Hi again, George.
With respect to the theory part...to finish up on my earlier post.
The battle with making a vertical work well is fighting off loss. The
classic multi-band maxi-loss vertical consists of an iron pipe driven into
the ground, multi-band vertical mounted to it, effectively sharing lossy
dirt and the coax shield on the ground as a counterpoise. It is memorable
with broad 50 ohm matches, quality of SWR an utter inverse of on-the-air
results.
There are three major loss categories in the above, all of which are
countered in various degrees by the traditional dense buried radial field,
e.g. 60 buried 1/4 wave radials.
1) series IR loss in the current sink (counterpoise) fed by the shield.
Raised radials avoid some of this, with serious caveats.
2) Induced media loss in the area immediately below the feedpoint. Consider
this an open microwave oven for the dirt within ~ 1/8 wavelength.
3) "First bounce" loss. Ground basically does not reflect vertically
polarized energy. So energy radiated away from the vertical below the
horizon is lost, absent salt water or a conductive ground screen.
Regarding 1) In your above the roof version, the radials are ONLY there as
the current sink to complete the feedline connection. You have two easy
choices here one radial or two opposed, more or less per band. More below.
Regarding 2) This is what the roof does for you, and it's significant.
Regarding 3) The roof helps some, but because of the practical nature of
the problem, more below horizon energy is lost than above the dense 1/4 wave
radial field. This, alas, is the vertical antenna quandary
One can see how the maxi-loss vertical takes it hard on all three counts.
Regarding one or two radials, in my location in North Carolina, and doing
most serious DX contest operations elsewhere at multi-op stations, I can
"lean" antenna design to the NW, because the SE is "vacant" for USA
contests.
If your usage is hemispheric like mine, then pull a single radial roughly
toward the center of the desired hemisphere, and you will reduce loss 3) in
the desired hemisphere by creating an incident wave that is not 100 percent
vertical, therefore not 100% absorbed below the horizon. I would suspect
the wave to be 45-60 degrees instead of 90. This method does NOT work as
well absent the ground screen (tin roof) below because this presents an
unscreened high current knee close to the ground, seriously increasing loss
2).
If your need is non-hemispheric, then pull two radials to opposite
quadrants. The horizontal "lean" will be cancelled out. In either case you
can prune the radials to assist with the feed impedance. Some radials may
serve on more than one band. At my DC location I had radials for 40, 20,
10, using the 40 radials for 15.
Regarding Pete's method below, this would seem to be radials with "vague"
end points. Without a few more specifics on how it was done, and some
measurements, it would be hard to speculate whether it was lossy or not. If
that is still around, Pete, I would be very interested in a HF spectrum
graph on it's Z treated as if the two were a dipole, leaving out the
vertical. Would need to measure that right on the roof.
73, Guy.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: "Guy Olinger, K2AV" <olinger@bellsouth.net>; "George Warnock"
<george_warnock@hotmail.com>; <antennaware@contesting.com>
Sent: Saturday, February 28, 2009 6:18 AM
Subject: Re: [Antennaware] vertical with metal roof as groundplane
>I had decent performance from a Butternut 9-band vertical installed on a
> metal roof. Because I was skeptical about the connections between sheets,
> I connected a couple of 100-foot wires to the radial terminal and snaked
> them over the roof to couple to each sheet. Don't know how goos the
> theory
> is but it seemed to work.
>
> 73, Pete N4ZR
>
> At 11:20 PM 2/27/2009, Guy Olinger, K2AV wrote:
>>Using the tin roof itself for the ground plane is rife with potential
>>connectivity problems.
>>
>>Back in my Washington DC just out of college days I lived in a row house
>>block with parallel copper roofs couple hundred feet either direction up
>>the
>>street. I mounted the antenna on a sewer vent pipe and put regular
>>radials
>>suspended a foot or so above the roof. The results for that setup were
>>excellent. I used it on 40-10. I was severely disappointed when I took
>>the
>>same physical setup and ran it just above sod at my suburban home a few
>>years later.
>>
>>Use your building as a ground screen underneath but not connected to your
>>tuned radial system. Use a current block between the vertical/radial feed
>>and the feed coax. Ground the coax shield to the roof on the shack side of
>>the current block. Do not use the roof as an active part of the antenna.
>>
>>73, Guy.
>>
>>----- Original Message -----
>>From: "George Warnock" <george_warnock@hotmail.com>
>>To: <antennaware@contesting.com>
>>Sent: Friday, February 27, 2009 6:27 AM
>>Subject: [Antennaware] vertical with metal roof as groundplane
>>
>>
>> >
>> > Hello folks...am preparing the ground to build a workshop, when
>> > completed
>> > it will be 40 foot long by 30 foot wide, there will be a box profile
>> > tin
>> > roof, the pitch of the roof will be approx 20 degrees.There will be a
>> > metal truss in the centre of the roof to support metal purlins and then
>> > the roof its self....to me this sounds like a likely candiate to
>> > support a
>> > "support" for a 40m vert antenna and use the tin roof for a
>> > groundplane.I
>> > would be interested in what you folks think of the idea, and perhaps
>> > some
>> > one could see perhaps a way of taking even more advantage of the
>> > situation...am thinking along the lines of phased verticals perhaps
>> > with
>> > additional ground wires or antenna for 80m or 40/80....the only down
>> > side
>> > I can see is there is a tower about 50 feet from where the shed is
>> > going
>> > to be so that may have some interaction...the "mechanics" of bonding
>> > the
>> > individual sheets of tin together to form a ground plane will be taken
>> > care of during the build..
>> >
>> > reguards george
>> >
>> > gi0vgl...
>> >
>> > _________________________________________________________________
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>> >
>>
>>
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