Owen,

The antenna is a short loaded vertical. The base is about 1.1 meters
long and 90mm in diameter, the coil is about 160mm long and about 80mm
in diameter, the top is 3 meters. The coil wire is 12 gage and the
spacing is about .5. As a model cross check, the impedance of the coil
measures about 60 uH.

- Dan

Owen Duffy wrote:
Owen Duffy wrote in
:

dansawyeror wrote in
:

...

I think a summary is that at 3970KHz, the feedpoint Z looks like about
45 +j0 and you reckon the radiation resistance should be around 4+j0,
suggesting the earth system contributes around 40 ohms of resistance.
Observations at a single frequency provide a limited view of what
might be happening.

Dan,

I asked your for the details of your antenna and measurements, and how
you did your calculations, but I am still left wondering how you have
what appears to be a purely resistive feedpoint impedance and a radiation
resistance of 4 ohms. The second implies a short vertical, and if that is
the case, the first implies some form of loading... but you didn't
mention loading of any kind. Loading, if you have used it, may introduce
an equivalent series resistance at the feedpoint.

Once again, a dansawyer problems leaves us guessing to fill in the
missing dots before attempting to joint them up to make a picture.

Often, solving a problem is about being able to draw the picture, once
the picture is draw, the answer becomes trivial.

Owen

dansawyeror wrote in
:

Owen,

The antenna is a short loaded vertical. The base is about 1.1 meters
long and 90mm in diameter, the coil is about 160mm long and about 80mm
in diameter, the top is 3 meters. The coil wire is 12 gage and the
spacing is about .5. As a model cross check, the impedance of the coil
measures about 60 uH.


So, what do you think its impedance would be? 1500+j??

This is probably accounting for somewhere between 5 and 15 ohms of
additional resistance, depending on Q.

Owen

Owen Duffy wrote in news:Xns990EA1652F44Enonenowhere@
61.9.191.5:

So, what do you think its impedance would be? 1500+j??

Duh, ??+j1500 is more like it.

Same outcome, it is probably accounting for 5 to 15 ohms of resistance that
you haven't included in your calcs.

Owen

Owen,

Are you referring to the coil alone? You are correct the coil accounts
for a significant part, probably between 4 and 8 Ohms.

I am measuring real values between 52 and 56 Ohms with 0j. This varies
mainly with moisture.

So let's say the total antenna is 12 Ohms. If the measured value is 52
that says the ground is 40. This puts us back to the original question:
why is the ground so high?

- Dan

The circuit

Owen Duffy wrote:
Owen Duffy wrote in news:Xns990EA1652F44Enonenowhere@
61.9.191.5:

So, what do you think its impedance would be? 1500+j??

Duh, ??+j1500 is more like it.

Same outcome, it is probably accounting for 5 to 15 ohms of resistance that
you haven't included in your calcs.

Owen

dansawyeror wrote in
:

Owen,

Are you referring to the coil alone? You are correct the coil accounts
for a significant part, probably between 4 and 8 Ohms.

I don't recall that you have told us where the coil is located, and I am
not going to build a model to discover something that is relevant and
that you could have told us.

You have told us the coil had an "impedance" of 60uh. If it had an
inductance of 60uh, it would have an impedance of 1500/Q+j1500 where Q at
3.9MHz for a practical coil is likely to be above 100 and less than 300.
The resistive component of the coil has to be referred to the feedpoint
so that you can deduct it from the total feedpoint Z. The best way to do
that is your NEC model.

You should be able to form a better range for the equivalent coil loss at
the feedpoint than you stated above (since you seem to have the means of
measuring the coil and inserting the values in your model).

I am measuring real values between 52 and 56 Ohms with 0j. This varies
mainly with moisture.

So let's say the total antenna is 12 Ohms. If the measured value is 52
that says the ground is 40. This puts us back to the original
question: why is the ground so high?

You also previously said "... Even if the antenna were 6 to 8 Ohms the
ground loss would be at least 42 to 44 Ohms."

I read this to mean total system R is 50 ohms.

In this post it is reported between 52 and 56.

The other issue that Frank raised is the elevated feedpoint and whether
you modelled that correctly. The radiation resistance you quoted seems
(without checking) reasonable for a short monopole over ideal ground, but
one expects it would be higher for an elevated feed point. Have you
modelled the antenna you built, or have you build an antenna you cannot
model accurately and are applying model results incorrectly to the thing
you have built?

Though you see only one question, "why is ground 40 ohms", you haven't
disclosed enough information in your posts to convince me that it is 40
ohms. If you ask the wrong question, you might not get a useful answer.

Is ground 40 ohms?

Owen

I believe everything is stated:

The antenna is a short loaded vertical. The base is about 1.1

meters long and 90mm in diameter, the coil is about 160mm long

and about 80mm in diameter, the top is 3 meters. The coil wire

is 12 gage and the spacing is about .5. As a model cross check, t

he impedance of the coil measures about 60 uH.

The order is base, coil, and top.

- Dan


Owen Duffy wrote:
dansawyeror wrote in
:

Owen,

Are you referring to the coil alone? You are correct the coil accounts
for a significant part, probably between 4 and 8 Ohms.

I don't recall that you have told us where the coil is located, and I am
not going to build a model to discover something that is relevant and
that you could have told us.

You have told us the coil had an "impedance" of 60uh. If it had an
inductance of 60uh, it would have an impedance of 1500/Q+j1500 where Q at
3.9MHz for a practical coil is likely to be above 100 and less than 300.
The resistive component of the coil has to be referred to the feedpoint
so that you can deduct it from the total feedpoint Z. The best way to do
that is your NEC model.

You should be able to form a better range for the equivalent coil loss at
the feedpoint than you stated above (since you seem to have the means of
measuring the coil and inserting the values in your model).

I am measuring real values between 52 and 56 Ohms with 0j. This varies
mainly with moisture.

So let's say the total antenna is 12 Ohms. If the measured value is 52
that says the ground is 40. This puts us back to the original
question: why is the ground so high?

You also previously said "... Even if the antenna were 6 to 8 Ohms the
ground loss would be at least 42 to 44 Ohms."

I read this to mean total system R is 50 ohms.

In this post it is reported between 52 and 56.

The other issue that Frank raised is the elevated feedpoint and whether
you modelled that correctly. The radiation resistance you quoted seems
(without checking) reasonable for a short monopole over ideal ground, but
one expects it would be higher for an elevated feed point. Have you
modelled the antenna you built, or have you build an antenna you cannot
model accurately and are applying model results incorrectly to the thing
you have built?

Though you see only one question, "why is ground 40 ohms", you haven't
disclosed enough information in your posts to convince me that it is 40
ohms. If you ask the wrong question, you might not get a useful answer.

Is ground 40 ohms?

Owen

dansawyeror wrote in
:

The order is base, coil, and top.

That wasn't clear to me, it looked to me like you were feeding it between
the base and the top. I think Frank may have formed the same view.

Owen