Date: Sun, 21 Nov 2010 08:05:31 -0800 (PST)
From: Bob Gibson <w5rg@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
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