[Moon-Net] 2M antenna matching Data

Edward R Cole kl7uw at acsalaska.net
Mon Nov 7 23:14:10 CET 2016


I sent Curt a direct e-mail prior to this one.  I repeated some 
observations others have made.

Of course measuring and cutting each phasing line to <1 degree phase 
difference is admirable, I think in practical sense a little overkill.
I have four M2 XP20 so talking about a different antenna and I use 
two M2 4-way dividers, so phasing lines only need to be the same 
length.  I didn't sweep them to find electrical length but were cut 
from same cable.  All are 18-foot which comes out to 3.29 electrical 
WL.  Since dividers are 50-ohm it isn't significant.

I thoroughly re-tested all in 2011 after 13-years in service and 
replace 3 of the 8 cables that showed excessive loss (water incursion 
upon examination).
I tested each cable with 50-ohm term before reconnecting and 
rechecking impedance match into each antenna (4 vertical & 4 
horizontal).  Some antenna matches were out of what I considered 
optimum but found the T-match with frozen Allen screws prevented 
adjusting so I left as-is and documented.  Cost to replace T-match is 
more than 50% antenna cost.  Should have used anti-seize on screws.

Final match at input of both 4-way splitters was still good 
SWR<1.2.  I originally chose 144.300 as match freq but recent 
measurement in the shack shows 144.48 to be lowest SWR.  Still get 
1.26 at 144.100 which I figure is good enough considering there are 7 
separate cables in the 214-foot run from amp to driven element.  Most 
of this is either LMR-600 or AVA7-50 Heliax.  Rest is LMR-400 or LMR-400UF.

Array has <2.0 SWR 144-146 MHz and usable on 146.52, though high 
SWR.  I run lower power (120w at 146.52) on my 170w RFConcepts 
amp.  It is run at 55w to drive my 8877.  The 18-year old 8877 has 
survived transmitting into a shorted transmission line pumping 
out >1800w (for a very short time till over-current protection shut 
it down);  one nice thing about these tough old tubes.

The important issue is using the same test cable for making 
measurements; ideally it would be odd half-wave so measurements would 
reflect actual impedance at antenna terminals.  But if match is good 
the effect is minor.

A common thing for those starting out is over analyzing -  at some 
point just hook it all up and start operating (any real issues will 
soon show up).  Eme is a learning experience we all have to go 
thru.  Some is very significant and other is not so.

73, Ed-KL7UW


At 11:53 AM 11/7/2016, Terry Price wrote:
>I think maybe you are over-thinking this. If the electrical length of the
>phasing lines that connect from the antennas to the power divider are the
>same (as measured on a network analyzer) and the distance from the center
>pin of the N connector to the Tee arm on the antenna is the same, the phase
>can't be off - as long as you have both driven Tee arms on the same side.
>
>If the antennas are the original design by Steve (K1FO) and sold by Tom
>Rutland using 1/2" driven elements and 3/8" Tee arms, the shorting bars will
>be almost at the very ends of the Tee arms. Even if one antenna has the
>shorting bars 2 or 3 inches from the other antenna, I'm not sure what if any
>difference it would make except in the match.
>
>After making a LOT of phasing lines in the last three years, I will say
>there is a difference in coax. Even the beloved Times LMR-400. In two 20
>foot pieces I've seen an inch difference to make the phase difference less
>than 1 degree @ 2m. This is from contiguous cuts on the same reel. I've
>actually found the Advanced Digital cable that the RF Connection sells is as
>repeatable as LMR-400 if not more so.
>
> From a practical point of view, at 2m, I can't imagine any of this could
>cause a problem except on paper. As my dear friend and past editor of the
>World Above 50 MHz, Gene Zimmerman (W3ZZ SK) used to say " The DX will never
>tell the difference".
>
>Cut the phasing lines as close to the same length as you can.
>Get the SWR on the antennas as low as you can, the original design was VERY
>broad, easily from 144 to 145 with greater than 20 dB of RL.
>Put them up and get on and make contacts!
>
>73,
>
>Terry - W8ZN
>
>
>-----Original Message-----
>From: moon-net-bounces at mailman.pe1itr.com
>[mailto:moon-net-bounces at mailman.pe1itr.com] On Behalf Of Steve Bell
>Sent: Monday, November 07, 2016 3:19 PM
>To: moon-net at mailman.pe1itr.com
>Subject: Re: [Moon-Net] 2M antenna matching Data
>
>Curt,
>
>I have the same question---I have a 13 El 2m Yagi per K1FO design and wish
>to build a second one, with added phasing harness to improve my EME
>capability.
>
>It looks like the different phase angles which you measured may cause
>trouble with the total array gain and also affect the antenna pattern.
>
>Please keep this topic on Moon-net for others to ponder.
>
>Steve Bell KJ7OG
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73, Ed - KL7UW
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