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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 06-10-2017, 7:54 AM
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Default Omni antenna in attic, concern with air conditioning ducts

I will be taking the ham technical test next week and am trying to plan on my first antenna, which will be in the attic for 2m/70cm. Looking at the diamond x200a or x300a
My question is should I be concerned if the antenna radials are actually below the air conditioning ducts in the vicinity or should they be above the ducts or does it really not make a difference so long as nothing touches any part of the antenna.
My attic can fit the x300a (10 footer) but the radials will be very close to the blown in insulation and below the ducts. Or I can go with the X200a (8.5 footer) and mount the whole antenna above the ducting.
I have so many questions and my local club is not being much help.
I appreciate any help.
73
Pete, no call sign yet…
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Old 06-10-2017, 8:05 AM
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I have a somewhat similar situation that I've experimented with. I have the x50 and have mounted it in a few different places. Two of which were in proximity to the galvanized steel cap on my chimney. Having the radials above the steel cap by about 1/4 wavelength seemed to impact my ability to both reach and hear distant repeaters. What you'd gain with the shorter antenna would likely offset what you'd lose from having that metal too close to the antenna. Just my 2 cents... Good luck on your test! Take the general too when they offer it!

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Old 06-10-2017, 7:05 PM
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In general the father away you can keep metal the better. actually if you can keep things about a half wave (like 3 1/2 feet on 2 m). you will be all right. think of the duct work as part of the antenna -it will interact if it is in the 'field' of the antenna and how much is somewhat dependent on where in the field it is.

IF you can, set up the antenna so the duct work is 'part' of the ground plane (on the same level). You may want to think about a smaller antenna (say 1/4 wave) over the duct work rather than a 'gain' antenna that needs more room. The instructions for your antenna may have some information on how close other objects can be. If you have metal too close, the antenna 'pattern is affected -sometimes drastically with a huge effect on directionality or range.

Experiment and remember to have fun.

73
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Old 06-10-2017, 8:41 PM
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the radials need to be ABOVE the duct work

any metal above radials will severely effect the antenna
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Old 06-11-2017, 9:24 AM
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Quote:
Originally Posted by hawkeye10 View Post

snip...

Or I can go with the X200a (8.5 footer) and mount the whole antenna above the ducting.

snip...

73 Pete, no call sign yet…
I would go with the 200 above the duct work. Looking at the repeaterbook for your location you have a nice mix of repeater types. Depending on distance to repeater and your roof material a lesser antenna may work to.

The attached is a copy from repeaterbook dot com.
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Old 06-11-2017, 1:53 PM
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If you have separate radios or can use separate antennas, I would just build a couple of 1/4 wave antennas out of wire-you can likely barrow a SWR meter or just cut to 19 1/4 inch for 2 m and 6 1/4 on 440. If you have an SWR meter leave it a little long and 'trim' for minimum SWR. With all the metal in your attic, there will be some 'detuning' which will likely wipe out any gain of a fancy antenna anyway. If you are running 20 watts and use good coax (RG213 or better) or have a short (30 feet or less) run, odds are you will never notice the difference.

The 'fancier' antennas are also more sensitive to their surroundings and will likely only be near spec if they are 1/2 wave or farther from anything. Given the compromise of putting you antenna in the attic, I would take advantage of the fact that it will be out of the weather and go simple. If you put a loop in the wire at the top, you could just hang the antenna from a rafter with fishing line and keep in 'in the clear'. The extra money could go for other equipment(SWR meter or antenna analyzer). making your own antennas can be fun and is not as difficult as most believe-most of us don't 'push' our stations to the point where a couple of db make a difference- most squelches require more than that to 'open'.

Have fun-like driving now that you have a license, the real learning can start.

73
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