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

"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

"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

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

"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