"Reg Edwards" wrote in message
...
One of the most serious sources of error will be pick-up on the long
line between the small loop and the receiver. With a coax line there
will be a greater signal pick up on the coax braid than there is in
the loop. They are both located in the same field.

So best to use very low impedance balanced pair line such as 50 ohms
perhaps with a screening braid. A good choke balun or a 1-to-1 wound
transformer would be advisable between the line and receiver input.

Also, depending on frequency, length and impedances, there may be
standing waves on the line which could make a mess of your
calculations.

A change in line length is a good way to check for errors of this
sort.


Wouldn't it be better to use a pre-amp at the loop feed? The gain of the
pre-amp could make line pickup a negligible effect, and the pre-amp would
match the coax very well.

--
Ed
WB6WSN
El Cajon, CA USA

"Owen" wrote in message
...
Frank wrote:

In the BPL report at
http://www.ofcom.org.uk/research/tec...line/ascom.pdf
I noticed the system noise floor at about 10 dBuV/m (in 9 kHz). For the
tests they used an active bi-conical antenna. (By my calculations 10
dBuV/m is about 9 uV(2.5 kHz BW) from a 40 m dipole at 7 MHz.) In the
previously

Yes, Fig 8 shows about 10dBuV/m in 9KHz which interpolates to 5dBuV/m in
3KHz, and their measurements used a peak detector. On white noise, the QP
value would probably be 2 to 3dB lower.

I have made a large number of measurements at my home QTH (in a
residential neighbourhoos) using a half wave dipole and assuming an
average gain of -1.2dBi or an AF of -11.6dB/m and I regularly get ambient
noise readings down to around 0 to 3dBuV/m QP in 3KHz or extrapolated to
9KHz BW, 5 to 8dBuV/m QP. Ambient noise is probably lower than indicated
by ITU P372-8!

mentioned report most of the BPL signals -- even at 1 meter from the
source -- is 60 dBuV/m. It seems your system with the loop will be
much less sensitive at about 100 uV/m (+40 dBuV/m).


See my response to Reg re the noise floor for the setup, I make it around
8dbuV/m or 2.5uV/m. I don't pretend it can measure ambient noise, but it
can and has measured BPL interference at 40dBuV/m to 70dBuV/m.


What detector do you think should be used to evaluate the interference
potential of BPL?

I had thought that the QP detector was designed to the "annoyance" effect to
AM or SSB modulation. CISPR has standardized this detector, and it's been
adopted for many legal compliance standards world-wide. Yet the USA &
British military insist on use of a Peak detector. Perhaps a dual level is
needed, with a QP value for comparison of harm to the older analog
modulation techniques, and a Peak value, for comparison of harm to digital
modulation techniques.

--
Ed
WB6WSN
El Cajon, CA USA

Ed Price wrote:
"Owen" wrote in message
...

Ed Price wrote:


So practically, since the average ham has a receiver with a sensitivity
in the order of a microvolt, then your antenna limits your minimum
discernable signal level to around 65 uV/m. Maybe 100 uV/m to be on the
safe side.

In fact, the technique calls for measuring signals on the rx from the
noise floor to about 20dB above it. The noise floor for receivers today is
typically -135dBm.


No, the technique does not use an S-meter. In a nutshell, it uses Ed
Hare's (W1RFI) technique for calibrating the noise floor of the receiver,
using an external attenuator to keep the rx input below the AGC threshold,
and measuring the audio output with signal and the audio output from rx
internal noise as inputs to a calculation of the input signal power.
Applying external attenuator losses, feedline losses and antenna factor
allows calculation of field strength.


Owen


IS Hare's technique published somewhere on the web?


Yes it is, see http://www.arrl.org/~ehare/aria/ARIA_MANUAL_TESTING.pdf .

That paper outlines the principle of using the known rx noise floor as a
baseline for measurements. I have developed a piece of software for
making the associated audio power measurements and automating the
calculation / documentation process.

Thanks for taking the time to review the loop model, it is appreciated.

Owen

PS: I saw your other response and Richard's suggestion that I use the
units capability of Mathcad. Sometimes the unit capability gets in the
way of readability, for instance I think you could not take the log of E
in volts / meter divided by V in volts and get dBV/m, I think you would
need to split E into two variables (say E' and l) and say
AF=20*log(E'/V)/l. Additionally, you can spend more time trying to get
the units to work, so that they don't collapse to fundamental units of
MLT etc, than solving the numerical side of the problem.

"Owen" wrote in message
...
Ed Price wrote:
"Owen" wrote in message
...

Ed Price wrote:


So practically, since the average ham has a receiver with a sensitivity
in the order of a microvolt, then your antenna limits your minimum
discernable signal level to around 65 uV/m. Maybe 100 uV/m to be on the
safe side.

In fact, the technique calls for measuring signals on the rx from the
noise floor to about 20dB above it. The noise floor for receivers today
is typically -135dBm.


No, the technique does not use an S-meter. In a nutshell, it uses Ed
Hare's (W1RFI) technique for calibrating the noise floor of the receiver,
using an external attenuator to keep the rx input below the AGC
threshold, and measuring the audio output with signal and the audio
output from rx internal noise as inputs to a calculation of the input
signal power. Applying external attenuator losses, feedline losses and
antenna factor allows calculation of field strength.


Owen


IS Hare's technique published somewhere on the web?


Yes it is, see http://www.arrl.org/~ehare/aria/ARIA_MANUAL_TESTING.pdf .

That paper outlines the principle of using the known rx noise floor as a
baseline for measurements. I have developed a piece of software for making
the associated audio power measurements and automating the calculation /
documentation process.

Thanks for taking the time to review the loop model, it is appreciated.

Owen


Thanks; looks like I have a lot of reading to do!

--
Ed
WB6WSN
El Cajon, CA USA

Wouldn't it be better to use a pre-amp at the loop feed? The gain of
the
pre-amp could make line pickup a negligible effect, and the pre-amp
would
match the coax very well.

--
Ed
WB6WSN
El Cajon, CA USA

It might be. But the extra complication of powering an amplifier would
bring another load of things to worry about. Simplicity is a
wonderful thing.

By far the best way of improving performanc and reducing possible
measuring errors, is to increase size of loop relative to length of
feedline. Doubling dimensions would make a world of difference.

I suppose he had a good reason for choosing a 1/2-metre square loop.
----
Reg, G4FGQ

Reg Edwards wrote:

I suppose he had a good reason for choosing a 1/2-metre square loop.

Sources suggested variously that the model's assumption of uniform
current distribution was reasonable if the side was from 0.1 to 0.03
wavelengths. I chose the go with the more conservative value at this time.

Owen

"Reg Edwards" wrote in message
...

Wouldn't it be better to use a pre-amp at the loop feed? The gain of
the
pre-amp could make line pickup a negligible effect, and the pre-amp
would
match the coax very well.

--
Ed
WB6WSN
El Cajon, CA USA

It might be. But the extra complication of powering an amplifier would
bring another load of things to worry about. Simplicity is a
wonderful thing.

By far the best way of improving performanc and reducing possible
measuring errors, is to increase size of loop relative to length of
feedline. Doubling dimensions would make a world of difference.

I suppose he had a good reason for choosing a 1/2-metre square loop.
----
Reg, G4FGQ

I have done some more calculations on a square loop fed at the corner. I
agree with your results of input impedance. NEC 2 shows Zin at 0.388 +
j109. The radiation efficiency of such a loop is 2.88%. I have made a very
careful analysis of the currents in the loop when in the presence of a know
E field. The current appears to vary in a sinusoidal manner around the
loop, with very slight discontinuities at the corners. I assume the
variation in current is due to the fact that the induced current is
different on those conductors normal to the dipole axis.

Using the RMS current through the 50 ohm load resistor at the corner, and
more careful calculations, I obtain an antenna factor of 60 dB, or 24 dB
more than your findings. It will be interesting to find why we have such a
large difference.

Regards,

Frank

Ed Price wrote:
....
What detector do you think should be used to evaluate the interference
potential of BPL?

I had thought that the QP detector was designed to the "annoyance" effect to
AM or SSB modulation. CISPR has standardized this detector, and it's been
adopted for many legal compliance standards world-wide. Yet the USA &
British military insist on use of a Peak detector. Perhaps a dual level is
needed, with a QP value for comparison of harm to the older analog
modulation techniques, and a Peak value, for comparison of harm to digital
modulation techniques.


I understand that the CISPR 16-1 QP detector and 9KHz bandwidth are
rooted in a series of subjective listening tests done somewhere around
the 1930s.

(cue storyteller here...)

Firstly, the bandwidth survives, and you are right that it is embedded
in all sorts of EM standards.

I understand that part of the tests I referred to was to discover a
instrument response that fitted well with subjective assessment of the
impact of interference (I presume on an AM broadcast transmission).

I think EMC measurement equipment often contains some of Average, RMS,
QP and Peak detectors.

I have seen several recent reports on BPL radiation that have not used
the QP detector and the reasons have IIRC been that on a scan in xyz
planes over a wide range of frequencies, the EMC receiver is too slow
using the QP detectors.

Ed Hare suggests that the AGC on a receiver acts similarly to the QP
detector, and he is probably right. So the effect being that in an
impulse noise scenario, your receiver will reduce gain roughly in line
with the QP value (rather than say the RMS or the Peak), so it may be a
good measure of gain reduction due to interference. As to interference
with the detection process, the subjective tests to arrive at the QP
detector did not assess impact on digital modulation. It seems to me
that the impact on digital modulation / encoding systems would depend on
the peak value / repetition scenario in concert with the encoding
system's capacity for error detection / correction, but that is just me
thinking aloud.

Measurement bandwidth and extrapolation / interpolation is an issue, and
possibly a bigger one than the detector response. Again there is a
disconnect between 9KHz MBW (below 30MHz) for the standards and the 2KHz
wide receivers in use for SSB. In my opinion, there is no better way to
demonstrate the impact of interference in a 2KHz wide receiver than to
measure it on a 2KHz wide receiver, so I suggest that (for us amateurs)
there may be value in measuring both where possible.

Owen

I have done some more calculations on a square loop fed at the corner. I
agree with your results of input impedance. NEC 2 shows Zin at 0.388 +
j109. The radiation efficiency of such a loop is 2.88%. I have made a
very careful analysis of the currents in the loop when in the presence of
a know E field. The current appears to vary in a sinusoidal manner around
the loop, with very slight discontinuities at the corners. I assume the
variation in current is due to the fact that the induced current is
different on those conductors normal to the dipole axis.

Using the RMS current through the 50 ohm load resistor at the corner, and
more careful calculations, I obtain an antenna factor of 60 dB, or 24 dB
more than your findings. It will be interesting to find why we have such
a large difference.

I see nobody picked up my deliberate mistake! Using Terman's simplified
formulas on pp 813, and 814, I get identical parameters of induced voltage,
and antenna factor. I completely agree with the results of the Mathcad
file, although have not gone through the more intricate equation development
yet.

Using NEC 2 I found out, the hard way, that maximum pick-up takes place off
the ends of the loop. Given these constraints: With a field strength of
5.5 V/m, the voltage across the 50 Ohm resistor is 0.091 V RMS, or an
antenna factor of 35.1 dB. Within 0.4 dB of Owen's results.

Interesting exercise, and I actually learned something.

73,

Frank

Frank wrote:

Using NEC 2 I found out, the hard way, that maximum pick-up takes place off
the ends of the loop. Given these constraints: With a field strength of
5.5 V/m, the voltage across the 50 Ohm resistor is 0.091 V RMS, or an
antenna factor of 35.1 dB. Within 0.4 dB of Owen's results.


That is great Frank, I like it because it agrees of course, but if its a
valid model, that is even better.

My model was much simpler in just trying to establish the Z of the loop,
I didn't do what you have done and irradiated the loop, well done. My
only residual concern is whether your receive loop was far enough from
the exciter to be truly operating under free space conditions. I suppose
if you double the distance and get nearly identical results, that would
be sufficient confirmation.

Two questions:
1. can I have a look at your model;
2. would you permit me to publish it (with attribution) on a web page
that I am drafting on the antenna design for BPL interference and other
interference measurement purposes.

BTW, although the gain is really low, I have also used it effectively
for getting bearings on interference from afar (though, DFing HF signals
is problematic, especially at high path angles).

Interesting exercise, and I actually learned something.


Life is fun, isn't it.

There is more to ham radio that "logging-in" to content-free nets,
talking about what is on the menu for dinner. Indeed, I suspect that
more "real" ham radio takes place off-air than on-air.

Owen