[Antennas] Stacked beams

[Bob Wilder]

Thanks Billy for the great reply.  I have a better understanding of why 
stacking is used.

73

Bob AF2HD




At 08:14 PM 7/12/2002, Billy Cox wrote:
>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

Bob Wilder, AF2HD/AFA2HD
6032 Idlemoore Court
Theodore, AL 36582-4036
251-653-5274
http://home.earthlink.net/~bwild
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