Hi again John and all.

Sorry, I made an error in my description. For minimum interference (or max gain) you need to stack so the apertures just 'touch'. So for two identical antennas, if the aperture is 9 feet then the 'edge' of that aperture is 1/2 the aperture diameter, or 4.5 feet. That was what I was thinking of - but of course the other antenna also needs 4.5 feet, so the correct stacking distance is the actual diameter of the aperture of a single one (or 9 feet in this example).

In the case of two different band antennas, then the best stacking distance would be 1/2 the aperture of the smaller antenna plus 1/2 the aperture of the larger one.

Regarding the experiments carried out by Kent Britian, I have not discussed it with him or heard his presentation. I am fairly sure he is right when you do not take into account the pattern distortion. So if your objective is to work fairly strong signals, and if you are in a quiet environment, then very close stacking will work OK.

But if you are trying to hear very weak signals and you have significant man made noise to reject, then you need as tight a pattern from your array as you can get. In that case close stacking will create all kinds of what are called 'grating lobes', that will pick up noise from random directions and mask those weak signals.

It is particularly bad for EME because you are pointed up, where there is no (or a lot less) noise, but those grating lobes still pick up noise from your neighbors house, electric poles and wires, etc.

Thanks to Alan Larson, WA6AZP for catching my error.

73, Russ K2TXB

-----Original Message----- From: owner-vhf@w6yx.stanford.edu [mailto:owner-vhf@w6yx.stanford.edu] On Behalf Of Russ Pillsbury Sent: Thursday, December 16, 2010 3:54 PM To: 'John Geiger'; vhf@w6yx.stanford.edu; amsat-bb@amsat.org Subject: RE: [VHF] Horizontal stacking distance question

Hi John. That old rule of thumb does not apply for modern antennas (and never did except for fairly short antennas). You need to know the aperture size of the antenna, and stack them at half of that measurement - for maximum gain, but somewhat closer if you are interested in a better pattern (less side lobes), at the expense of some gain.

Most modern antenna manufacturers have done this for you and will specify the horizontal and vertical stacking distances. You can also look up the antenna lists maintained by VE7BQH and it shows the stacking information for most antennas. There are other sources on the web as well.

Generally the stacking distance will be wider than it is high. IIRC the difference is about 1 foot for my ~4 wavelength 2 meter antennas.

The above information is for stacking two identical antennas. For stacking different band antennas, to completely avoid interference between them, the stacking distance should be the same as for the antenna with the larger aperture (normally the lowest band). However if you need to stack them closer, be aware that the greatest interference will be to the lower frequency antenna. The higher band antenna will not be affected until the lower antenna is close enough to be inside of it's aperture.

Basically you can think of aperture as a three dimensional oval shape that surrounds the antenna. The idea is to keep other objects outside of that oval as much as possible.

All the above said, the major problem with too close spacing is pattern distortion - not gain reduction. So if you have to stack closer, don't be afraid to do so - the antennas will still work reasonably well unless you are working EME with them.

73, Russ K2TXB

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-----Original Message----- From: owner-vhf@w6yx.stanford.edu [mailto:owner-vhf@w6yx.stanford.edu] On Behalf Of John Geiger Sent: Thursday, December 16, 2010 3:21 PM To: vhf@w6yx.stanford.edu; amsat-bb@amsat.org Subject: [VHF] Horizontal stacking distance question

I know that when stacking antennas for different bands on the same mast, the general rule of thumb is to space them at least

1/2 of the

...

boom length for the higher frequency antenna. Does this rule also hold true for stacking 2 antennas on a horizontal crossboom? If I want to put a 2m and 70cm yagi on the same crossboom, how far apart should they be spaced?

73s John AA5JG

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