[amsat-bb] Re: [VHF] Horizontal stacking distance question
russk2t at comcast.net
Thu Dec 16 12:53:39 PST 2010
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
> -----Original Message-----
> From: owner-vhf at w6yx.stanford.edu
> [mailto:owner-vhf at w6yx.stanford.edu] On Behalf Of John Geiger
> Sent: Thursday, December 16, 2010 3:21 PM
> To: vhf at w6yx.stanford.edu; amsat-bb at 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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