2-Meter Yagis Stacking Distance?

[lar...@teleport.com]

I recently picked up a used pair of M-Squared 12-element 2-Meter Yagis. They came with the M^2 power divider, RG-8 coax cables to go between antennas and power divider, and a 9-1/2 foot boom for stacking them side by side. These antennas were up in the air for over 10 years down in the Southern California San Joaquin Valley, so they've had plenty of sunshine and heat. All the years of 110+ degree heat has thoroughly baked the cables and made them very stiff - I imagine they should be replaced before putting them back up in the air. I don't have the measurements here at work, but I think the RG-8 cables were each something like 14 feet, 1/2 inch long, with type "N" connectors. I plan to use these antennas as they were used by the previous owner - vertically polarized, side by side for FM operation.

Looking at the M^2 manual for these yagis, it says that the stacking distance is 10 feet. The supplied stacking boom was only 9-1/2 feet long. M^2 emailed me and said it should be 11 feet. Which is correct? Or does it even matter? If it's 11 feet, is that distance measured from the center of the two - 2-Meter antenna booms, or just an 11 foot long piece of 2-inch O.D. aluminum, and the antennas mounted out at the ends as far as possible?

I have a refurbished Hy-Gain boom-to-mast clamp that will be a good fit for the 2" O.D. stacking boom, and a 2" aluminum boom from the remains of an old 20-Meter monoband yagi that I can cut down when I'm sure what would be the optimum length. I'm trying to do this project "properly" the first time so that I can be done with it, and have a mechanically stout antenna system with new cables that will hopefully last a few years.

For making a pair of replacement RG-8 cables, I'm trusting that I can just use any equal lengths of RG-8/RG-213/RG-214 or even 1/4" superflex heliax that will reach between the power divider and each antenna? I'd use regular 1/2" heliax or superflex but it would be too inconvenient to work with, compared to the other kinds of cables.

Thanks in advance for any suggestions.

Larry K7LJ

[wb6...@klitzing.org]

Larry,

You could also use LMR400 for the stacking harness; the losses are low,
and it's light and easy to work with.

On the stacking distance, the M2 recommendation is probably your best
bet. It's a compromise...too close and you give up some gain; too far
and the grating lobes come up and the main lobe gets excessively
narrow. You can calculate the optimum distance yourself with the
generally accepted formula "d=57 divided by the 3db beamwidth in
degrees". The answer will be in wavelengths, which you can convert to
inches. You'll need to know the H-plane beamwidth of your antenna,
assuming you will stack the way you described.

Jim
W6PQL

[W7CE]

Larry,
I ran simulations of a pair of M2 2M12 antennas, vertically polarized and
stacked side by side. Here are the results for various stacking distances:

Spacing Gain Sidelobe
-------- ----- ---------
9.5' 17.49 dB -15.1 dB
10' 17.62 dB -13.4 dB
10.5' 17.73 dB -12.0 dB
11' 17.81 dB -10.8 dB

As you can see, the forward gain on the primary lobe doesn't change that
much over the range of 9.5 to 11'. However the secondary sidelobe at +/- 28
degrees from the main lobe increases rapidly as you increase the spacing.

I hope this helps.

73,
Clay W7CE

[wa7...@gte.net]

To all

Back in April of 1985 Joe Reisert W1JR wrote a two part article on
antenna spacing of which I only have part one (HR April 1985 page 129).
In it he describes the general principles of antenna stacking,
effective apertures and stacking patterns.

On page 134 he states:
"Now that we have determined the beamwidth and side lobes of our
antenna, how do we determine the optimum stacking distance? For
antennas with very low side lobes (at least 18 dB below the main beam):

57
Stacking distance = ----------------------
beamwidth

where stacking distance is in wavelength and beamwidth is in degrees.

If the side lobes are typically 13 -17 dB down, the usual situation, my
test have verified that:

51
Stacking distance = ---------------------------
beamwidth

If the side lobes are only 12 dB down (or less) (typical of the H plane
of many Yagis):

45
Stacking distance = -------------------------
beamwidth"

I personally own 4 old KLM 2M-16LBX's that I had misplaced the
instructions for and that's when I went looking for Joe's article that
I remembered him once writing.

>From the article I learned that my antennas have an E-plane bandwidth
in degrees of 26 and an H-plane of 29 and that the Side lobe data is 18
degrees for the E-plane and 15 degrees for the H-Plane. From the
formulas above I calculated:

E-Plane

57
Stacking distance = ------------------------
26

SD = 2.192307692 Wave lengths

11808
One Wave length at 144.200 Mhz = ----------------- x K
Freq. in Mhz

0r 81.88626907 inches.

Multiply this times the free Space velocity factor (K = .92 for air)and
you get = 75.33536755 inches. Many of us use 80 inches as a standard
for One wavelength at 144.2 Mhz.

Thus for my KLM antenna stacking distances would be an E-Plane of
2.192307692 Wave lengths or 165.1583057 inches or divided by 12 (for
the number of feet) equals 13.76319215 feet.

After doing some needed cleaning I found the instructions for my KLM
antenna and they recommended a stacking distance of 12.5 to 13.0 feet
in the E-Plane so I guess Joe's article was pretty darn close for
amateur work. Had I used 51 instead of 57 in the equations I would have
come up with 12.31443508 feet. The difference is that KLM never
published the Side lobe data and even the author Joe had to make an
educated guess.

That's my two cents worth hope it helps.

DE 73 Gary
WA7BBJ CN97