Hi Yuri and all,
Another long and never ending thread, radials! I might bail out of
this tar-baby on this one.
> I suspect that in all mentioned cases you are talking about radials on the
> ground. Your experiences confirm what would be expected (surprise).
> Was any comparison done with elevated radials few vs. many? That is
> really what I am after. Thinking and analyzing the situation, there should
> be improvement if using many (>60) elevated (or on ground) radials vs. few
Fact Yuri. When the radials are less than .025 to .05 wl apart at
the open ends, they look like a solid screen. Using more radials
than that is a waste of wire.
> (<8) elevated radials over salt water "ground." The question is how much
> improvement there is, or is it worth while the effort of going from few to
> many on the beach. This is what I am after, not speculations what should
> happen, just the facts ma'am!
Fact. All time-varying current-carrying conductors without close
opposing currents in another conductor will radiate. They all have
induction fields that are both electric and magnetic, as well as
radiation.
These fields concentrate losses in the media below the radial,
unless you use enough wires spaced close enough that it looks
like a "wall". Moving the wire away from ground reduces this
requirement, but the wire has to be a large portion of a wavelength
and a large portion of it's physical above ground.
The reason multiple radials work close to earth is they divide the
current among so many wires that each wire has practically no
current or voltage!
I can go out and touch any point on any of my 100 radials while
pumping 1500 watts into my vertical, and not feel a thing. When
walking underneath the elevated radials at a BC station, and while
20 feet away from the radial, a wire tied to my pant's belt loop
burned a hole in my trousers and me! Now think about putting that
junk near all your other wires, telephone lines, and antennas.
Why would anyone serious about having a decent problem-free
station do that?
It is utter nonsense to think moving a wire that is 130 feet long six
feet in the air suddenly makes the earth below that wire not be
coupled to that wire, or that a second wire 50 or 100 feet away from
that point will somehow magically cancel the earth coupling.
Moving wire up certainly does help reduce earth currents, but the
effect isn't large enough to eliminate losses until the wire is at least
1/8 wl above earth with eight radials. The fewer the radials used,
the higher above ground they must be.
We were all smart enough to know that at one time, when people
measured antennas instead of modeling them. If you look at early
copies of the ARRL Antenna Handbook you will see they caution
that a groundplane antenna is ONLY a groundplane when 1/2 wl
above earth.
I really can't see why we depend on models that assume the earth
is homogeneous at all depths (Reg on the Antenna Newsgroup
does the same). Such a model certainly has errors.
In a paper presented at AGARD in 1989 in London, Belrose made a
comparison using NEC-2 of a dipole at low heights and compared
that model to a REAL dipole measure by Hagn-Barker (1970
Stanford Research Institute). This data shows close agreement of
the real dipole and a model until a height of .02-.04 wl is reached.
Then the real dipole quickly falls below the model.
Keep in mind the earth conductivity was adjusted to provide the
best compliance between the model and the actual antenna.
If a model can not do a simple dipole correctly at a low height, it
would also be suspect on radials at the same height. The model
underestimated loss.
A height of five feet on 160 meters would put the dipole and model
about 5 dB out of compliance.
I also made some current measurements on a Beverage, and
Lewallen modelled the antenna. There was a notable error in
currents predicted by NEC, in a direction that would underestimate
earth loss.
This doesn't mean the models are "bad", but it does indicate we
should be careful assuming we can just plug in some numbers and
get great results. To the best of my knowledge the only verification
of elevated radial models have been based on FCC data from
proofs, and not A-B comparisons of systems at one site. Anyone
who has done FCC proofs knows very well you can get almost any
answer you like depending on what points you select and how you
apply the data through several steps. It is actually easy to "waddle"
the data 3 dB or so, because the FS slope along radial directions
from the tower is never constant.
As I've looked at data, the engineers have taken the most
"compliant" readings with a goal of 100% efficiency, rather than the
mean value of conflicting points. The correct procedure would be to
take more readings into and out of areas of "flyers" (where the
reading suddenly moves off an unexpected amount) but that is
never done. The curve is simply (in the cases I looked at) moved to
a favorable point in the desired results.
If the system was A-B compared, all these variations would be
meaning less. You would have a direct comparison at every point of
a change between two systems.
When that was done at WVNJ, six elevated radials were down
anywhere from 2 to 6 dB, with the average just under 5 dB, at ten
reference points.
At my QTH I had almost the same results.
Now this is off-topic for saltwater, but it leads me to one important
point. Unless someone A-B's the system properly with proper
equipment and procedures there will always be an element of
doubt. I'm not the only one in this camp. As someone once said,
"some people swear at elevated radials and some people swear by
them".
With such little work involved to install 30 or so radials 1/8 or so wl
long, I wonder why anyone would go through the bother of installing
the supports necessary to hang four wires in the air and not know if
the thing works right or not.
Not only are you assured with a conventional ground (even a
modest one) that the system works within a dB or less of perfect,
you are totally out of the headaches of decoupling the feedline and
radials from your other equipment. You can not feed a small
elevated radial system directly with coax (no choke) without adding
loss, and with out having RF follow the feeder. You can not put
anything around the radials without it being part of the antenna
system.
As an additional advantage, you can ground the ground system for
lightning.
> Modeling and speculation shows that half wave vertical dipole close (<
> .01wl) to salty ground seems to be the best/simplest radiator for the
> beach. It gets efficient "return" for currents from it's lower half and is
> looking at the salt water quarter to half wave out from the base, where it
> sees the salt water for low angle for far field pattern.
The concept of "return currents" is misleading. It gives a false
impression of how complex the problem is. The radial system does
two things:
1.) It provides something for common mode currents in the antenna
to be pushed against. (You can't make anything flow up without the
same amount "flowing in" from the ground connection. Folded
unipole lovers take note, you don't gain a thing by folding the
antenna so far as ground losses are concerned. The SUM of
currents in both conductors is the same as a conventional
monopole, and the loss is identical)
2.) It shields the earth from the concentrated fields surrounding the
antenna. Both induction fields, the electric and magnetic,
contribute to loss.
As was pointed out to me on this reflector a few years ago,
sticking the ends of an inverted V close to dirt increases loss. I
even use counterpoises under my dipoles, so I often forget about
losses like that.
73, Tom W8JI
w8ji@contesting.com
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