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Author Topic: phasing harness 2 meter quads  (Read 1132 times)
KF4AGD
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Posts: 51




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« on: June 09, 2013, 05:59:52 AM »

I have 2 five element identical quads that I just finished homebrewing from PVC.  I would like to stack them side-by-side. My question is concerning the phasing harness. how? what lengths? type? of coax to use.  I will be feeding the antennas from the rig with rg-11 ( 75 ohm) coax.  Any ideas greatly appreciated.
Thanks,
Anthony
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KB4QAA
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Posts: 2407




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« Reply #1 on: June 09, 2013, 12:20:50 PM »

The ARRL Handbook and Antenna Book have plans for phasing harnesses.
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WB6BYU
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Posts: 13336




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« Reply #2 on: June 09, 2013, 08:54:08 PM »

Why 5? 

How do you plan to stack them together?

What is the feedpoint impedance of each quad?

4 antenna is fairly easy - one way is to take a piece of square aluminum tubing and mount
4 coax jacks around one end, all connected to a common center conductor.  The input jack
is mounted 1/4 wavelength down the tubing, and connected to the other 4 with a conductor
whose size is chosen to provide the proper transformation ratio (for 4 50 ohm antennas this
would be 25 ohms.)  Then run equal lengths of coax to each of your antennas.  You can buy
such splitters commercially. 

Squeezing a 5th connector on the output becomes problematic - it can't go on the end because
that is where you need to reach in to solder things.

Or you can do it all with coax:  run your main feedline to a coax T connector.  On each end
of that put a jumper that is 1/4 electrical wavelength long (that means you have to correct
the physical length for the velocity factor of the coax.)  Put another T on the end of each of
those and connect your 4 antennas.

Now, if you want to try 5 antennas, you'd need to connect them all together with equal
lengths of coax:  assuming 50 ohm antennas and 50 ohm coax, that would give 10 ohms.
Then transform that 10 ohms up to 75 ohms using an electrical quarter wavelength of
sqrt( 10 * 75 ) = 27 ohm coax.  I'll bet 25 ohms is close enough, which could be done using
a pair of 50 ohm cables in parallel - that would give you an SWR of 1.2 : 1 on a 75 ohm
feedline.
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KF4AGD
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Posts: 51




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« Reply #3 on: June 10, 2013, 04:05:33 PM »

Thanks for the info.  To clarify though.... I am using only 2 quads containing 5 elements each.
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WB6BYU
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Posts: 13336




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« Reply #4 on: June 10, 2013, 04:38:59 PM »

Sorry - I misread the original post.  (Fife antennas did seem like an odd number!)

Are the quads designed for 50 or 75 ohms?

With 75 ohm antennas, connect them to a T with equal lengths of 75 ohm coax.
Then cut an electrical quarter wave of 50 ohm coax to the common leg of the T,
and 75 ohm coax from there to the radio.

For 50 ohm antennas used with a 50 ohm main feeder, a common method is to use
equal lengths of 75 ohm coax on each antenna that are cut to 3/4 or 5/4 electrical
wavelengths (depending on what is required for the antenna spacing.)  These can
then be connected in parallel with a T connector and 50 ohm coax to the rig.

For 50 ohm antennas with a 75 ohm main feeder, connect them to the T with equal
lengths of 50 ohm line, then use a quarter wave piece of 50 ohm coax between the
T and the main line.

These don't give a perfect match, but usually are close enough.  If you want to get
more elaborate then you can build your own power splitters using square aluminum
as described previously, but with just 2 outputs rather than 4.
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