Length & number of radials
 

Hasan et al,

Tom says current can be detected in radials well beyond the 20dB
attenuation limit. This is easily explained.

The total current flowing in the system at a distance is in the soil
due to its far greater cross-sectional area. Especially when soil
resistivity is low. Nothing in particular happens in the soil at the
end of the 20dB limit.

The small current in a radial is INDUCED in it by the relatively
larger total current flowing in the soil in parallel with it. The
radial current is NOT generated by the voltage at its input. Its high
attenuation isolates it from its input.

What current flows in a radial has a progressively less effect on the
total current (which is what matters) as distance increases.
Eventually, it doesn't matter whether the radial is there or not.

The limit is reached when the radial input impedance converges on Zo,
the radial's characteristic impedance. This occurs when radial
attenuation is around 18 or 20dB. Beyond that distance the current
flowing in the ground carries on, as usual, unaffected whether the
radial is there or not.

Resonant effects, small peaks and troughs in the impedance-frequency
curve, also die away at the 20dB or even lower limit. There's not much
left even at 14dB.

Radial attenuation increases rapidly with frequency. So shorter
radials can be used at 14 MHz than at 1.9 MHz. When 30 MHz is the
lowest frequency of use, and soil resistivity is high, a dipole,
without radials, is more likely to be used than a vertical.

(Comment: I guessed correctly I would be accused of trolling when I
introduced the subject of radials as transmission lines.)
----
Reg, G4FGQ.

Length & number of radials
 

Wash your hands before opening that next bottle.

73's
Richard Clark, KB7QHC

======================================

Like Bob Hope, when in your company, I always drink out of a dirty
glass.
----
Reg.

Length & number of radials
 

Richard Clark wrote:
I have to ask, is it written in sonnet form in middle English?

Almost as bad, it is written in PASCAL. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Length & number of radials
 

hasan schiers wrote:

Are you saying (for example), that the feedpoint R of a 1/4 w vertical
against perfect ground cannot be reliably estimated at 37 ohms?

No. I'm not saying that.

If it can,
then isn't 37/R a measure of efficiency?

No. A ground can have transmission line effects. As such, it can modify
impedances.

Is it really possible to reduce ground losses to the absolute minimum and
not have a corresponding increase in field strength?

You are assuming a reduction of ground loss or an increase of ground
loss always indicates a certain resistance change at the feedpoint.
That assumption is not correct Hans.

For example, I can measure feed resistance of a mobile antenna on my
truck. By moving the antenna around I can vary the "apparent" ground
resistance from a few ohms to perhaps 30 ohms with very little change
in ground loss.

All I'm saying is the feedpoint reistance change when using a 1/4 wl
series fed radiator does not correspond to change in field strength. I
know that to be absolutely true, because I and others have seen that
happen.

73 Tom

Length & number of radials
 

Reg,

I certainly don't think you are trolling. You have aroused a very
interesting discussion. I'm absolutely fascinated. The issue isn't whether
Tom can detect the current at a point beyond your description. The point is
will that current be quite a bit larger than the 20 dB down your approach
predicts.

This is getting pretty simple...either the current is or isn't substantial
beyond the wire lengths you describe. If it isn't, you have hit on
something big. If it is, then the model you are using or the application of
that model is in error. I'm just trying to learn which of these two cases is
true. I find your analysis breathtakingly interesting. It's just hard to
resolve the apparent contradictions....fun nevertheless! Thanks again.

....hasan, N0AN
"Reg Edwards" wrote in message
...
Hasan et al,

Tom says current can be detected in radials well beyond the 20dB
attenuation limit. This is easily explained.

The total current flowing in the system at a distance is in the soil
due to its far greater cross-sectional area. Especially when soil
resistivity is low. Nothing in particular happens in the soil at the
end of the 20dB limit.

The small current in a radial is INDUCED in it by the relatively
larger total current flowing in the soil in parallel with it. The
radial current is NOT generated by the voltage at its input. Its high
attenuation isolates it from its input.

What current flows in a radial has a progressively less effect on the
total current (which is what matters) as distance increases.
Eventually, it doesn't matter whether the radial is there or not.

The limit is reached when the radial input impedance converges on Zo,
the radial's characteristic impedance. This occurs when radial
attenuation is around 18 or 20dB. Beyond that distance the current
flowing in the ground carries on, as usual, unaffected whether the
radial is there or not.

Resonant effects, small peaks and troughs in the impedance-frequency
curve, also die away at the 20dB or even lower limit. There's not much
left even at 14dB.

Radial attenuation increases rapidly with frequency. So shorter
radials can be used at 14 MHz than at 1.9 MHz. When 30 MHz is the
lowest frequency of use, and soil resistivity is high, a dipole,
without radials, is more likely to be used than a vertical.

(Comment: I guessed correctly I would be accused of trolling when I
introduced the subject of radials as transmission lines.)
----
Reg, G4FGQ.

Length & number of radials
 

On Fri, 21 Jul 2006 11:28:02 -0500, "hasan schiers" wrote:


"Reg Edwards" wrote in message
...

In the case of RADIAL_3 the obvious purpose of the program is to
assist with choosing an economic length and number of radials to be
used with a given test antenna height. It is also educational in that
after reading the introductory notes and using it, the user will have
a better understanding of how radials work.
snip

================================================= ======
We need to know: does the predicted attenuation of current along a radial
wire happen as quickly as you predict? This can be measured. This can be
modeled. That's what makes this fun. Let's find out. Let's see what agrees
with what and what doesn't. Then we can conjecture as to why, and which
approach is to be "believed".
================================================= =======
snip
...hasan, N0AN

I've been reading this thread, and sent the following msg to hasan. I then
decided to post it here for others to see.

Walt, W2DU

Hi Hasan,

I've been reading the radials thread on the rraa, with the works of BLE bandied
about. I have a copy of BLE in PDF that I can put on a CD and mail it to you if
you don't have a copy, which I'd be pleased to do.

Incidentally, I worked for many years with Jess Epstein, the 'E' of BLE, in
Brown's antenna laboratory at the RCA Labs in Princeton, where Brown is the 'B'
in BLE. I also know Bob Lewis, the 'L' in BLE, as we've spent many hours
together as hams. Bob is W2EBS.

Bob and I were attending a ham meeting in NJ in the 1960s where Jerry Sevick was
demonstating how radials worked with verticals. He had annular rings of wire
connecting all the radials at various radial distances from the center. He even
had a wire connecting the ends of all the radials together. I asked him what the
annular rings were for, and he replied that they kept the currents in each
radial equal. I asked him if he was acquainted with the BLE paper, and he said
he had heard of it but was not familiar with it. So I asked him if he'd like one
of it's co-authors to explain it He agreed, but was totally shocked to know
that Bob Lewis was in his audience. Bob then proceded to straighten Sevick out
on how radials worked. A fun night, indeed.

The graphs reporting BLE's measurements are pretty conclusive. In addition, the
BLE paper is the basis on which the FCC set the requirements for the ground
systems on all AM BC stations since 1939. So there's thousands of empirical
proofs of the correctness of their measurements in every situation where field
strengh measurements were required for proof of performance.As I'm sure you
already know, for every AM BC station that uses a directional antenna system the
FCC requires field strength measurements. There has never been any such
measurements that disagreed with those of BLE. Even Tom's (W8JI)

Please let me know if you'd like me to burn you a copy of BLE.

Walt,W2DU

PS--If any others reading this would like a copy of BLE let me know and I'll
burn it for you.

Length & number of radials
 

wrote in message

No. A ground can have transmission line effects. As such, it can modify
impedances.

Bummer! I had no idea.


Hasan: Is it really possible to reduce ground losses to the absolute
minimum and
not have a corresponding increase in field strength?

Tom: You are assuming a reduction of ground loss or an increase of ground
loss always indicates a certain resistance change at the feedpoint.
That assumption is not correct Hans. (Hasan)

Bummer again! The field strength does change, but you are saying the
feedpoint Z may not track it.

All I'm saying is the feedpoint reistance change when using a 1/4 wl
series fed radiator does not correspond to change in field strength. I
know that to be absolutely true, because I and others have seen that
happen.

Then we are left with no use for monitoring feedpoint resistance (other than
matching). Bummer.
All we can do is keep adding radials and watch the R drop until it gets
boring. (Or is that not possible now, either?). Every vertical antenna (1/4
w), I've ever made and played the radial game with has behaved predictably
with increasing numbers of radials...the feedpoint Z has always dropped
asymptotically towards the Rrad of the vertical. Now I have to discard all
that...or are you quoting the exception that doesn't invalidate the general
nature of things? I'm getting that "too many variables to deal with" black
magic feeling again. Things looked so reasonable for a while there...now it
appears for all but the brave, it becomes nothing more than cramming a lot
of wire into or onto the ground and hoping for the best.

Not what I was hoping for at all. Bummer.

Thanks for taking the time to explain parts of this, Tom. (even though it
wasn't what I wanted to hear)

73,

....hasan, N0AN


73 Tom

Length & number of radials
 

"Walter Maxwell" wrote
The graphs reporting BLE's measurements are pretty conclusive. In
addition, the
BLE paper is the basis on which the FCC set the requirements for the
ground
systems on all AM BC stations since 1939

==========================================

Walt, where've you been lately?

I have no doubt that BLE measurements are good and valid at LF and
below. But to extrapolate conclusions up to HF, where amateurs reside,
and where funny things happen to radials, is somewhat dangerous.

I understand BLE forgot to measure ground resistivity and permittivity
of the site. Perhaps because they thought it didn't matter very much.
But such things certainly matter above about 3.5 MHz. At HF radials
behave very differently from behaviour at LF if only because the
ground 'constants' have changed from their DC and LF values (which are
the values usually inserted in HF computer programs.)
----
Reg.

Length & number of radials
 

"Walter Maxwell" wrote (in part):
I've been reading the radials thread on the rraa, with the works of BLE
bandied
about. I have a copy of BLE in PDF that I can put on a CD and mail it to
you if
you don't have a copy, which I'd be pleased to do.
___________

All,

Not to minimize the fine offer of Mr. Maxwell, any of you who might prefer
to download a PDF of this BL&E "benchmark" paper with empirical data on
buried radial ground systems vs radiation system efficiency can do so from
the link following the text below, which I posted last December to some
broadcast-oriented websites.

N. B. for/to REG EDWARDS (G4FQP): I hope that you will be motivated to
follow through on one or the other of these offers, and that you will post a
comparison of the results of your ready-to-run, "radial_3" DOS program as
compared to the BL&E datum, for equivalent conditions.

RF

+++

Link: Brown, Lewis & Epstein Paper on MW Ground Systems
Richard Fry (rfry at adams.net )
Fri Dec 9 14:54:23 CST 2005

--------------------------------------------------------------------------------

After some "back and forth," IEEE just gave me permission to post the
benchmark 1937 paper by Brown, Lewis and Epstein about MW ground systems.

Quite a few on RT and BC asked me earlier for this, but I had to hold off
until it was possible. Now I hope all those who asked will see this posting,
eventually, and view/download it from the link below. There are some
restrictions on its use, as shown in the PDF.

The file size is about 3.3 megs, so a fast connection will help.

http://rfry.org/Software%20&%20Misc%20Papers.htm

RF

Length & number of radials
 

Hi Hasan,

I have not forgotten your model. I am very interested in verifying
the findings of Reg's program, so will get around to it. I will be
very busy this weekend, so may not have a chance until the
beginning of next week. I have been investigating some of
the limitations of NEC, and 1 mm below ground is one of
them (Not necessarily because of the depth, but segmentation
issues). I will try and get as close to your model parameters
as possible. This afternoon I was sidetracked by a challenge
from Reg, and spent about 90 minutes running a model
similar to yours. Interestingly enough there appears to be
a very large discrepancy between the programs.
NEC 4.1 indicated only 30.5 % efficiency.
(See later thread).

Frank


"hasan schiers" wrote in message
...
Hi Frank,

I think the general question became "can one use this Rrad value in
calculating efficiency". I'm waiting for Tom's response to my last
posting.

On the other issue, radial length vs. usefulness, (I tried a diect mail
to you and it didn't make it cuz I forgot to take out the nospam part),

here is what I want to know from NEC-4:

Radial wire is #14 THHN inslulated wire. I approximated it at 2mm. The
antenna wire is 4 mm. For these purposes, you can probably forget that the
wire is insulated.

Now...looking at radial length (assuming 26 radials), and given the
constants I previously provided, how long does a radial in this
configuration have to be, before it is no longer valuable to increase its
length. Tom says he measured significant current in a radial well beyond
where Reg's program says the current had diminished to insignifcant
levels.

I would be MOST interested if you can confirm Tom's measurements. If NEC-4
says there is substantial radial current where Reg's program says there
isn't, then that is an important contradiction, putting Reg's model into
question. I'm giving more credibility to NEC-4 (properly used) than I am
to Reg's own design. If, however, we have two sources (one measurement
based: Tom, one model based: NEC-4), that say Reg's theory that radials
quickly approach maximum effectiveness over a MUCH shorter run than has
been previously understood (in moderate to very good soils), that
contradict Reg's algorithim.

Having only looked at conclusions from BL&E, I can't say what their
measurements indicated in terms of radial current vs. length. Ian has
suggested that they did measure the radial current vs length and they
concur with Tom. So, if BL&E and Tom (both empirical), as well as NEC-4
(model based), all say that important levels of current are present in
radials well beyond where Reg's program predicts, then there's only one
conclusion left. (Unless I'm missing something).

This, to me, is much more interesting stuff than a month long peeing
contest over precipitation static.(which may be rearing its ugly head yet
again in the "double bazooka" thread. God help us!

73, and thanks for your comments and efforts to help me understand what is
going on.

...hasan, N0AN
"Frank's" wrote in message
news:ZO5wg.115459$A8.61548@clgrps12...
I understand there are measurement issues (and certainly assumption
issues
for Rrad). Isn't is fairly certain that increasing the number of radials
(of proper length) until the feedpoint R (at resonance, at the antenna)
no longer drops, is a reasonable approximation of "high efficiency"? The
only issue I see, is determining the target Rrad to compare it to when
trying to "estimate" efficiency.

Are you saying (for example), that the feedpoint R of a 1/4 w vertical
against perfect ground cannot be reliably estimated at 37 ohms? If it
can, then isn't 37/R a measure of efficiency?

Again, I'm thinking of the efficiency of the ground system... I have no
way to look at field strength.

Is it really possible to reduce ground losses to the absolute minimum
and not have a corresponding increase in field strength?

This is starting to turn into "black magic" for me. I can understand
questioning a particular "number" for efficiency based on the simplistic
Rrad/R formula. If the implications go further...indicating there is no
meaning to Rrad/R, then I'm lost.

Perhaps the issue is that it's known how to maximize efficiency, it's
just completely unknown what that efficiency really is, and there is no
simple way to measure it. If that's what your saying, then I understand.

That position does seem to muddy up the "how many radials and of what
length" efficiency info presented in ON4UN's book and referenced in
other texts. They all seem to acccept some sort of accuracy for the
Rrad/R formula with 1/4 w verticals. If I understand you correctly, the
formula is rejected outright as hopelessly simplistic, and of no
particular value.

Do I have it now? If so, I'll refrain from using it in the future.

I had always assumed that a NEC model of a perfectly conducting
monopole above a perfect ground would provide the radiation
resistance. For example, considering your antenna of 18.3 m
at 3.62 MHz, the input impedance is 27.5 - j 64.7. The radiation
resistance would therefore be 27.5 ohms. This appears to be
fairly close to your estimate of 25.4 ohms.

Frank