Mounting an antenna off the side of the mast
Posted 13 January 2021 - 07:05 PM
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?
Posted 13 January 2021 - 07:35 PM
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.
Posted 13 January 2021 - 07:50 PM
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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