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Mounting an antenna off the side of the mast


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#1 daschnoz

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Posted 13 January 2021 - 07:05 PM

I know this question itself is about ham radio, but it can also apply to GMRS equally.


I'm going to be mounting a BIG antenna (Tram 1481) at the top of the pipe, but I would also like to mount a smaller antenna off the side of the mast pipe. The 1481 covers the 2 meter and 70cm bands (and GMRS quite well). The additional antenna is an Ed Fong J-Pole that covers the 1.25 meter band.

The question -
How far from the mast pipe should I mount the 2nd antenna so that the steel mast does not interfere with the antenna?

#2 mbrun

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Posted 13 January 2021 - 07:35 PM

The simple answer is, the further away the better. Practically speaking however, at least 1/4 the wavelength of the lowest operating frequency.

Any metal in the path of the antenna radiation pattern will affect it, the closer it is the more pronounced the effect. Do expect a bit of a null on the back side of the mast, so position your antenna where the null occurs in a direction of least concern to you.

I am prepped to do exactly the opposite of you. GMRS collinear at the top of 46’ mast and amateur dual-band collinear side mounted at 30’. Lower antenna will be offset by 20-24”. In my case the mast is actually fiberglass so my only metal of concern is the shield of the coax feeding the upper antenna.


Michael
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Michael

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#3 daschnoz

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Posted 13 January 2021 - 07:50 PM

Thank you. I expected that it would be related to wavelength.


To keep this related to GMRS:

Wavelength (in meters) = 300 / frequency (in MHz)
Longest GMRS wavelength = 300 / 462.550 = 64.85cm

Convert to your preferred measurement unit.




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