TopBand: Elevated GP vs. Vertical Antennas
Eric Gustafson Courtesy Account
n7cl@mmsi.com
Mon, 16 Mar 1998 18:45:50 -0700
>From: km1h@juno.com (km1h @ juno.com)
>Date: Sat, 14 Mar 1998 15:56:28 EST
>
>I will try and be brief.
>
Hi Carl,
Thanks, but I don't feel constrained to be limited to brevity on
this subject - yet. Probably soon though...
>Eric are you then saying that the BCB requirements/measurements
>have pretty much no bearing on DX oriented 160M operators? If
>you read published info on "optimum" angles, it can then be
>assumed that those below 10-15 degrees are not usually necessary
>on 160. I am NOT claiming that those articles are accurate.
>
No. I wasn't saying that at all. The only published info I have
seen regarding "optimum" angles refers to the measurement of
_arrival_ angles of signals being received. I have seen nothing
which even purports to determine the optimum takeoff angle for
transmit. The elevation angle at which the wave was launched has
precious little to do with the elevation angle from which it
arrives at the other end of the link.
My own experience has been that the lower the takeoff angle I
could achieve, the more DX I could work (By that I mean the more
DX could hear me - pretty much regardless of band).
Note that pattern improvements at the lower elevation angles take
nothing away from the pattern at higher angles. We are simply
making use of energy that would otherwise be lost. Have you ever
seen or heard a report from a ham with a waterfront site saying
that he had less success over the water azimuths where his
takeoff angle was in the 2-3 degree range than he did over the
azimuths which traveled away from his site over land where he had
a 24 degree takeoff angle? I haven't.
And to reiterate. Ground level far field (one to several
wavelengths out) field strength measurements that show a change
in field strength in response to change in the antenna's ground
system indicate that there has been a corresponding change in the
strength everywhere in the pattern. This statement is true
unless the changes were made in the ground system more than 1/2
wavelength away from the antenna (still talking about full or
nearly full size vertical radiators here).
>So then for 160M or 80M, what is the optimum angle field strength
>for say a 6 elevated radial system. What is the angle of maximum
>radiation for a 6 radial system, 20' high over average ground?
>
I can't answer that one without more information about the
antenna over the radials. How tall is it physically? Is it
resonant in the band? How is it resonated (inductive loading,
sloper loading, capacitive top loading, etc.). Is it base fed at
the radial system? Is the portion of the tower which extends to
ground below the radials decoupled from the antenna system
somehow? Is the tower base isolated from ground?
>If I understand you correctly, then measuring FS at some XXX
>point at zero degrees elevation is a waste of time since all you
>are confirming is the FS at the lowest point of the lobe, and you
>still have no idea what it is doing at useful skywave angles.
No, Im saying exactly the opposite. See above.
>
>I do know that my minimal elevated system is doing something
>right. There are just too many times when 9M2AX calls me year
>after year while I am working EU at my evening. Later, on a
>higher band he often tells me I was the only USA station that he
>could hear and I was at similar signal strengths as the
>EU. Optimal angle or just blind luck?
>
I'm sure that your elevated radial system is indeed doing
something for you. Without it, your system would no doubt be
many dB down from where it is now. You are competitive because
NOBODY (Maybe 1 or 2 but I don't know about them.) has a full
size full density ground screen under their topband vertical.
And even fewer (not living on an island which is 1 wavelength
across) have improved ground far enough out to lower the takeoff
angle significantly.
The following few slightly edited paragraphs are excerpted from a
message I recently sent (off reflector) in response to very
similar questions. I include them here because I think they are
relevant to this discussion.
-------------
Four (or six) elevated radials over poor earth are superior to
zero radials over poor earth. I was trying to get people
thinking about the losses (all of them) properly. If you start
off with a vertical that is 15 dB down from ideal because of
ohmic and near field losses, and you vastly reduce the ohmic
losses with 4 elevated radials, you have indeed made a 10 dB
improvement. The antenna will work MUCH better. But it will
still be 5 dB short of what would be possible with a LOT more
effort and expense. And you will still have the same takeoff
angle for the main lobe in either case.
I'm glad to see that we're finally causing the discussion to be
steered in what I think is the right direction. That is what can
PRACTICALLY be done by an amateur at his existing site. And what
can he reasonably expect to be the consequences of any necessary
compromises he must make.
I usually stay out of these discussions untill I'm triggered by
someone propagating a blatantly erroneous myth (This statement is
not aimed at you Carl.). This time it was the implication that 4
elevated radials close to the ground is as effective as a full
size ground screen. The plain truth is that they are not. They
are not _ineffective_. They are just not _AS_ effective as the
fulll screen. And people should not be lead to believe that they
are.
I was also triggered by the implication in some of the posts that
there is anything short of moving to the beach that an amateur
(unless he is unbelievably rich and landed) can do to affect the
takeoff angle of the signal from his topband ground mounted
full sized vertical.
I have been waiting to hear someone ask: "OK then, what would you
do if you were working on a 100 foot by 100 foot lot?" Of
course, here the answer would necessarily vary greatly depending
on the other constraints imposed by the site. However most of
them would probably involve trading physical size of the radiator
for bandwidth and taking as much care to screen the near fields
off the local earth as possible.
In my own case, I'm having to face the same problem at a new QTH.
Although not particularly space constrained, the thought of the
work and expense of the full screen under a full size radiator is
daunting. I will definitely be making compromises and
tradeoffs. But I will be trying to understand the real
consequences of doing so.
The real question is: "Is 4 - 6 dB improvement significant in
terms of being able to work the stations I want to work?" The
answer is probably not most of the time when they are workable at
all (unless I _MUST_ work them over the top of anyone else who is
calling at the same time). But sometimes, it will definitely be
the difference. Otherwise nobody would ever buy an amplifier
that provides only about that much strength improvement.
So the next real question is: "Will my site permit me do install
the `full screen'?" _AND_ "If so, is the 4 to 6 dB improvement
worth the money and effort to me?" Everyone has to answer those
questions for himself. Mostly, including my own case, the answer
is NO.
>Reply directly if you do not want to clutter up the reflector.
No, these are useful enough questions that the reflector should
be cluttered up with them.
>
>73 Carl KM1H
>
73, Eric N7CL
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