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. 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

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

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

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

"William Mutch" schreef in bericht
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.

William,
I have done some work on local QRM reduction during the last
few years. Summarized on:
http://home.plex.nl/~jmsi/
Most important is avoiding any coupling with the coax/feedline.
With small magnetic loops this is easy to accomplish and my
guess is that this is why loops are less susceptible for local QRM.
That is why I choose small loops instead of small dipoles.

73 de Jan PA0SIM

On Tue, 13 Jul 2004 17:57:21 -0500, "Crazy George"
wrote:

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.

In the _far_ field both the E and H fields are inversely proportional
to distance and have the 120 pi (377 ohm) relation (impedance) between
the fields. However, in the _near_field_ ( 1 lambda) the 377 ohm
relationship is no longer valid and the magnetic field is inversely
proportional to the square of the distance, while the electric field
is inversely proportional to the cube of distance.

Summarising the graph from an article by Lloyd Butler VK5BR in Amateur
Radio, August 1990: The output voltages from both E and H field
antenna system are calibrated to the same value at 1 lambda (i.e. in
the far field). The antennas are moved closer, when the E and H
antennas are moved to 0.05 lambda, the E antenna delivers 50 dB and
the H antenna 40 dB (relative to 1 lambda) i.e. the H-field is 10 dB
quieter. At 0.005 lambda, the E field antenna output is 110 dB and the
H-field 80 dB, i.e. the H field antenna is 30 dB is quieter.

Thus, with same far field sensitivity, the sensitivity to very local
interference is attenuated considerably when _only_ the H field is
used. However, at 3.5 MHz and 80 m wavelength, 0.05 lambda corresponds
to 4 m and 0.005 lambda to 40 cm, so we are talking about really close
noise sources. At even higher frequencies the number of potential
interference sources is dropping within the 0.05 (or even 0.1) lambda
radius from the receiving antenna, in which the H antenna has an
advantage.

However, on the 135 kHz LF band (lambda 2.2 km), the distances would
be 110 m resp. 11 m, thus much more unwanted interface sources could
be eliminated.

Shielding the H-loop simply prevents the stronger E field from
entering the loop and thus destroying part of the advantage of using
the H-antenna.

Paul OH3LWR