Jim,
I have only a years experience with my 5 over 5 stacks but have observed
that the top antenna only can be much quieter at times. I haven't
experienced heavy p-static but that is often cited as a reason to select
a lower antenna, as is shorter paths. Otherwise I'm in BIP mode except
when antennas are in different directions for contests (all are
independent on K0XG rings). So, extrapolating that to a 3 high stack
rotating tower, I would probably do top only, bottom only, and all.
What I've read agrees that BOP is pretty useless, so I bought stack
matches without that capability.
Grant KZ1W
On 7/13/2016 9:27 AM, Jim Thomson wrote:
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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