[TowerTalk] STACKED 40 METER BEAMS MATCH

[Jim Thomson]

Date: Sun, 21 Nov 2010 08:05:31 -0800 (PST)
From: Bob Gibson <w5rg at yahoo.com>
Subject: [TowerTalk] STACKED 40 METER BEAMS MATCH

Hi All...I'm having a hell of a time trying to match a pair of 40 meter M2 4 ele 
beams..both antennas have 1.5 swr or less at 7.150..high or low..switching to 
both it goes up to 2.1 swr..I'm using a ft-240 core..I have tried 4 turns or 5 
turns with not much change..any ideas?? I don't want to go buy a stack match!? 
Bob W5RG

##  a buddy of mine had 2 x m2 40m yagi's  at 90'  and 161'.  He used a stack match on em. 
It was extremely rare that the bottom yagi was ever louder than the top one.   Both, driven in phase,
was always louder than the top one alone.   Later on, the stack match box was dumped, and replaced 
with a hb LC box. [ at input connector to the LC box, there is  a 445 pf NPO cap  from hot side of connector
to chassis.   Then  from  the same hot side of input connector..is a series .56 uh coil.   At the far end of this
coil it terminates in  TWO  coax connectors.    Equal length 50 ohm coax to each 40m yagi. 

##  OK, now with the hb LC box I designed for him, the swr is a lot flatter across the entire band.  The
original AS stack match box is based on 22 ohms...so it provides a compromise if  2 or 3 yagi's  are used. 

##  with the new LC box,  both  40m yagi's  are driven in phase [BIP] all the time, and NO switching between
top, bottom both, no relays used etc.   The thought here was, after years of use, the stack match box is a waste
of time on 40m.   The 2 x yagi's in phase were always louder than the top one alone...and the bottom one by itself
was a total dead loss.  

##  On his new tower, the m2 40m yagis  are at  89'  and 180'.    OK, this time around, I designed the same LC box... but
added  BIP/BOP capability.  IE:  for BOP mode, an electrical  1/2 wave of  50 ohm coax is inserted into the bottom yagi
[you can insert it into EITHER yagi, makes no difference].   This BIP/BOP function required one DPDT relay, and 2 x more
connectors....[for the coiled up  1/2 wave of coax], plus the control cable for the 12 vdc relay.  OK, so now we have
1 x input connector, 2 x output connectors,  and 2 x more connectors  for the 1/2 wave of coax. 

##  The thought here was.... BIP  will  blow away the top ant by itself...... and  BOP  should  blow away the bottom
ant by itself.   After hundreds of recent tests, the BOP  mode is a dud  for the most part. Even when listening to these
daytime 40m nets,  BIP is louder than  BOP.     In perhaps  3-5 x RARE cases, [ out of hundreds],  BOP was a few db
louder than BIP.     IE:  BIP was 10 over S-9.... and BOP was 13 db  over S-9.   In the extreme rare cases  where BOP
out did BIP, the signals were already loud.   And these were close in stations.   As  soon as any station was more than
150-200 miles away, the BIP mode  reigns supreme.    This experience mirrors the dismal performance of the original
AS  stack match box, which had top/bottom/both capability. [no BOP].   With the stack match box, the bottom yagi never
outperformed either the top or both yagi's. 

##  IMO, the simple LC  box [suggested by Jay at  AS]   works the best,  110%  reliable, can't be blown up, won't drift, and
offers  a true  25 ohm output Z   [which consists  of the 2 x equal length  50 ohm coax's in parallel at the output of the LC box]. 

## I used Bings RF software to design the simple LC network.   The .56 uh  tubing coil consists of just a few turns  of  either 1/4"  or
3/8"  cu tubing, with ends flattened.   The   445 pf NPO  cap assy  consist of a bunch of paralleled  5 kv NPO  HT-50/58  type  TX 
doorknobs . [ these came from allen bond at max gain systems]. 

##  per Bings RF software....even with 15 kw applied, the loss in the coil is only 15.0 watts, and the loss in the caps totals 15.0 watts. 
[and  1.5 w for the both the coil and caps assy...with 1.5 kw CCS applied].  
The 3db BW of the LC network is huge, 3.58 mhz  to 10.73 mhz.   I can guarantee, with the above LC box,  you will  not  have any more
SWR problems with the pair of M2  40m yagi's.  Now, whether your 40m choke baluns are up to the task is a side issue.  To test the completed
box, we terminated each of the 2 x output ports with a 50 ohm dummy load,  and then swept it  from 7-7.3 mhz.... dead flat.   If you want pix
of the completed LC  box, contact me offline. 

Later... Jim   VE7RF