[Antennas] Stacked beams

[Billy Cox]

Harvey does gets close with his answer ... for some combinations.

That's the issue ... as there no 100% rules that apply in every
situation.  Let me try to give a "snapshot" on this ...

First lets look at antennas stack vertically ... on the same tower.

>Stacking 2 antennas results in 3 dB gain (over the single antenna).

UP TO 3 dB would be more accurate ... LOTS of variables to
consider ... stack two antennas too close ... you lose ... too far
apart ditto ... stack two different types of antennas ... all bets are off.

Bigger, longer boom antennas (like a 6 over 6) require wider
spacing. And ... gain may increase, but F/B may decrease.

Don't forget that 3 dB is roughly <key word> 1/2 a  S-unit, something
that would be very hard to detect in most cases.

The lobe(s) also change ... as in the horizontal pattern may get narrower,
while vertical pattern may get wider. That's 1 reason why you see contest
stations use 'stacks' ... it's not just increased gain. It can mean more
of a "foot print" into the desired area.   A 100' high yagi on 20m has
it's main lobe at 10 degrees. A 50' high yagi has it's lobe at a higher
angle. Stack them correctly ... and now the stack has a main lobe
that is somewhere between the individual antennas ... so the actual
coverage is 'wider' that either antenna would be when used alone.
<That can also help with QSB> Notice that going to a stack does
NOT lower the main lobe, it actually increases it.

This is where much of the amazing "stack gains" come from, being
able to 'steer' the RF angle to best suit conditions ... not in actual
gain from stacking the antennas.

Examples: Into Europe from here the 4 over 4 stack does better
as a rule ... when the band is barely open, the top is better, when
the band is wide open, the lower is better ... What do I do? I leave
it in BOTH most of the time? Why ... because as one part of the
path moves from "just barely open" (high antenna better) to being
wide open (low antenna better), another area is now "just barely
open", ... so the stack allows me to cover both of those areas, in
a manner better than either antenna alone. Like SP and G3 ....

Now having the stack also allows your to turn the antennas in
different directions ... and "spray RF" in more than one direction.

Like a W5 in Texas, who point one antenna NE and the other one
NW ... Does doing this give any 'stacking gain' ? NO ... but the
signal is stronger to both areas at the same time, as if the stack
was pointer NW, then the signal would be much weaker to the NE
due to being OFF of the main lobe and the F/S characteristics.

Back to the 4 over 4 stack here ... what I do many times is point
the lower antenna to Europe and the top antenna to Japan. And
keep working the stations ...

There's a EA8 station, and on their tower are three identical yagis,
each pointed at different direction, and at a different height on the
tower. Any stacking gain from that setup? No ... Any increased
coverage from doing that ? Yes ... as in being "loud" in three
different directions at the same time.

Another reason for 'stacks' is to fill in the nulls of the high antenna.

Example ... recall the 20m 100' yagi's main lobe is 10 degrees.
It also has a NULL at around 20 degrees ... If you have a 50' 20m
yagi ... it's main lobe is at 20 degrees or so ... now it can "fill in
the holes" of the high antenna. Again, being able to run BOTH
of those antennas in phase solves that problem also.

Higher is NOT always better ... Many times the LOWER antenna
of the 6/6/6 may be the best one to use !  That's also you don't
see stacks hardwired with only one combination at most stations.

There's many 'tricks' ... such as feeding the antennas OUT OF
PHASE to RAISE the main lobe up higher than the low antenna
would normally be ... for more on that ... check out the various
antenna handbooks ...

> Putting them on two separate towers and pointing them in 2 different
> directions would result in 3 dB loss, all other things being equal.

Yes, and no ... as in it depends on if the antennas are in phase
or not, or some combinations. That's why some folks swear by
a certain combination, and other swear at them!

Side by side ... in phase ... on two different towers ... at the right 
spacing ... can give gain ... but the horizontal lobe gets narrower ... 
in fact sometimes too narrow to be of practical use. That's another 
reason why more 'vertical' stacking is seen rather than horizontal.

Again ... for deeper digging ... pickup a copy of the various antenna
handbooks ... this topic makes for some great reading and pondering
of all the possible combinations, and what works best for a set of
defined expectations.

73 Billy AA4NU