[Antennas] Antennas Digest, Vol 103, Issue 1

Sun Nov 4 18:52:11 EST 2012

>
> Jim, feeding two 50 ohm antennas is easy if you understand the use of 1/4
> wave coax lines.
>
>From each 50 ohm antenna run a 1/4 wave 70 to 75 ohm coax, that is 492/2
times Velocity factor divided by frequency in Mhz, from each antenna to a
TEE connector. The 1/4 wave line will raise the impedance from 50 to 100
ohms. The two 100 ohms end of the 1/4 wave lines will be paralleled in the
Tee connector and be 50 ohms again. The 50 ohm feed line coax will go to
the common terminal of the Tee connector.

Later when you add two more antennas you do the same on the two new
antennas. Then add a 1/4 wave 70 ohm coax from each Tee to a third Tee
Again you raise the 50 ohm up to 100 ohms and then parallel the two sets of
antennas and it will match the 50 ohm feed line again.

All this is based on all antennas being resonant at the same frequency, All
being the same impedance, close to 50 ohms, and nearly zero reactance. You
should actually use a bridge , like a mfj 259 ot similar, to measure each
antenna and each 70 ohm line to assure each 1/4 wave line is a 1/4 wave
including velocity factor. I would not use the manufacturers VF except as a
starting point. Manufacturing tolerances are usually not tight enough to
assure each cable is the same.

Use a good high quality coax cable, not Rad Shack, and good silver
connectors and teflon inserts. Quality will pay off big time at VHF. Check
out the wireman products for a start. He carries a good line of coax and
connectors.
Do the job right and you will have a good working system

Don W4BWS

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> Message: 1
> Date: Sat, 3 Nov 2012 23:02:21 -0500
> From: "Jim Miller" <JimMiller at STL-OnLine.Net>
> Subject: [Antennas] FW: Feeding stacked antennas
> To: "Antennas Reflector" <Antennas at Mailman.QTH.NET>
> Message-ID: <[email protected]>
> Content-Type: text/plain;       charset="us-ascii"
>
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> Subject: Feeding stacked antennas
>
>
>
> How do I match four 50 ohm driven elements (in phase) to 50 ohm feed line?
>
>
>
> For years I have been considering placing a 2 meter directional antenna
> into
> my attic (because that is the only place where I can have one).  I find
> that
> for 2 meters that the Moxon antenna would rotate between my roof braces and
> possibly even a pair that are two Moxons wide.  I want to make it 2 tall
> first and later add another pair alongside the first ones later.  The two
> tall stack would be about 78 inches tall overall not including rotor and
> that will fit in my attic.  Two over two would be separated by 19 inches(or
> so) horizontally and still be 78 inches tall.  The driven element to
> reflector would be about 11 inches so if carefully placed may also be able
> to turn between my 24 inch spaced roof braces.
>
>
>
> OK, so with the two Moxons vertically stacked, I would try the 75 ohm cable
> to a tee and then 50 ohm to the shack for matching.
>
>
>
> The question, HOW do I feed the two over two stack and get to 50 ohms ????
> If this isn't too stupid, can I just run all four together with same length
> 50 ohm coax or short ladder line pieces and then a 4:1 balun or toroid?
>
>
>
> I am excited with the modeling results (I am in Kindergarten on modeling)
> of
> this and the small size and ability to rotate inside my attic to give me
> gain and directionality.  I have had the modeling results for over three
> years and only started to build a prototype in that time.
>
>
>
> I intend to build these two vertically very soon (weeks-month) but want to
> double the stack in the future.
>
>
>
> Comments please.
>
>
>
> Thanks es 73, de Jim KG0KP
>
>
>
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> End of Antennas Digest, Vol 103, Issue 1
> ****************************************
>