I had a 3 stack of C31's in Arizona and was able using a StackMatch to
select any combo (no out of phase selection though)... I can tell you on 20
meters there was never a case where anything other than all 3 was better. On
15 once in a while there might be a selection, like the bottom antenna, that
was better... and on 10 it was a little more often with a few rare cases
where the bottom one was MUCH better... as far as BOP my plans on stacks for
10,15, 20 (2X) include this ability - but when you model it note that it has
marginal benefit on 20 as that lobe is pretty darn high - maybe for SS
stateside contacts close in... on 15 and especially on 10 it starts to model
quite nicely. As you've seen indeed on 40 it would have no value doing BOP.
Also note for other reference the 'likely' best combinations of a 2 stack
from best to least best are: BIP, Bottom, BOP. The top alone is never (very
rarely and then marginally) any better than BIP and just isn't needed as a
selection.
Gary
K9RX
-----Original Message-----
From: Jim Thomson
Sent: Wednesday, July 13, 2016 12:27 PM
To: towertalk@contesting.com
Subject: [TowerTalk] 20m stack switching question
A buddy has 3 x 20m monobanders, stacked at 172, 117, 62 ft.
Currently, all 3 are driven in phase,
via a L network. ( 50 ohm to 16.66 ohms, consisting of a shunt 317.7 pf
cap from input 50 ohm connector to chassis, then
a series .2646 uh coil. Coil goes from input connector...to the 3 x
paralleled output connectors) Its just a mess of 5 kv NPO doorknob caps
in parallel + some cu tubing. Pretty straight forward. In actuality,
325 pf was used... and slightly less coil.
The plan was to install some relays, and then be able to switch top,
middle, and bottom.... and all 3 in phase. We can pull that off
with just 3 x SPST relays...and 1 x SPDT relay.
Then we thought, why not be able to use any 2 x yagis at a time, like top +
middle... middle + bottom.... and also top and bottom.
That could be done with a 2nd L network + 1 x additional SPDT relay.
Ok, is it even worth it to be able to use any 2 yagis ?? For DX, all 3
in phase work pretty good. For local, say within 3000 miles, perhaps
the lowest yagi alone would suffice? I cant see the top and bottom
buying him anything..... vs all 3 in phase. I cant see the top and middle
outperforming all 3 in phase. That only leaves the middle and bottom..and
even that is dubious... but we have zero experience with this.
Even with the 4 x relays and just one L network box, we could still easily
select any 2 yagis..and use the 50: 16.66 ohm L network... however
the swr will not be dead flat, but it would be < 2: 1 25 / 16.66 =
1.5:1 swr.... which is still useable.
Are we wasting our time ? On a side note, the pair of 40m yagis, 180 + 89
feet, are also driven in phase.... with no provisions for top + bottom
switching. However, BOP was added to the BIP. Easily done with a
switchable 1/2 wave of coax in either leg. Then countless hrs spent
switching between
BIP...and BOP. BOP was a disaster. Only once was BOP a bit louder... and
that was on 40M during the daytime..and just one station, aprx 200 miles
away.
Looking at some notes here and there, it appears that BOP has a higher
take off angle than the lower 40M by itself...which ends up way too high.
Latest plan is to be able to switch top /bottom /both on the pair of 40m
yagis......and ditto with the pair of 15M yagis ( 127 + 79 ft) . We could
also add
the BOP function, but after the 40M failed experiment, I think BOP is a
total write off.
Any thought on any of this would be greatly appreciated. BTW, all yagis
are pointed in the same direction on a rotating tower. In the original
config,
non rotable tower, tic rings were used on 40M + 20m.... then all ants could
be pointed in different directions. That came in handy at times, but the
tic rings
were a constant head ache.
Jim VE7RF
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