Mounting an antenna off the side of the mast

[daschnoz]

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?

[mbrun]

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

WRHS965

KE8PLM

[daschnoz]

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.

[AdmiralCochrane]

So, with my back to a body of water where I do not expect significant GMRS use, I could offset a bracket 17 or 18 cm off my VHF antenna mast and mount a GMRS antenna on the side away from the water and probably get acceptible performance (at least according to theory) ... 

[daschnoz]

That's how it would work. In my case, there is little 220MHz activity to my west, so I will mount the antenna on the east side of the mast. 1/4 wavelength at 220MHz is about 13". I will probably go with 2 feet.

 

Since you're only talking about 6-7 inches for GMRS, personally, I would double that distance and find a "nice" number that is close (12"-14" ... 12" or 18" are nice numbers, a clean 1.0 or 1.5 feet).

 

I think 1/4 wavelengh is minimum, but as we know, having more distance between the radiating element and a ground element is better.

[mainehazmt]

My Tram  antenna suffered a catastrophic failure in the 1st section and I strapped an Ed Fong antenna just about  4” off the side of my tower at 30’. Knowing that one side is blocked (toward Canada) I have not had any issues.  I can’t work on the tower now due to weather ( not worth freezing my butt off over an antenna).  I may never change it as it works every bit as good as the tram so far.  No swr issues and no complaints from others hearing me via repeater or simplex.  It shouldn’t work that good in theory but......  and it is cheaper!

[AdmiralCochrane]

That's how it would work. In my case, there is little 220MHz activity to my west, so I will mount the antenna on the east side of the mast. 1/4 wavelength at 220MHz is about 13". I will probably go with 2 feet.

 

Since you're only talking about 6-7 inches for GMRS, personally, I would double that distance and find a "nice" number that is close (12"-14" ... 12" or 18" are nice numbers, a clean 1.0 or 1.5 feet).

 

I think 1/4 wavelengh is minimum, but as we know, having more distance between the radiating element and a ground element is better.

 

I didn't mention that I may do the same with my own 220Mhz antenna in addition to a separate GMRS antenna.  Right now I'm running a Comet triband that's probably not too far off for GMRS and have a separate 220Mhz J-Pole on its own (shorter) mast.   Time to buy more coax!