the 50 ohm impedance of the dipole to
about 100 ohm into the barrel but two 100 ohm in parallel becomes 50 ohm as
seen from the coax line connected to the barrel PL connector and going down
to the shack.
The 75 ohm line connected to the polarization switching relay is also 1/4
electrical wavelenght long but including the electrical lenght of the relay
To switch polarization from RHCP to LHCP the relay only switches the inner
conductor of it's 75 ohm coax cable from one to the other stud i.e. from
one to the other screw of the same folded dipole.
The 70 cm KLM antenna uses the same basic criteria but to switch
polarization from RHCP to LHCP the relay add to one folded dipole a 1/2
wavelenght delay line wich is included into the aluminum box and is made
with a 50 ohm strip line over a fiberglass printed circuit.
I hope this helps.
----- Original Message -----
From: <w6zkh at comcast.net>
To: "amsat-bb" <amsat-bb at amsat.org>
Sent: Monday, July 13, 2009 4:51 PM
Subject: [amsat-bb] Polarity switching question
> I recently obtained a pair of KLM yagi's for satellite work. They both
have the KLM polarity switching box mounted and wired up. Since I havent put
them up yet, and not well versed in the subject, I got out my ohm meter just
to see what was what on this item. I noticed that both driven elements have
continuty to the center pin of the coax at the same time. I was under the
impression that the relay switched back and forth the driven elements to
give either RH or LH polarity. I havent applied the 12 volts to the box yet
to check that out, but was just curious. One thing I have noticed while
using the downlink yagi is that I get very little fade due to polarity when
I turn the antenna. I hope that is a good thing?? I havent checked the
uplink yagi to see if it has the same results.
> I hope someone can enlighten me abit.
> 73 de John W6ZKH
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