Side vs Top mounting a 2m/70cm dual band antenna

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LakeMan2

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I am considering a dual band 2m/70cm antenna something like the Comet GP-9 (17ft antenna). I have several mounting options and I am trying to decide which one is best.

I have a 30ft tilt down tower (Universal Towers 12-30) with a 10ft, aluminum 1 1/2 inch diameter 1/4 in wall mast on top of that for a total height of 40ft. At the top of the 30' tower I have a large TV antenna (I kept it low due to wind loading). At the top of the 10ft mast I have a Cellular (850MHz) Yagi. I know that the best place to mount something like the GP-9 would be above the Yagi, but I am not sure I have enough room to bring the tower down (or actually even get it up) with the Comet extending for a total of 57ft. Or actually I should say I am pretty sure I don't have room. I have trees that are about 60ft from the tower and I am pretty sure I don't have 57ft of clearance to the branches to raise/lower the tower. From what I remember the 40ft just cleared, but I might have another 5-10ft of clearance.

So, assuming I don't have clearance to mount the antenna at the top, I was looking at side mounting it further down the 10ft mast based on how much clearance to the trees I end up having for raising/lowering it. I looked into that and see that when side mounting to a tower you want, I believe it was, a 1/2 wavelength separation between the tower and the antenna. For the 2m band that is 39in which is quite a bit. I don't think I can do that.

My question is if mounting it along the side of a 1 1/2in mast, is just as bad as mounting it close to a full tower side? I suspect that mounting it right along a mast will be bad for tuning/VSWR, but I am not positive. I don't care if the only impact would be a null in that direction. But I would care if there were side nulls 30deg from there or so.

A second option would be mounting a much shorter (maybe 5-6ft) dual band at the top of the tower just above the 850Mhz Yagi (I think I have the clearance for that). That would put the base of the antenna at 40ft and the tip at ~ 45ft.

A third option would be to mount the larger GP-9 on top of a ~25ft 1 1/4 steel mast that I have in a cement base next to the house and a bracket at the peak of the roof that attaches the mast to the house. That would put the base of GP-9 a few feet above the peak of the roof and the tip a total of about 42ft high. The complicating issue there is that I have a metal roof (nothing is ever easy).... Now the base of the antenna would be above that, but not by much.

So it breaks down to:

1) Put the larger, higher gain antenna at the highest point, but some or most of it will run close and parallel to an aluminum mast.

2) Put a shorter, lower gain antenna on top at 40ft.

3) Put the larger antenna on top of a lower mast, but the total height to the tip is about the same as the shorter antenna on the tower, but the base is only a few feet above a metal roof.

Any ideas on will #1 work at all? If not would #2 or #3 be better?


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mmckenna

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You would want to space the antenna away from any metal by at least 1/4 wavelength, more if you can do it. Metal in close proximity to the vertical radiator is going to not only affect the radiation pattern, but also couple the antenna to the grounded tower. This would likely show up as a change in SWR.

Personally, I'd go for a lower gain antenna on the top of the tower. Lower the 800MHz Yagi a bit to make it all fit.

Either that, or trim the trees accordingly.

Antenna gain is nice, but don't make the drive for it do something detrimental to it's performance, like mounting it next to the tower leg or mast without sufficient stand off.
 

LtDoc

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I think you have the general idea, if that antenna (any antenna) is mounted to the side of a tower/mast, then the further away you can keep the two the better. From there, discounting the 'null', it'd just be a matter of matching input impedances, or tuning the thing. Big 'just' there, huh? Yes, but certainly possible.
Not caring much for TV, I'll bet you can guess where I'd recommend you put that dual-band antenna... :)
- 'Doc
 
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