On 20m I have a 4 high and a 3 high 20m stack. All towers can have any
combination using modified stack match style boxes with a 25 to 50 ohm
unun. The mismatch, even using 4 antennas in // is quite acceptable to me.
Then each tower goes thru a 2 position stack box.
My thoughts...
It is essential here to take the top antenna out of the mix when rain or
snow static hits.
At this location the low 20m yagi in the stack seems to (if it does
anything) reduce a weak signal or add noise, either way the effect isn't
useful.
If contesting is a major interest, having all the antennas in the same
direction is a BIG disadvantage. Geographical diversity is most
beneficial, esp from this signal starved part of the world. Our normal 20m
contest set is top 2 on Eu, top 2 on other stak into JA, and lower yagis
into east and west coast areas. All lower yagis are on side mounts that
cover the aforementioned areas.
73 Don
VE6JY
On Wed, Jul 13, 2016 at 4:27 PM, Jim Thomson <jim.thom@telus.net> 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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