[TowerTalk] STACKED 40 METER BEAMS MATCH

[Jim Thomson]

Date: Mon, 22 Nov 2010 16:11:07 -0600
From: Larry K4AB <larry.k4ab at gmail.com>
Subject: Re: [TowerTalk] STACKED 40 METER BEAMS MATCH

I use a Stackmatch to feed dissimilar antennas in order to spray RF in two
directions.
For example, during this past Sweepstakes, I fed a tribander at 60' pointed
NW along with a different type of tribander pointing NE.

 Both with different types/lengths of feed line.

###  Since the  2 x feed lines are different types, and esp lengths, 
they will be out of phase, when pointed in the same direction. Since 
you have them pointed in different directions.. [ I think this is called orthogonal, or offset]
its'  probably a moot point. 





All worked well, but despite claims, the Stack match did not evenly
distribute power to each antenna.  In my case, 600 watts to one antenna, and 350 watts to the
other.

###  Was  each ant  50 ohms and 0 reactance   at freq XXX ?    If not, that could easily
account for the   600/350 w  split in pwr. 





This is probably by design, since I know of no broad banded device (1.8-30
mHz) that can effectively distribute to vastly different loads.

## key word here is... "vastly different loads".   I can't see how any broadband
device  will evenly distribute power  50-50..to 2 x vastly different loads ! 
Now if you put 50 ohm dummy loads and wattmeters on each output port,
you will get ur 50-50 split..[ or very close to it].   The  'fix'  may be to install a 
simple L network on the ends of each coax.... just prior to plugging the input ends
of each L network into the stack match.   At least that would  keep the stack match happy,
so it 'see's '  50 ohms + 0 reactance  on the output of each of it's ports.   But then if you qsy,
they may each / both have to be re-tuned. 




However, the Stack match did keep my amp happy by presenting it with a nice
VSWR.
Someone on this reflector (I forgot who) mentioned that the Stack match has
proven design flaws.  Could someone elaborate on them?

## other than the obvious  compromise, where the output is  designed 
for 20 ohms  [ sorta mid point between  16.66 and 25 ohms]  to handle 
2/3 x yagi's... I really don't see anything else obvious.   To make a broadband
matching device that handles  40-6m, and 1-2-3-4 x yagi's  is  tough enough
as is.   There may well be some design trade off's,  but for the most part, the
basic design is very good.   What  AS  does NOT have control over...is the load Z
presented to the output ports.  When you start installing  dissimilar  ants, complete
with dissimilar  coax's..and  dissimilar lengths,   the stack match will see a real 
dogs breakfast on it's output ports.  To then expect the power to split 50-50,
may be asking for too much.  What does the manual say ?

later... Jim   VE7RF  





73,
Larry K4AB