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Author Topic: VHF Antenna Phasing Harness  (Read 1690 times)
K4JC
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Posts: 76




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« on: March 27, 2008, 11:07:39 AM »

I am experimenting with a multi-element dipole array for VHF, similar to the old Cushcraft AFM-4DA or the DB Products DB224. What I need is the dimensions for the phasing harness. I've searched online but can't find any information. Can anyone help? Thanks!
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WB2WIK
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Posts: 20612




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« Reply #1 on: March 27, 2008, 11:29:13 AM »

It's normally 67.5" per branch (five quarter wavelengths at 146 MHz adjusted for 0.66 V.F.) of 75 Ohm solid dielectric coax (RG-216/U double shielded normally used) times six branches, using three M359 coaxial "T" adapters and three PL258 coaxial "barrel" adapters.  This forms three power dividers:  One divider for the upper two dipoles, one divider for the lower two dipoles, then one divider to feed both of those sets.

This puts them all in phase and adjusts feedpoint impedance to 50 Ohms.

67.5" is longer than they need to be to "reach," but is the shortest length that will do the job electrically because three quarter wavelengths won't reach.

WB2WIK/6

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WA3SKN
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« Reply #2 on: March 27, 2008, 12:35:27 PM »

Use either 1/2 wavelength or 1 wavelength transmission lines (remember velocity factor in determining length!)
Its the easy/logical way to do this!
73s.

-Mike.
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KE4MOB
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« Reply #3 on: March 27, 2008, 12:51:41 PM »

You have to use odd quarter wavelengths for impedance transformation.  Even quarter wavelengths yield impedance repetition.

Here's a pretty good example:

http://www.cebik.com/gup/gup24.html
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WB2WIK
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« Reply #4 on: March 27, 2008, 01:06:46 PM »

>RE: VHF Antenna Phasing Harness  Reply  
by WA3SKN on March 27, 2008  Mail this to a friend!  
Use either 1/2 wavelength or 1 wavelength transmission lines (remember velocity factor in determining length!)
Its the easy/logical way to do this!
73s.

-Mike.<

::That won't work.  The phasing lines should be odd quarter wavelengths of 75 Ohm cable to act as impedance transformers, to step up the impedance of each dipole to 100 Ohms; that way, when they're combined at the "T" (splitter) the network impedance is 50 Ohms (two 100 Ohm branches in parallel).  If you use 1/2 wave or 1 wave lines, that won't happen.

WB2WIK/6  
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K4JC
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Posts: 76




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« Reply #5 on: March 27, 2008, 01:10:13 PM »

That helps a lot. Thanks all for your replies!
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WA3SKN
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« Reply #6 on: March 28, 2008, 06:56:38 AM »

I was not sure whether he was looking for "in-phase" or "out-of-phase"... I stand corrected!... Sorry!
(I should have asked!)

-Mike.
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