Effectiveness of decoupling radials
 

I have been playing with some NEC-2 models of a multiband vertical with
radials.

The vertical is an unloaded vertical of 13m height, and it is mounted on
a 6m high grounded metal mast, and an ATU installed at the feedpoint
(base of the radiator). I have fitted a pair of opposed nominal quarter
wave radials for each of the 80, 40, 30 and 20m bands.

To simulate ground loss, I have modelled a 20 ohm resistance in the
bottom of the mast, and used a MININEC ground.

An interesting observation is the sensitivity of this model to length of
the radials. Properly adjusted, each pair of opposed radials near
eliminate current on the mast (more than 20dB below the current into the
radiator). The exception to this is the 30m radials which seem to suffer
some interaction with the 80m radials (near third harmonic). Without the
appropriate radials, current in the mast to ground is large, and losses
can be 10dB or more.

The ideas I take away from the modelling excercise is that:
- inadequate decoupling exacerbates ground loss;
- decoupling is very dependent on the length of the radials;
- one pair of opposed radials is enough for a narrow band;
- the radials for different bands have some interaction; and
- the optimum length may be quite a deal longer than the expected length
of legs of a half wave dipole in the same place.

I am grappling with some other way to optimise such a system, other than
measuring the mast current (which often isn't easy).

These effects probably also apply to a trapped vertical with similar
counterpoise, and the traditional wisdom of tuning either the length of
the vertical or radial length to achieve low VSWR is probably less than
optimal, there is an optimal length for each of them.

The traditional wisdom that elevated radials are generally significantly
lower loss than buried radials probably depends on careful "tuning" or
isolation of feed point ground to minimise current flowing to the real
ground.

Comments, thoughts?

Owen

Effectiveness of decoupling radials
 

On Mon, 30 Apr 2007 03:53:06 GMT, Owen Duffy wrote:

The traditional wisdom that elevated radials are generally significantly
lower loss than buried radials probably depends on careful "tuning" or
isolation of feed point ground to minimise current flowing to the real
ground.

Hi Owen,

As Reggie would have pointed out at this juncture, tuning of radials
in very close proximity to the ground would have been an exercise in
futility (that is, expecting a sharp resonance as would be evidenced
in elevated radials). As was his wont, his description of radials as
lossy transmission lines would have held sway in this analysis.

This, of course, is contingent upon my having understood the
implication of the quoted text above relating to tuning radials near
the ground (instead of on high).

73's
Richard Clark, KB7QHC

Effectiveness of decoupling radials
 

Richard Clark wrote in
:

This, of course, is contingent upon my having understood the
implication of the quoted text above relating to tuning radials near
the ground (instead of on high).

Richard,

The radials in this case are 4m to 6m above the ground, so should exhibit a
fairly clear resonance.

Owen

Effectiveness of decoupling radials
 

On Mon, 30 Apr 2007 03:53:06 GMT, Owen Duffy wrote:


I have been playing with some NEC-2 models of a multiband vertical with
radials.

The vertical is an unloaded vertical of 13m height, and it is mounted on
a 6m high grounded metal mast, and an ATU installed at the feedpoint
(base of the radiator). I have fitted a pair of opposed nominal quarter
wave radials for each of the 80, 40, 30 and 20m bands.

[snip]
Comments, thoughts?


Owen.

My first thoughts are while such a situation can be modeled, in the
real world I would doubt you can build a system as you describe where
the radials are balance. Just as one can not install a dipole in a
typical back yard (garden) that is truly balanced.

My two cents worth. G

Danny, K6MHE

Effectiveness of decoupling radials
 

Owen Duffy wrote:
I have been playing with some NEC-2 models of a multiband vertical with
radials.

The vertical is an unloaded vertical of 13m height, and it is mounted on
a 6m high grounded metal mast, and an ATU installed at the feedpoint
(base of the radiator). I have fitted a pair of opposed nominal quarter
wave radials for each of the 80, 40, 30 and 20m bands.

To simulate ground loss, I have modelled a 20 ohm resistance in the
bottom of the mast, and used a MININEC ground.

An interesting observation is the sensitivity of this model to length of
the radials. Properly adjusted, each pair of opposed radials near
eliminate current on the mast (more than 20dB below the current into the
radiator). The exception to this is the 30m radials which seem to suffer
some interaction with the 80m radials (near third harmonic). Without the
appropriate radials, current in the mast to ground is large, and losses
can be 10dB or more.

The ideas I take away from the modelling excercise is that:
- inadequate decoupling exacerbates ground loss;
- decoupling is very dependent on the length of the radials;
- one pair of opposed radials is enough for a narrow band;
- the radials for different bands have some interaction; and
- the optimum length may be quite a deal longer than the expected length
of legs of a half wave dipole in the same place.

I am grappling with some other way to optimise such a system, other than
measuring the mast current (which often isn't easy).

These effects probably also apply to a trapped vertical with similar
counterpoise, and the traditional wisdom of tuning either the length of
the vertical or radial length to achieve low VSWR is probably less than
optimal, there is an optimal length for each of them.

The traditional wisdom that elevated radials are generally significantly
lower loss than buried radials probably depends on careful "tuning" or
isolation of feed point ground to minimise current flowing to the real
ground.

Comments, thoughts?

Owen

Just as an aside, Owen, have you
considered just a single pair of
non-resonant, opposed radials for all
bands? The ATU can just as easily "tune"
a single radial pair + vertical
element as it can the vertical element
alone.

Any additional benefit of separate
radial pairs for different bands may be
slight.

Chuck

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Effectiveness of decoupling radials
 

Chuck wrote:
Just as an aside, Owen, have you considered just a single pair of
non-resonant, opposed radials for all bands? The ATU can just as easily
"tune" a single radial pair + vertical element as it can the vertical
element alone.

I have such an antenna and am very satisfied with it.
It is a 22 foot vertical with the feedpoint at 22 feet.
Four 22 foot radials slant down at a 45 degree angle
and double as guy wires. An SG-230 is installed at the
feedpoint.

EZNEC says it has a radiation pattern ranging from
0.5 dBi at 22 deg TOA on 40m to 4.2 dBi at 11 deg
TOA on 10m. Come on sunspots!
--
73, Cecil http://www.w5dxp.com

Effectiveness of decoupling radials
 

"Owen Duffy" wrote in message
...

I have been playing with some NEC-2 models of a multiband vertical with
radials.

The vertical is an unloaded vertical of 13m height, and it is mounted on
a 6m high grounded metal mast, and an ATU installed at the feedpoint
(base of the radiator). I have fitted a pair of opposed nominal quarter
wave radials for each of the 80, 40, 30 and 20m bands.

To simulate ground loss, I have modelled a 20 ohm resistance in the
bottom of the mast, and used a MININEC ground.

An interesting observation is the sensitivity of this model to length of
the radials. Properly adjusted, each pair of opposed radials near
eliminate current on the mast (more than 20dB below the current into the
radiator). The exception to this is the 30m radials which seem to suffer
some interaction with the 80m radials (near third harmonic). Without the
appropriate radials, current in the mast to ground is large, and losses
can be 10dB or more.

The ideas I take away from the modelling excercise is that:
- inadequate decoupling exacerbates ground loss;
- decoupling is very dependent on the length of the radials;
- one pair of opposed radials is enough for a narrow band;
- the radials for different bands have some interaction; and
- the optimum length may be quite a deal longer than the expected length
of legs of a half wave dipole in the same place.

I am grappling with some other way to optimise such a system, other than
measuring the mast current (which often isn't easy).

These effects probably also apply to a trapped vertical with similar
counterpoise, and the traditional wisdom of tuning either the length of
the vertical or radial length to achieve low VSWR is probably less than
optimal, there is an optimal length for each of them.

The traditional wisdom that elevated radials are generally significantly
lower loss than buried radials probably depends on careful "tuning" or
isolation of feed point ground to minimise current flowing to the real
ground.

Comments, thoughts?

Owen

Owen, It may not be too critical, but would not the Sommerfeld/Norton
method improve accuracy?

Frank

Effectiveness of decoupling radials
 

"Frank" wrote in news:u9qZh.14500$JF6.4868
@edtnps90:

Owen, It may not be too critical, but would not the Sommerfeld/Norton
method improve accuracy?

Hi Frank

My understanding was that the MININEC ground model was the better to use if
the model caused current to flow into ground (as mine does).

The draft model is at
http://www.vk1od.net/multibandunload.../13MVERT01.nec if you want to
play with it.

Owen

Effectiveness of decoupling radials
 

Chuck wrote in
:

....
Just as an aside, Owen, have you
considered just a single pair of
non-resonant, opposed radials for all
bands? The ATU can just as easily "tune"
a single radial pair + vertical
element as it can the vertical element
alone.

Any additional benefit of separate
radial pairs for different bands may be
slight.

That would seem the case if you just regard the radials as providing a
counterpoise, the "other" connection that provides a return path for
current to the source. I have modelled this scenario where the source is
at the feedpoint (ie no transmission line) and the radials and radiator
are suspended above ground by a non-conducting structure, and you are
right that the radials need not be resonant, residual reactance being
dealt with by the auto-tuner at the feed point.

However, if you connect the radials to ground by some conductor (eg feed
line and / mast) that conductor is not part of the picture, and as I
modelled a conducting mast with a lossy ground connection, the big
picture is very different.

Here is a plot of modelled system losses with the configuration that I
described: http://www.vk1od.net/multibandunload...al/13mEV03.gif .
Not the large losses at 5MHz, this loss is mostly in the 20 ohms
equivalent earth resistance. The high ant+gnd loss at 1.8MHz can be
reduced to less than a dB with a pair of ~40m long radials (but tuner
losses increase to 3+dB).

So it seems that one could do as you suggest and effectively isolate the
radiator and radials from ground, or the radials need to be carefully
adjusted to minimise the mast / feedline current to ground, especially
where the feedpoint resistance is small wrt the equivalent mast to eart
resistance.

Owen

Effectiveness of decoupling radials
 

Danny Richardson wrote in
:

....
My first thoughts are while such a situation can be modeled, in the
real world I would doubt you can build a system as you describe where
the radials are balance. Just as one can not install a dipole in a
typical back yard (garden) that is truly balanced.

I agree that practical antennas on suburban blocks are a challenge. But
that doesn't eliminate the effects, and I think the key issue that the
models raise is the value in effectively decoupling the mast / feed line.
Tuned radials are not the only method, and as I suggested in my post, short
of measuring the mast current, I cannot see any other obvious method of
"tuning" the radials for maximum decoupling.


Owen

Effectiveness of decoupling radials
 

On Apr 30, 3:56 pm, Owen Duffy wrote:


So it seems that one could do as you suggest and effectively isolate the
radiator and radials from ground, or the radials need to be carefully
adjusted to minimise the mast / feedline current to ground, especially
where the feedpoint resistance is small wrt the equivalent mast to eart
resistance.

Owen


Any elevated radials need to be resonant, and show a low Z at
the freq being used. If they show a high Z, they are basically
useless. You have to think of them more as the lower half
of the antenna, rather than ground radials. I read a post of
Roy's the other day that kind of touched on this. There really
is no "RF ground" when elevated, according to him, and I agree.
It's all one total antenna same as any other.
Of course, the number of radials used for the lower half of the
low GP antenna will greatly effect the ground loss, and to some
extent the decoupling of the feedline.
I've noticed that adding more than four radials to a 1/4 wave GP
on 2m did improve decoupling.
So even if ground losses are not an issue, IE: the 2m example,
the performance can still be improved by improving the decoupling
of the line. And it's noticable too.
But with your low band, low height antenna, ground losses are
a larger issue than decoupling. 2 radials are going to be lukewarm
at best, even elevated at your low height in wavelength.
Your ground losses are going to be pretty high.
MK

Effectiveness of decoupling radials
 

"Owen Duffy" wrote in message
...
"Frank" wrote in news:u9qZh.14500$JF6.4868
@edtnps90:

Owen, It may not be too critical, but would not the Sommerfeld/Norton
method improve accuracy?

Hi Frank

My understanding was that the MININEC ground model was the better to use
if
the model caused current to flow into ground (as mine does).

The draft model is at
http://www.vk1od.net/multibandunload.../13MVERT01.nec if you want
to
play with it.

Owen

I was not thinking Owen. I forgot that some versions of NEC support the
MININEC
ground. I have loaded your program, but noticed my version of NEC does not
support a "GN" entry of "3" in the "I1" field. It thinks it is a
Sommerfeld/Norton
ground, but does not recognize the conductivity and permittivity fields.
About the
only way I could get the program to run is to extend "GW 10" below
round --
at a guess about 5 segments should be ok. I am also concerned about some
discontinuity with the large diameter change from GW 9 to GW 10. Also GW
1 to GW 2. Initially I will set all diameters to 1 mm, and see what I get
by
running the AVG test.

Frank

Effectiveness of decoupling radials
 

"Frank's" wrote in
news:4OxZh.8833$Dq6.8346@edtnps82:


"Owen Duffy" wrote in message
...
"Frank" wrote in news:u9qZh.14500$JF6.4868
@edtnps90:

Owen, It may not be too critical, but would not the
Sommerfeld/Norton method improve accuracy?

Hi Frank

My understanding was that the MININEC ground model was the better to
use if
the model caused current to flow into ground (as mine does).

The draft model is at
http://www.vk1od.net/multibandunload.../13MVERT01.nec if you
want to
play with it.

Owen

I was not thinking Owen. I forgot that some versions of NEC support
the MININEC
ground. I have loaded your program, but noticed my version of NEC
does not support a "GN" entry of "3" in the "I1" field. It thinks it
is a Sommerfeld/Norton
ground, but does not recognize the conductivity and permittivity
fields. About the
only way I could get the program to run is to extend "GW 10" below
round --
at a guess about 5 segments should be ok. I am also concerned about
some discontinuity with the large diameter change from GW 9 to GW 10.
Also GW 1 to GW 2. Initially I will set all diameters to 1 mm, and
see what I get by
running the AVG test.

Hi Frank,

I built the models in 4NEC2 and EZNEC, both using the MININEC ground
feature.

My guess is that the radials are far enough clear of the ground that NEC-
2 should be adequate for modelling, but it would be interested to see
what results you get from NEC-4.

Re extending wire 10, don't forget it is loaded, so you need to deal with
that.

Owen

Effectiveness of decoupling radials
 

I built the models in 4NEC2 and EZNEC, both using the MININEC ground
feature.

My guess is that the radials are far enough clear of the ground that NEC-
2 should be adequate for modelling, but it would be interested to see
what results you get from NEC-4.

Re extending wire 10, don't forget it is loaded, so you need to deal with
that.

Owen

Owen, GNEC reports the following. Note I have taken the ratio
of the current magnitudes without regard to the phase relationship.
I can send you a zipped copy of the NEC output file if you are
interested.

Freq TAG 10 TAG 1 Ratio
ABS SEG 169 SEG 1
(MHz) (mA) (mA) (dB)
3.8 1.43 6.5 13
7.1 0.59 5.4 19.2
10.1 0.25 1.0 12
14.1 0.036 3.4 39.5

Large currents in the 3.8 MHz radials are evident
on 30 m. I have removed the loading from TAG 10,
and EK is not required in NEC 4. Also I show
TAG 10 at extending 3 m below ground. Probably
not realistic, but I am always confusing meters with
feet!

Frank

Effectiveness of decoupling radials
 

"Frank's" wrote in
news:ay2_h.21$Vi6.12@edtnps82:

I built the models in 4NEC2 and EZNEC, both using the MININEC ground
feature.

My guess is that the radials are far enough clear of the ground that
NEC- 2 should be adequate for modelling, but it would be interested
to see what results you get from NEC-4.

Re extending wire 10, don't forget it is loaded, so you need to deal
with that.

Owen

Owen, GNEC reports the following. Note I have taken the ratio
of the current magnitudes without regard to the phase relationship.
I can send you a zipped copy of the NEC output file if you are
interested.

Freq TAG 10 TAG 1 Ratio
ABS SEG 169 SEG 1
(MHz) (mA) (mA) (dB)
3.8 1.43 6.5 13
7.1 0.59 5.4 19.2
10.1 0.25 1.0 12
14.1 0.036 3.4 39.5

Large currents in the 3.8 MHz radials are evident
on 30 m. I have removed the loading from TAG 10,
and EK is not required in NEC 4. Also I show
TAG 10 at extending 3 m below ground. Probably
not realistic, but I am always confusing meters with
feet!


Hi Frank,

A plot of this current ratio shows very steep slope around the design
frequencies.

A plot of my model results is at
http://www.vk1od.net/multibandunload...al/new_pa1.gif .

Ratio of mast current to radiator current at junction:

(MHz) NEC-4(dB) NEC-2
3.8 -13 -14.6
7.1 -19.2 -17.6
10.1 -12 -9.9
14.1 -39.5 -38.3

Note that for the NEC-2 model, these were not the optimal frequencies.

Owen