A very interesting question. I've had two verticals on metal roofs but
the roofs were galvanized steel over wood frame. Thus the roof acted as
an elevated ground plane. One worked very well for me with an 80m top
loaded vertical on a barn about the size and geometry of your building.
This insulated roof structure can be analyzed with EZNEC.
Now, I have an all metal building 56 x 70' which is a very tempting
target for an elevated vertical. However, the metal walls are connected
to Ufer grounds in the footings, which goes against the (correct I
believe) advice to not ground an elevated set of radials. I know EZNEC+
5 can't analyze this, maybe something else can. Advice?
I've gotten advice to use tuned insulated radials suspended a foot above
the roof. Makes some sense, since the ground loss from the roof will be
quite low. However, with the coupling I wonder if they can be
"tuned". I just added an 8th elevated 120' radial to my 160m T and got
a bit of surprise in how much the swr changed, I think because of
coupling to the steel building. Radials 6 & 7 are farther away and had
little influence on swr & resonance as they were added.
OTOH, if you think about elevated radials, at some number (above 4 or so
I think) the concept of tuning them makes no sense. Extrapolating to
large numbers of radials and then to a uniform sheet conductor, is like
asking "what is the resonance of a perfect ground plane?" N6LF's
extensive QEX, QST, & his web site elevated radial analysis shows this
property. With enough (8 or more) elevated radials, the length can vary
quite a bit without much change in performance. The goal is to shield
the lossy earth from the field.
So another thought experiment is to start with your roof on the ground.
Would you not use it as a ground plane? Of course not, but you might
add some wire radial extensions if the metal area was small. Now start
raising the roof, and in an all metal building, keep it electrically
connected all around to earth. Would antenna performance degrade? If
so, at what height and why is the *really* hard question? Or imagine a
fantasy, a vertical on top of a bump of salt water. Or if the earth
around the building was covered with steel.
So, IMO verticals on metal roofs as a ground plane are excellent low
angle radiators if the dimensions of the roof are significant e.g. ~1/4
wl. There are also the advantages of fewer objects to interact with the
near field and the slope of the roof improving the match as it does for
VHF ground plane verticals.
With a SPDT relay, it wouldn't be too difficult to try a real time 4
"tuned" elevated radials over the roof vs direct roof connection say for
20m. I've not seen anything published, anybody have any links?
Grant KZ1W
On 11/14/2013 5:51 AM, Patrick Greenlee wrote:
I have a Hy-Gain Hy-Tower on top of an all metal barn. Base is about
23 ft AGL. I have no radials. It works pretty well. I got a
recommendation to put at least 4 radials on the roof (2.5:12 pitch
gable roof) and preferably 4 cut for each band that can stay on the
roof (roof is about 38x74 feet and ant is dead center.) Thoughts?
Patrick AF5CK
-----Original Message----- From: GARY HUBER
Sent: Wednesday, November 13, 2013 10:59 PM
To: Michael Murphy ; TT TowerTalk
Subject: Re: [TowerTalk] HF2V Elevated or On Ground
Mike, I forgot to say the top loading can give your HF-2V a low VSWR
across
the entire 40 M band or across the entire phone or CW portion....
great for
contests!
73 ES DX,
Gary -- AB9M
-----Original Message----- From: Michael Murphy
Sent: Wednesday, November 13, 2013 9:25 PM
To: Mike & Coreen Smith VE9AA
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] HF2V Elevated or On Ground
I have an HF-2V ground mounted with 32 - 40 foot radials under it.
When I
went from 16 to 32 radials, 80 became significantly more touchy to tune.
That said, it works very well on 40 and 80 into EU.
This weekend I plan to add 4 -12 foot wires for a top hat for 80.
According to Bencher, it will improve performance on 80 as well as a
little more bandwidth. Anyone tried this?
Thanks
Mike - KI8R
On Wed, Nov 13, 2013 at 6:51 PM, Mike & Coreen Smith VE9AA <
ve9aa@nbnet.nb.ca> wrote:
James,
I recently installed an HF9V at around 8' AGL. I use 2 elevated
"ground"(counterpoise) radials per band, sloping from 8' down to
about 5'
AGL.
I find it works VERY well on 40m and up and adequate on 80m.
Over the past month, I've compared it to a host of other wire
antennas at
similar heights and always found the Butternut to be as good or better
than anything else in the yard here. I believe the raised radials have
less
loss, but it was twitchy to tune. An HF2V ought to be a cake-walk to
tune.
It's just my opinion that using only a few ground radials is lossy and
that's why everyone preaches ground mounting it. (a lot easier to
tune w/
better/wide 2:1 SWR curves) (I use the term "better", but I don't really
mean better, but it is better for the match, but I think it's like
dumping
power into the ground to heat worms.)
So far I have around 2500 Q's with it, contesting every weekend and I am
sure it works well , raised up like it is.
p.s.- subscribe to the Yahoo group for Butternut antennas and then check
the
files section for "VE9AA" or "AD5X".a couple good ideas
how to get the whole 80m band out of the antenna.
YMMV,
73 Mike VE9AA, NB
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