[NLRS] Mounting 1296 Mhz loop antennas to metalic masts?

[John P. Toscano]

Carl Estey wrote:
> 
> I have contemplated putting up a DSS 1296 Mhz 45 element loop yagi and
> mentioned that to Dave, N0KP, during a chat today.  He explained that the
> metallic mast will very seriously degrade performance of the antenna,  

The DS loopers should not have a vertical mast extending above and below 
the antenna's main boom, since it will pass between loops and be 
physically "inside" the width of the loops at the crossing point, which 
is likely to be detrimental to performance.  Keep in mind, however, that 
most people that stack antennas for the 4 "low" bands (50 - 432 MHz) do 
just that with traditional yagis, and they still work.

I would hazard a guess that most people actually do have a metallic mast 
of some sort that runs perpendicular to the boom, although not in the 
manner described above.  Normally, with the Directive Systems loop 
yagis, the antenna must mount at the very end of the vertical mast 
section, unless you are willing/able to re-design the boom-to-mast 
mounting plate.  This is probably intended to keep the mounting mast on 
the opposite side of the boom from the loops -- if the antenna is at the 
top, the loops point up and the mast extends down below the boom -- if 
the antenna is at the bottom, the loops point down and the mast extends 
up above the boom.  Because of this, plus the fact that the mast section 
is vertical while the antenna is horizontally polarized, the performance 
is generally quite acceptable.

You may be wondering what I mean by mounting the antenna at the bottom 
of the mast -- the bottom of the main mast is normally inserted into the 
rotator!  What I mean is that when you have a lot of antennas that need 
to go into limited space, a common practice is to build an H frame.  The 
very center, horizontal part of the H attaches to the main mast that is 
inserted into the rotator, and the four tips of the H (two up and two 
down) can mount up to 4 loop yagis, one on each.  If you have only two 
loopers, they could attach to the two downward pointing sections of the 
H frame, with the loops pointing down.  This is the preferred 
orientation if you are not using both the top and bottom ends of the H 
frame, to minimize the effects of birds sitting on your loops and 
mashing them down.

Another option is to use one of the REAR MOUNT yagis, for example the 
2324LYRM.  It has 24 elements, so half the gain of the 45-element model, 
(17 dB vs. 20dB), with half the boom length (6' vs. 12').  Because the 
mast is behind the reflector, it is functionally invisible to the 
business end of the antenna.  These can be mounted right onto a metallic 
mast without trouble.

> When stacking a 1296 Mhz loop antenna how do people mount the antenna?  I am
> wondering how a 32" long 1" diameter PVC water pipe might work clamped to
> the metallic mast and extending horizontally to a 90 degree elbow with a
> short vertical piece of 1" PVC pipe to mount the loop antenna.

If, by stacking, you mean placing two antennas for the SAME frequency 
band stacked over one another to get about double the gain, the H frame 
description above is the way to go.  For example, two 1296 antennas at 
the top and bottom of the left side of the H, and two 902 (2304, 3456, 
or name your poison) stacked at the top and bottom of the right side of 
the H frame.

I would be extremely wary of using PVC pipe for mounting the antennas 
for any long-term installation.  I once built a PVC antenna mounting 
system for roving, and it didn't last the whole trip.  Of course, 
driving down a bumpy road underneath low-hanging tree branches is a 
challenge to any mounting system, but your home tower is not necessarily 
a benign place for your antennas either, and your goal is to put them up 
and have the installation last for years rather than 1 or 2 days of a 
single contest weekend.  PVC is not as strong as some people think.  I 
did try PVC mounting one more time for the rover, but the second time I 
very carefully reinforced the PVC pipes with snug-fitting oak closet 
rods inside of the PVC.  The rods gave strength and the PVC kept the 
wood dry.  That one held together better, but I'd still rather not trust 
it to last through a dozen Minnesota winters and summers.

With horizontally-polarized antennas (the de facto standard for 
terrestrial weak-signal work on these bands), if using an H frame, the 
vertical sections can be made of aluminum mast.  The horizontal part of 
the H frame would be the part that would be more important to make of a 
non-conductive material (fiberglass is far superior to PVC, although not 
cheap), but if your stacking distances are correct, even the horizontal 
part of the H frame can be aluminum.  The optimal spacing of the 
antennas, whether stacked on the same band for double gain, or stacked 
on different bands with minimal interference with one another, is to 
have the "capture area" of each antenna just barely reach the horizontal 
boom.  In the case of same-band pairs, having the two antennas' capture 
areas just touch one another should give optimal gain and pattern, yet 
the oval shape of the capture area means that very little of the boom is 
actually within the capture area.  In the case of different-band pairs, 
stack them so that the capture areas don't quite reach the horizontal 
cross-boom, to minimize the interaction while also minimizing the amount 
of space needed to hold multiple antennas.

So how do you figure the capture area?  I am not aware of a simple 
universal formula to compute it, but there is often an extremely simple 
trick to get the information if you're talking about high-quality 
commercial antennas such as those sold by M Squared or Directive 
Systems.  Look in the spec sheets for the vertical and horizontal 
stacking distances.  For the 2345LY that you were interested in using, 
the vertical stacking distance is 24 inches, and the horizontal stacking 
distance is 26 inches.  When vertically stacked, the capture areas 
should just touch, so if a pair of them are 24 inches apart, that means 
that the capture area of each one extends about 12" above and 12" below 
the boom, and likewise extends about 13" left and 13" right of the boom. 
  Doesn't get much simpler than that.

For my home antenna array, I happen to use square fiberglass tubing for 
the horizontal cross-booms and aluminum pipe for the vertical booms in 
the H frame for 902, 1296, 2304, and 3456 MHz.  I also have plans for a 
much larger H frame on the same tower whose horizontal cross-boom is a 
very heavy-duty round fiberglass tube that passes through an elevation 
rotator, with a 12 foot long vertical aluminum boom on each end of the H 
frame.  The left side of the H will hold a pair of 2M12 antennas 
(12-element 2 meter band antennas from M-Squared) at a stacking distance 
of just under 12 feet, and the right side of the H was originally 
planned to hold a pair of 432-9WL antennas (9 wavelength-long antennas 
for the 432 MHz band, also M Squared) half-way up and half-way down the 
boom for a 6-foot stacking distance.  The top and bottom ends of that 
side of the H were reserved for a 1269 MHz and a 2400 MHz loop yagi 
satellite antenna.  I'm now re-considering the arrangement.  I might put 
the two satellite loopers at the ends of the horizontal cross-boom 
(yeah, vertically polarized, but satellite polarity is a mixed bag most 
of the time), and put a pair of 222-5WL antennas (5 wavelength long 
antennas for 222 MHz) at the tips of the H for a 12-foot stacking 
distance.  This arrangement is pretty optimal for both 144 and 432 MHz, 
and a bit less than optimal for 222 MHz, because the two 432 MHz 
antennas are inbetween the two 222 antennas and therefore within their 
capture areas, but I still think it will be superior to a single 222 MHz 
antenna.  The four microwave loopers will be on a much smaller H frame 
and not part of the elevation system.

Anyway, I hope that gives you some ideas.

73 de W0JT