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

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

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

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

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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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

"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

"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

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

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