[TowerTalk] need help identifying a vertical antenna...........

[Grant Saviers]

I've had excellent results with a metal roof as the ground plane for a 
two different single band verticals on two different structures. You 
have a real asset if it is of decent size - 1000 sq ft +.  Check out the 
recent QEX articles re elevated ground planes.  It is possible to 
extrapolate a bit from that data to see that an elevated metal roof will 
work very well as a "radial".  I've tried several EZNEC alternative 
representations and it seems that the NEC2 engine does not handle 
elevated ground planes well.

So a traditional trapped vertical (several makes, I have no opinion) 
that needs radials should work well and fit your length constraint 10 to 
40m.  Perhaps there is a top loaded one that also works on 80m but I 
haven't come across one.  Otherwise a 80m base loading coil will get you 
on 80m but without much bandwidth.

When asking tower talkians a few years ago about metal roofs as ground 
planes a lot of concerns were expressed about PIM - passive 
intermodulation.   It is caused by partially conducting metal 
connections, possible in metal roofs, metal gutters, tower bolts, coax 
braid, etc. etc.   PIM is a big concern in the cellular industry where 
there are many full duplex radios on a single tower. So, unless you are 
a multi-multi (unlikely from your question) I think it is highly 
unlikely that PIM will be a concern.  I had no problems with an old barn 
(50 yoa) somewhat rusty galvanized roof, an extreme test case, but there 
were no multi-multi, SO2R, or full duplex operations at my QTH.  On a 
second structure I had to connect two electrically isolated roof 
sections together and did that with #16 wire jumpers and would recommend 
those about every 18" if you have such a situation.  I could see the 
difference on an AIM4170 as jumpers were added.

A hip or mansard roof also helps raise the feedpoint impedance a bit, 
the same affect as drooping radials on a VHF ground plane vertical.  You 
want your antenna to be on a ridge line, of course.

Grant KZ1W


On 8/23/2012 9:01 AM, rbig at cox.net wrote:
> 1.   I'm in a situation where I need to try to use a different kind of antenna.   I would like ot try a vertical, and see what changes it makes in my transmitting and receiving ability.
>
> I would like an antenna (commercial or homebuilt) which:
>
> *  Is somewhere around 30' in vertical length
>
> *  Base to be mounted 25' above ground level
>
> *  Uses few radials (4 or so)  <the radials will be insulated wire, and also be guys>
>
> *  does not need a base tuner (one in the shack is fine)
>
> Would greatly appreciate links to an antenna similar to this.
>
> 2.   We have a metal roof.   Is there any particular advantage mounting this centered on my roof?    Or, are radials the trick?
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