[TowerTalk] 20m stack switching question

[StellarCAT]

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... 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 at 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