Say, for purposes of illustration, that the near field ends at 1 wavelength.
At 2 MHz, that is very roughly 530 feet . At 14 MHz it is about 64 feet.
At 30 MHz, it has shrunk to ~32 feet.

--
Crazy George
Remove N O and S P A M imbedded in return address
"Harold E. Johnson" wrote in message
news:Qf_Ic.82101$Oq2.21575@attbi_s52...


But, that seems to only be effective up to 3 or 4 MHz, due to the
wavelength
factor, i. e. the near field shrinks as you go higher in frequency.

REALLY? How does it do that?

W4ZCB

"Crazy George" wrote in message
...
Say, for purposes of illustration, that the near field ends at 1
wavelength.
At 2 MHz, that is very roughly 530 feet . At 14 MHz it is about 64 feet.
At 30 MHz, it has shrunk to ~32 feet.

-- Why would the near field end at 1 wavelength? It ends
when the wave front arriving at the receiving antenna becomes planar. ie, to
function efficiently in the far field, the receiving antenna needs to
intercept a planar wavefront. That is, the individual rays need to be
arriving in parallel. If the distance between antennas is very great, that
is very nearly the case.

If the capture area of the receiving antenna is great relative to the
distance to the source, the received energy arrives as non parallel rays
that basically reach the receiving antenna out of phase with each other and
partially cancel. So, the gain of antennas measured in the "near field",
where the received energy is not a planar wavefront, will be in error. The
distance to the end of the near field is highly dependent on the gain of the
antenna and with UHF and SHF antennas often exhibiting very high gain, their
near fields can be and often are very large.

The power collected by a receiving antenna within the transmitters near
field is very nearly constant with distance. In the far field, recovered
power varies inversely with the square of the distance.

Regards

W4ZCB

Harold E. Johnson wrote,
It ends
when the wave front arriving at the receiving antenna becomes planar. ie, to
function efficiently in the far field, the receiving antenna needs to
intercept a planar wavefront. That is, the individual rays need to be
arriving in parallel. If the distance between antennas is very great, that
is very nearly the case.
If the capture area of the receiving antenna is great relative to the
distance to the source, the received energy arrives as non parallel rays
that basically reach the receiving antenna out of phase with each other and
partially cancel. So, the gain of antennas measured in the "near field",
where the received energy is not a planar wavefront, will be in error. The
distance to the end of the near field is highly dependent on the gain of the
antenna and with UHF and SHF antennas often exhibiting very high gain, their
near fields can be and often are very large.

Balanis divides the near-field region into two parts: a reactive near-field
R0.62 square root(D^3/Lambda) where D is the largest antenna dimension,
Lambda is the wavelength, and R is the distance from the antenna surface,
and a radiating near-field region R2D^2/Lambda. The far-field he defines as
anything greater than 2D^2/Lambda. He gives exceptions to these rules, so
take them with a grain of salt.

73,
Tom Donaly, KA6RUH

Cecil wrote,

Bob Miller wrote:
Dumb question: how do you reduce noise without reducing the strength
of the signals you want to hear? How does the antenna know which is
which?

Beams seem to "know" how to receive a signal from one direction while
ignoring noise from the opposite direction. My horizontal dipole seems
to "know" how to ignore vertically polarized noise.
--
73, Cecil http://www.qsl.net/w5dxp



Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise
ratio in the directions of its maximum gain over say, a non shielded loop?
Moreover, how do you get your beam to be less sensitive to noise in its
favored direction? Are you robbing Peter to pay Paul?
73,
Tom Donaly, KA6RUH

In article ,
says...
OH, for Pete's sake. Loops are sensitive to the H vector. Wires receive
the E vector. Most near field noise tends to be predominantly E field.
But, that seems to only be effective up to 3 or 4 MHz, due to the wavelength
factor, i. e. the near field shrinks as you go higher in frequency. Fully
formed far field wavefronts of noise sources will be just like wanted
signals, and unless some polarization difference is available, then
directivity is the only way to improve S/N. Only in special circumstances
can you see much improvement above 5 MHz due to near field/far field
differentiation.

But, my point was that no improvement in S/N was reported in the original
post.

True; I didn't report it but it is there. Typically at most
frequencies the desired signal is reduced 1 to 2 S-units with respect to
the whip antenna (strong ones) or my high long wire weaker signal...156
feet AWG 16 up 45 feet fed off center w/ a 4:1 balun) but the noise
level is reduced by anywhere from 3 to 6 S-units...a very! worthwhile
tradeoff. Exact quantitative measurements are not possible on the
Sat800 RCVR because you can't turn off the AGC. My understanding of why
the shielded loop performs this way is that near field noise is
cancelled while far field signal is only attenuated by some factor
relating to capture area. In my temporary rooftop mount I was unable to
easily check out the effect of broadside null.


Only a decrease of noise accompanied by a decrease in signal. No
relative comparison offered. Are we supposed to *assume* that the signals
went down due to time of day, while the noise went down because it is a
loop? Maybe the opposite is true? Not enough data to prove either.

--
Crazy George
Remove N O and S P A M imbedded in return address

Hi William, All,

As is common with comparisons, the problems arise due to the shifting
sand these arguments are built upon.

On Wed, 14 Jul 2004 12:09:17 -0400, William Mutch
wrote:

But, my point was that no improvement in S/N was reported in the original
post.

True; I didn't report it but it is there. Typically at most
frequencies the desired signal is reduced 1 to 2 S-units with respect to
the whip antenna (strong ones) or my high long wire weaker signal...156
feet AWG 16 up 45 feet fed off center w/ a 4:1 balun) but the noise
level is reduced by anywhere from 3 to 6 S-units...a very! worthwhile
tradeoff.

Presumably, the comparison is loop vs. these others. It is not
explicit and that is one of the problems of reporting and subsequent
interpretation - hence the observation in the double quote above.

However, the "issue" is more has anything really changed? A loop
(dipole) compared to two verticals. Arguably the so-called off center
fed long wire is presumed to be a dipole, however (again poor
reporting) nothing says of this antenna being choked. Lacking that
choke offers every inducement of Common Modality (the antenna is,
after all, fully and admittedly unbalanced by its very description).
Common Modality is ever bit a noise hazard as any vertical (is
supposed to be - another nightmarish fantasy under the bed).

Hence, any perceived boon of noise reduction comes as a consequence of
the loop's faithfully performing as a - dipole! Wonders never cease.

Exact quantitative measurements are not possible on the
Sat800 RCVR because you can't turn off the AGC.

I don't know how this got started as a unnecessary evil - AGC is what
drives the S-Meter. AGC is only an issue if you want to derive signal
strength via modulation levels - which nobody here does anyway.

My understanding of why
the shielded loop performs this way is that near field noise is
cancelled while far field signal is only attenuated by some factor
relating to capture area. In my temporary rooftop mount I was unable to
easily check out the effect of broadside null.

Tom has posted in this thread very simple metrics to obtain just what
constitutes near field. The incantation of near/far fields belies
simpler explanations. If there is any issue of noise that relates to
its nearness, it follows that you are the source. You being the
source means that you also have the capacity to correct (and building
a magic antenna is possibly the most superstitious response to that
problem). The loop simply has less coupling (and less signal - that
means there is a constant of proportionality in S/N) than a full sized
dipole sitting over this noisy domicile. I have a random wire antenna
that passes within 2 feet of an 80W Fluorescent fixture with a humming
ballast. I barely pull in S-1 worth of noise and a loop would stand
to do worse at that same distance. If I find that little noise
troublesome, I turn off the noise.

The fact that the shielded loop performs as a dipole is proof of its
efficient construction (many fail to achieve even this). There is
very little more that can be said about its qualities short of its
loss of sensitivity.

73's
Richard Clark, KB7QHC

Tdonaly wrote:
Cecil wrote,
Beams seem to "know" how to receive a signal from one direction while
ignoring noise from the opposite direction. My horizontal dipole seems
to "know" how to ignore vertically polarized noise.

Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise ratio in the directions of its maximum gain over say,
a non shielded loop?

Depends upon the source of the noise. I remember a small shielded loop
being effective against localized electrical noise in my college dorm.
--
73, Cecil http://www.qsl.net/w5dxp



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Depends upon the source of the noise. I remember a small shielded loop
being effective against localized electrical noise in my college dorm.
--
73, Cecil http://www.qsl.net/w5dxp

=======================================

Yes, but did it make any difference when you removed the shielding?

Tdonaly wrote:



Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise
ratio in the directions of its maximum gain over say, a non shielded loop?

Many claim this, but I didn't see it when I compared them. I found a
shielded coax loop just as susceptible to local, and not so local noise,
as a non shielded loop assuming both are balanced. This is not counting
the feedline, or any common mode currents unbalancing the loops. Both
are capable of very sharp nulls. No difference really, and both are good
at nulling a noise source. But a shielded coax loop quieter than a
regular loop? I don't see it. It's not the loop itself, or having a
shield. It's the keeping of good balance. The shielded loop design and
method of feeding forces a good balance. But if you have a regular loop
that is also just as balanced, I maintain it's just as "quiet". To me,
this "shielded loop being quieter" theory is an old wives tail of sorts.
My two favorite MW loops are both unshielded. Ones a 16 inch dia circle
with 12 turns, and my big one is a diamond with 44 inches per side. "5
turns". Both are on floor stands indoors, and rotate.
I've tried using shielded coax loops, and I saw no reduction of noise.
I've also compared using both shielded and non shielded coupling loops
to feed the loops. Again, no difference in noise levels. MK

--
http://web.wt.net/~nm5k

Mark Keith wrote,

Tdonaly wrote:



Yes, but does your small, inefficient, shielded loop improve the
signal-to-noise
ratio in the directions of its maximum gain over say, a non shielded loop?

Many claim this, but I didn't see it when I compared them. I found a
shielded coax loop just as susceptible to local, and not so local noise,
as a non shielded loop assuming both are balanced. This is not counting
the feedline, or any common mode currents unbalancing the loops. Both
are capable of very sharp nulls. No difference really, and both are good
at nulling a noise source. But a shielded coax loop quieter than a
regular loop? I don't see it. It's not the loop itself, or having a
shield. It's the keeping of good balance. The shielded loop design and
method of feeding forces a good balance. But if you have a regular loop
that is also just as balanced, I maintain it's just as "quiet". To me,
this "shielded loop being quieter" theory is an old wives tail of sorts.
My two favorite MW loops are both unshielded. Ones a 16 inch dia circle
with 12 turns, and my big one is a diamond with 44 inches per side. "5
turns". Both are on floor stands indoors, and rotate.
I've tried using shielded coax loops, and I saw no reduction of noise.
I've also compared using both shielded and non shielded coupling loops
to feed the loops. Again, no difference in noise levels. MK

--
http://web.wt.net/~nm5k



This pretty much squares with an article on shielded loops written by
Glenn S. Smith of the Georgia Institue of Technology in _The Antenna
Engineering Handbook_. He says the shield enforces symmetry so that
the pattern doesn't suffer, and that's what it's supposed to do. No mention
of noise at all.
73,
Tom Donaly, KA6RUH