Is there a homebrew instrument that I can make that will let me know when
I'm out of the near field and in the far field? I'd like it to work with
mobile whips as well as dipoles on HF. The ARRL Ant HBK formula for the
boundary D between the two fields is an approximation where D = 2L^2/w where
L is ant length in same units as w, the wavelength.
tnx

--
73
Hank WD5JFR

On Wed, 26 May 2004 14:10:01 GMT, "Henry Kolesnik"
wrote:

Is there a homebrew instrument that I can make that will let me know when
I'm out of the near field and in the far field? I'd like it to work with
mobile whips as well as dipoles on HF. The ARRL Ant HBK formula for the
boundary D between the two fields is an approximation where D = 2L^2/w where
L is ant length in same units as w, the wavelength.
tnx

Hi Hank,

That is an interesting question. So right off the top I will admit I
know of no such beast. However, the formula looks particularly
liberal. Normalized it appears to suggest for a common dipole that
distance would be 1 wavelength, but I would think it from the
literature that this distance would be closer to 5. It is all a
matter of degree in how thin you want to cut it.

At:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm
I have done a suite of 6 antennas and have illustrations of the near
field in terms of its Z expressed as SWR to free space. If nothing
else this will give you a picture (for what it's worth). In returning
to the delineation between near and far, my illustrations show only
the first quarterwave out and things have pretty well settle out in
this scale.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Wed, 26 May 2004 14:10:01 GMT, "Henry Kolesnik"
wrote:

Is there a homebrew instrument that I can make that will let me know
when
I'm out of the near field and in the far field? I'd like it to work with
mobile whips as well as dipoles on HF. The ARRL Ant HBK formula for the
boundary D between the two fields is an approximation where D = 2L^2/w
where
L is ant length in same units as w, the wavelength.
tnx

Hi Hank,

That is an interesting question. So right off the top I will admit I
know of no such beast. However, the formula looks particularly
liberal. Normalized it appears to suggest for a common dipole that
distance would be 1 wavelength, but I would think it from the
literature that this distance would be closer to 5. It is all a
matter of degree in how thin you want to cut it.

At:
http://home.comcast.net/~kb7qhc/ante...elds/index.htm
I have done a suite of 6 antennas and have illustrations of the near
field in terms of its Z expressed as SWR to free space. If nothing
else this will give you a picture (for what it's worth). In returning
to the delineation between near and far, my illustrations show only
the first quarterwave out and things have pretty well settle out in
this scale.

73's
Richard Clark, KB7QHC

Very interesting Richard. What did you use as a "ground screen" and how much
ground coverage did you provide in the dipole experiment?

Best regards,

Jack Painter
Virginia Beach, VA

I don't know of any such instrument.

One problem is the very definition of far field. It's a region in which
the properties of the radiated wave approach their final values. Some of
the characteristics of these final values (that is, the values the wave
approaches at very great distances) are (for propagation in air):

-- The field strength decreases inversely with distance
-- The ratio of E/H = the Z of free space (about 377 ohms, purely real)
-- The field has no component in the direction of travel

All these characteristics are approached as a limit, and the distance at
which one or more are "close enough" is strictly a matter of definition
and the particular application. It's at that somewhat arbitrary point
that you're said to be in the far field.

You can devise instruments to measure any or all of these properties,
although I'd think doing it with any sort of accuracy would be beyond
the reach of most amateurs. But if you did, you'd still have to decide,
on the basis of the measurements and your own criteria, where you'll
draw your own particular line.

Roy Lewallen, W7EL

Henry Kolesnik wrote:

Is there a homebrew instrument that I can make that will let me know when
I'm out of the near field and in the far field? I'd like it to work with
mobile whips as well as dipoles on HF. The ARRL Ant HBK formula for the
boundary D between the two fields is an approximation where D = 2L^2/w where
L is ant length in same units as w, the wavelength.
tnx

I had a couple of reasons for asking. One was to find out more which I
have. I use a field strength meter in the shack which is in the near fied as
a tuning aid for tuning my linear and wonder if I would see any difference
it I placed it in the far field and remoted the reading to the shack?
tnx

--
73
Hank WD5JFR
"Roy Lewallen" wrote in message
...
I don't know of any such instrument.

One problem is the very definition of far field. It's a region in which
the properties of the radiated wave approach their final values. Some of
the characteristics of these final values (that is, the values the wave
approaches at very great distances) are (for propagation in air):

-- The field strength decreases inversely with distance
-- The ratio of E/H = the Z of free space (about 377 ohms, purely real)
-- The field has no component in the direction of travel

All these characteristics are approached as a limit, and the distance at
which one or more are "close enough" is strictly a matter of definition
and the particular application. It's at that somewhat arbitrary point
that you're said to be in the far field.

You can devise instruments to measure any or all of these properties,
although I'd think doing it with any sort of accuracy would be beyond
the reach of most amateurs. But if you did, you'd still have to decide,
on the basis of the measurements and your own criteria, where you'll
draw your own particular line.

Roy Lewallen, W7EL

Henry Kolesnik wrote:

Is there a homebrew instrument that I can make that will let me know
when
I'm out of the near field and in the far field? I'd like it to work
with
mobile whips as well as dipoles on HF. The ARRL Ant HBK formula for the
boundary D between the two fields is an approximation where D = 2L^2/w
where
L is ant length in same units as w, the wavelength.
tnx

So far as using a field strength meter for tuning up the transmitter is
concerned, you will not notice the slightest difference between near and far
field. There is none. They are both produced by the same transmitter.

You'll get different strength readings at different distances from the
antenna, and in the near and far fields. But they'll all stay in
proportion as you tune your linear, so it doesn't matter where you put
your meter. The only exception is that you need to put the meter far
enough away from the linear that it won't be responding to radiation
leakage from the linear itself, which could be coming from the tank
components or parts of the circuit ahead of it. You could check this by
seeing what your field strength meter indicates when the linear is
feeding a good dummy load. If it's a significant part of your reading
when driving an antenna, you need to move the meter.

Roy Lewallen, W7EL

Henry Kolesnik wrote:
I had a couple of reasons for asking. One was to find out more which I
have. I use a field strength meter in the shack which is in the near fied as
a tuning aid for tuning my linear and wonder if I would see any difference
it I placed it in the far field and remoted the reading to the shack?
tnx

Thanks for the answers. I even got a personal reply from someone with some
Pinot across the pond confirming the above.
No clear boundary, no differnce and it's OK to have the FS meter in the
shack. Now why can't I find out this quick and easy if the final in a Tx
is dissipative or non-dissipative to reflections?

--
73
Hank WD5JFR

"Roy Lewallen" wrote in message
...
You'll get different strength readings at different distances from the
antenna, and in the near and far fields. But they'll all stay in
proportion as you tune your linear, so it doesn't matter where you put
your meter. The only exception is that you need to put the meter far
enough away from the linear that it won't be responding to radiation
leakage from the linear itself, which could be coming from the tank
components or parts of the circuit ahead of it. You could check this by
seeing what your field strength meter indicates when the linear is
feeding a good dummy load. If it's a significant part of your reading
when driving an antenna, you need to move the meter.

Roy Lewallen, W7EL

Henry Kolesnik wrote:
I had a couple of reasons for asking. One was to find out more which I
have. I use a field strength meter in the shack which is in the near
fied as
a tuning aid for tuning my linear and wonder if I would see any
difference
it I placed it in the far field and remoted the reading to the shack?
tnx

On Wed, 26 May 2004 14:23:59 -0400, "Jack Painter"
wrote:
Very interesting Richard. What did you use as a "ground screen" and how much
ground coverage did you provide in the dipole experiment?

Hi Jack,

The EZNEC files are available at the page. It looks like a fan of 64
radials, each a quarter wave long. Check the file if necessary.

73's
Richard Clark, KB7QHC