Stacking with Coax Cables as Transformation Lines (DK7ZB)
"The stacking cables are made of odd multiples of λ/4-coax-cables (3/4-, 5/4- or 7/4-λ).
They use the transformation attribute of an electrical quarterwave-line. This can be a coax-cable or power splitter.
I've been testing small antennas and found one that seems pretty good.
A question came up about stacking
these small antennas, one above the other, to increase gain by 3 dB.
I'm not sure what the vertical spacing between to of these little antennas would be,
but I think stacking with 75 ohm coax cable transformation lines
would be the best way to stack two very-small-collinears.
I use 50 ohm coax cable on my Ham tower, so my Coax Stacking Harness
would be a 'T' adapter fed with 50 ohms (from the radio room)
and the two 75 ohm matching sections would use low cost TV 'F' connectors on both ends.
I can build these antennas using a TV 'F' connector attached at the 50 ohm feed point. (see pic below).
Or solder the 75 ohm cable directly to the antenna. (Once the correct 75 ohm cable length is known).
I can tune these antennas to a 50 ohm match using my SWR test set..
This test rig can also be used to test/tweak the length of 75 ohm matching sections,
by terminating the antenna
end (of both sections) with a standard 50 terminator.
When the 75 cables are cut correctly, the SWR will be at it's lowest.