Eric, Hardy, Bill, Jeff, Yuri....etc. tnx for all the input.
Eric, I will seriously edit your post so as to keep W4ZV contented. I
also sent off some more questions yesterday to the reflector as I am
getting educated here.
>>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...
I was thinking about W4ZV's tolerance threshold!
>
>
>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.
Good point which I forgot about.
>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).
I feel the same way. Even if the guy on the ocean has 5-6dB better
efficiency that does not explain the 3-5 S units relative difference at
times. IMO the angle is a factor.
>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.
I'm pretty well convinced that it is a linear or close to a linear ratio.
The change I see at the horizon is duplicated at xxx degrees higher.
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.
Nope!
>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).
My verticals are a full 1/4 wave long with 130-150' radials.
>>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?
The verticals here are #12 wire supported from 2 of the upper guys of a
180' Rohn 45G tower. They are a full 1/4 wave long. They are sloping,
with the top separation about 50' and the bottom 130'. Base fed which is
about 12' above ground. The radials rise up at a 45 degree angle to the
20-25' level and are then supported thru tree branches. At present there
are 6 130-150' radials per antenna. The phasing is a very basic 78 degree
piece of RG-11; using the ON4UN suggestion. All feed lines are 75 Ohms.
Observable FB on TX/RX is in the 15dB area ( S Meter reports)
....consistently...sometimes better. The pattern is relay switched for
NE, SW or Broadside.
The tower guys are broken up with insulators every 58'...using the ARRL
chart. The base is not grounded and just sits on a pivot pin in the
concrete pad.
My site is fairly flat for about 250-300' in any direction and then
drops down 400' in anywhere from 1/2 to 1 mile depending upon direction.
>
>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.
There are at least 2 I know of within 30-40 miles...those are full size
towers with insulated bases. They are at higher elevations to the West of
me...W1JCC and KC1XX.
>
>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.
Does a noise bridge give a meaningful indication of Ohmic loss reduction?
A full size 1/4 wave should be ~37 Ohms with an ideal ground according
to the literature. I started out with one radial and added one at a time
until I reached the 37 Ohm point...which happened to be 4 radials. I
added 2 more for good luck and saw absolutely no difference on the
bridge. Is this not supposed to indicate that the Ohmic losses are
minimized....? again according to lots of published ham articles.
I understand your remark that measuring the FS is the only true test but
I have yet to see a plausible explanation why the noise bridge can be
misleading. I am talking about a full size antenna...no loading or other
excuse to confuse the discussion.
>
>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.).
Thanks but I'll take my hits too!
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.
Which brings us full circle and back to the Chrisman article. I'll ask
again, what does NEC-4 have to say about this? You have me 99% convinced
BUT...
>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.
What would you suggest for a practical compromise?
>The real question is: "Is 4 - 6 dB improvement significant in
>terms of being able to work the stations I want to work?"
If the 4dB takes a minimal effort then I opt for that. If the remaining
2dB is a real b--- breaker I'll pass. A new amp vs a few thousand feet
of surplus wire is a no brainer.
>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.
Every time you reply I learn but that generates more questions.!
I have 2 for now:
Would installing a high density mesh at the feed point help? By that I
mean a 2" x 4" grid of welded, galvanized and vinyl insulated rabbit
cage wire. It is available in 50' x 4' rolls. Should it be placed on the
ground and connected to the elevated radials with wide copper strap or
elevated along with the radials?
At a prior QTH I placed 5 rolls of that cage on top of 25000 feet of
on/in ground radials. The tower was a shunt fed 100' with a top load of
long boom 10-15-20M yagis (all grounded elements) . The mesh reduced my
2:1 VSWR BW from 50 to about 20KHz. The "apparent improvement" in pileup
busting was very noticable. The QTH was pretty much surrounded by swamp
out to a mile or so in most directions. The water table was 2-3' below
the surface. I still do not understand the reason for the improvement.
Would running a few.... 4 to 10 maybe.... very long radials help? Long
is 1000-5000' . Elevated or on the ground?
Tnx agn Carl KM1H
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