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[TowerTalk] 75 ohm coax matching

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Subject: [TowerTalk] 75 ohm coax matching
From: (C. Logan Dietz)
Date: Tue, 08 Jul 1997 17:24:01 -0500
Jerry NO2T wrote:
> Hi Chuck
>        Please send me a rough schematic, or anything. I am interested.
>                    73 de NO2T Jerry
> >Anyone trying to stack two monoband beams who needs more info on this,
> >please e-mail me.

                        Stacking two monobanders

       50 ohm
Ant #1--------|
              |           50 ohm       75 ohm            50 ohm
            Upper/Lower  Length A     Length B         Any Length
            or switches              Switch this
              |                      coax in when
              |                      both antennas
Ant #2--------|                      are selected
       50 ohm

Combining two 50 ohm antennas results in 25 ohms impedence which needs
to be converted back to 50 ohms.  Length A of 50 ohm (RG-8 type) coax
and length B of 75 ohm (RG-11 type) coax constitute a linear transformer
which can do this.  Length B is switched in when both antennas are

I use a molded aluminum box with two relays in it to select antennas A,
B or both with two toggle switches and pilot lights in the shack.  The
relays ground the antennas when they are not selected or when the power
is off.  Then Length A of 50 ohm coax connects the relay box to another
box with a relay that switches Length B of 75 ohm coax in automatically
when "both" is selected.  I use 12 volt relays and put 24 volts on from
the shack to make up for line loss and to get positive switching.

Length A and Length B are determined from the formula in the ARRL
Antenna Book.

I use a multiple of an electrical half wavelength of line between the
antennas and the relay box.  The lines between the antennas and the
relay or switch box must be the same length.  I like multiples of half
wavelengths so I can "tune" the antennas with my antenna bridge.

Chuck, KE5FI

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