Reg Edwards wrote:
If you find yourself short of sensitivity, try a tuned loop in the
style of a magloop and match the antenna to the receiver.


In concert with an SSB rx of noise floor -135dBm in 2KHz Effective noise
bandwidth (they are realistic figures for an IC706IIG for example), the
rx noise in 2KHz BW is 0.04uV. With an antenna with AF=36dB, the rx
noise floor translates to field strength of 2.5uV/m or 8dBuV/m.

Clearly, the "instrument" is not going to be suitable for measurements
below 11dBuV/m. However, measurements of the BPL systems on "trial" here
(Mitusbishi based on DS/2 45Mbps chipsets) showed field strengths of 45
to 65dBuV/m in 2Hz and a rx with this loop needs 20+dB of RF attenuation
to keep the interference below the AGC threshold (with the benefit of
stabilising the rx input Z somewhat).

But whatever you adopt, accuracy will be limited by the uncertainty in
the amateur's receiver input impedance. This will change from band to
band and its actual value will be a matter of guesswork.

Yes, I have been measuring the rx noise floor with 20dB of attenuation
to simulate the common measurement configuration.


A receiver's input impedance can be masked with an attenuator. But
this further reduces sensitivity.


As discussed.

With amateur grade equipment, facilities and environment, expect a
measuring uncertainty in the region of 4 to 7 dB at 7 MHz. Which is
good enough for most amateur purposes and makes your precision
calculations, including conductor diameter and conductivity, not worth
the trouble.

Given that the interference is 70dB above the ambient noise floor, we
don't need 1/10dB accuracy to demonstrate to regulators that there is a
problem


All you need for calculation is enclosed loop area, loop inductance,
receiver impedance and a pocket calculator.

The uncertainty of a measurement is just as important as the value
itself. The only way to assess uncertainty is to compare with
professional-grade equipment. In which case, if professional grade is
obtainable, you can dump the amateur stuff.

Understood. My view is that if professional grade EMC measurement kit is
available, we can use it to do a lower grade calibration of the amateur kit.


I do like the way your calculations appeared on my screen with one
mouse click.
How do you do it?

The model is in Mathcad, I copied it to the clipboard and pasted it into
Frontpage (my web editor) which finds only a useful format in the
clipboard and saves it as a GIF file (ie a graphics image).

Thanks for checking the model Reg, I think you are telling me it is more
precise than needs to be, but you haven't faulted it for accuracy.

Owen

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.

Incidentally, when I attempted to save your web page of math, it was saved
as an ".mcd" document. Obviously I was not able to open it with Mathcad,
but will have to type it in by hand.

No you won't, I have posted a later version of the mathcad file to
http://www.vk1od.net/bpl/loop02.mcd .

The file you downloaded is an image (.gif) called loop.mcd.gif, and it
looks like your download process dropped the extension, or you hide the
extension on your machine. (Some software thinks that the first dot
begins the file extension, whereas it is the last dot that does so.)


Might be interesting to replicate your results with NEC2.

I have modeled the loop in EZNEC and get very similar inductance and
resistances.

Thanks Fred, appreciate the review.

Owen

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.

Fortunately, field strength measurements are seldom needed to great
accuracy. Strength is usually required only to be less than or greater
than some specified value and there is an ample margin for error.

Personally, I think a tuned loop, in the fashion of a magloop, is a
better bet. With its small coupling loop the main loop can be
completely isolated from the line and the line can be ordinary coax
which matches a 50-ohm receiver.

A tuned loop is far more sensitive than the untuned variety. But its
operating frequency range is somwhat restricted.

Field strength measurements are essentially power level measurements
and, ideally, the pick-up loop should be impedance matched to the
receiver. Result : no reflections.
----
Reg.

Reg Edwards wrote:
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.


Agreed. Because of the inherent balance of the whole loop I have use a
"Voltage Balun", see http://www.vk1od.net/bpl/loop.jpg . I have made
observations of the received signal level when close to aerial telephone
lines carrying ADSL and the pickup level seems the same no matter which
side of the loop is nearest the aerial line.

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

I have made the assumption that the line is adequately terminated in 50
ohms (the attenuator, and there should be now standing waves. Doesn't
that seem reasonable?


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

Fortunately, field strength measurements are seldom needed to great
accuracy. Strength is usually required only to be less than or greater
than some specified value and there is an ample margin for error.

Personally, I think a tuned loop, in the fashion of a magloop, is a
better bet. With its small coupling loop the main loop can be
completely isolated from the line and the line can be ordinary coax
which matches a 50-ohm receiver.

A tuned loop is far more sensitive than the untuned variety. But its
operating frequency range is somwhat restricted.


Noted.

I have encouraged another ham friend to design an active loop with an AF
good enough to get the system noise floor below -10dBuV at 7MHz. That
is another alternative, and it has issues I know.

I am also considering trying to measure the performance of a portable
short dipole such as a buddipole ( http://www.buddipole.com/ ) for the
purposes of measurement down to ambient noise and a little lower.

Field strength measurements are essentially power level measurements
and, ideally, the pick-up loop should be impedance matched to the
receiver. Result : no reflections.

But if the rx terminates the line, does it matter whether the
"generator" impedance is matched? (I am not trying to bait anyone here,
but Reg, I think I understand the standing wave issue you are raising,
but my reasoning is that if the rx terminates the line sufficiently
well, then standing wave ratio will be small and the error contribution
negligible.)

(I think the lights have gone out on the other side of the big pond.)

Owen

Owen,
I gather you are interested in measurents only in the 40m band which
makes life easier. The photograph of the loop and line makes it more
clear what you are up to.

Yes, there will be no standing waves on the line if the line Zo is
equal to receiver input impedance. (I didn't make myself clear). If a
balun is used it doesn't matter much what Zo is, provided the balun
has the correct ratio. So it is necessary to know what Zo actually is
just as accurately as the input impedance is known.

With your setup it is impossible to match the loop to the line. But if
it WAS possible (eg., as with a magloop) it would NOT be to prevent
standing waves.

Your calculations take the loop/line mismatch loss into account. Its
only a few dB.

Incidentally, have you considered what effects increasing the number
of turns to 2 or 3 would have? They MIGHT possibly be beneficial. It
needs more calculations. As with just increasing the size of the loop
which almost certainly would be beneficial.

You have set yourself a most interesting and useful task. I wish you
well with it.
----
Reg, G4FGQ

On Sun, 19 Jun 2005 01:59:51 GMT, Owen wrote:

Hi Owen,

Between:

I have looked at it and I can't see that I said "power" in relation to
Antenna Factor. Perhaps I am blind. (You didn't confuse the units dB/m
(dB per meter) with dBm (db wrt 1mW) did you?)

and:

I have gotten sidetracked here, my real interest is the completeness /
accuracy of the loop model.

I have observed that your Mathcad design is short of providing
something of a self-check feature. No where in any of your formulas
do you use Units.

Yes, you label them as notes, but this is risky and has been revealed
in your first comments in response to Ed's comment:
Looks pretty decent, until the very end. Antenna Factor (AF) is the ratio of
the field strength voltage to the output VOLTAGE, not power, although you
did get the numbers right.

Folks who follow your math work, will be skimming it, and perhaps a
few will be transcribing it while others will have picked up your MCD
file. This is to say, very few will actually go the whole distance
for a sanity check.

That sanity check is to include the Units within the formulaic Mathcad
expression; that is, after all, one of the boons of using this
package, otherwise any spread sheet would do as well. This inclusion
would enforce a strict compliance with keeping every transformation
accurate, and you would not end up mixing terms which is very simple
to do - and later suffer from. Cecil's work with Photonics suffers
from this problem horribly such that expressions of power end up in
terms so bizarre and thoroughly out of whack that anything could be
proven, except the proof.

73's
Richard Clark, KB7QHC

No you won't, I have posted a later version of the mathcad file to
http://www.vk1od.net/bpl/loop02.mcd .

Yes, got it ok, thanks.

The file you downloaded is an image (.gif) called loop.mcd.gif, and it
looks like your download process dropped the extension, or you hide the
extension on your machine. (Some software thinks that the first dot begins
the file extension, whereas it is the last dot that does so.)

Guessed it was something like that.

Might be interesting to replicate your results with NEC2.

I have modeled the loop in EZNEC and get very similar inductance and
resistances.

Now this is where it gets interesting, and hope I can learn something from
it, as I am sure I have made a mistake someplace. I have not directly
attempted to verify your math, so don't know if you developed it from first
principals or got it from a book. I have a number of references including
Kraus' "Antennas", and also a text by Stutzman and Thiel, etc., so may try
to replicate your methods later.

Using NEC2 I set up a 40 m dipole in free space, and fed it with 1 kW (for
nice large current values in the loop). I placed a square loop, 0.5 m per
side and 40 m distance. with the plane of the loop parallel to the axis of
the dipole, also two sides parallel to the dipole. The dipole uses perfect
conductors, and copper for the loop, with 0.7 mm radius conductors. The
segmentation of the loop is significantly different than the dipole, but
thought it not important because of the large separation of the two
antennas. In the loop I am very close to the minimum segmentation allowed
in NEC at 0.001 wavelengths per segment -- i.e. 11 segments per side. One
segment, near a corner, has a 50 ohm load.

As is easily verified, the field strength from the dipole, at 40 m, is
5.5V/m (RMS). According to NEC the current in the loop varies from segment
to segment, ranging from 0.1 mA (peak), to 0.3 mA (peak). I took the
average (0.191 mA peak), and computed the RMS average current in the loop.
Multiplying by 50 ohms, gives me an output voltage 6.76 mV RMS.

The antenna factor is therefore 58 dB. Wonder if anybody has any idea where
the error lies. I have copied the code below.

Regards,

Frank

NEC Code:

CM Dipole antenna
CE
GW 1 41 0 0 0 20.25 0 0 0.0026706
GW 2 11 10 40 0.25 10.5 40 0.25 0.0007
GW 3 11 10.5 40 0.25 10.5 40 -0.25 0.0007
GW 4 11 10.5 40 -0.25 10 40 -0.25 0.0007
GW 5 11 10 40 -0.25 10 40 0.25 0.0007
GS 0 0 1
GE 0
EX 0 1 21 0 379.63 0.00000
LD 4 5 1 1 50 0
LD 5 2 1 11 5.8001E7
LD 5 3 1 11 5.8001E7
LD 5 4 1 11 5.8001E7
LD 5 5 1 11 5.8001E7
FR 0 12 0 0 7.15 0.0025
RP 0 181 1 1000 -90 90 1 1
EN

Reg Edwards wrote:
Owen,
I gather you are interested in measurents only in the 40m band which
makes life easier. The photograph of the loop and line makes it more
clear what you are up to.

Not necessarily, but the exploration of the loop has been done on lower
HF, and it happens that the BPL system that I have available for
measurement radiates on 7 and 10MHz in low HF.

7MHz is not the only band affected, these guys will use every scratch of
spectrum and power to maximise the speed / reach profile of their service.

....

Your calculations take the loop/line mismatch loss into account. Its
only a few dB.

Incidentally, have you considered what effects increasing the number
of turns to 2 or 3 would have? They MIGHT possibly be beneficial. It
needs more calculations. As with just increasing the size of the loop
which almost certainly would be beneficial.


I did.

It obviously increases the open circuit voltage. It also increases the
loop inductance, and this almost completely offsets the increased open
circuit voltage in terms of power delivered to the receiver input
depending on frequency). Calculation of the wire loss resistance becomes
more complex due to proximity effect, but that doesn't matter too much
because the dominant factor in determining the source Z is the
inductance of the loop, and even if tuned, the resistance is small wrt
the load.

Ofcom had the answer to measurements down to ambient noise level, the
antenna is shown in their recent reports on BPL radiation measurements.
However, it isn't a very portable answer.

As I said in an earlier post, an active loop and a portable short dipole
(such as the Buddipole) are avenues for investigation.

(A tuned loop obviously helps, but with the single frequency /
calibration issues.)

You have set yourself a most interesting and useful task. I wish you
well with it.

Thanks Reg, and I appreciate your help with the task in this discussion
/ review. Wish Amateur Radio well with it, because BPL is the greatest
risk to HF Amateur Radio that we have known. I don't say that from
having read or heard somone else's reports, I have stood on the streets
where BPL is deployed and measured it.

Though my measurement methodology has progressed from "calibrated
S-meter" readings, the calibrated S-meter is a reality check, and when I
last visited the trial site, set the receiver up and waved the 0.5m sq
loop (~-50dBi) about to see if they were still "on air", S-meter
readings of 5uV says they are, and it is seriously high in level.

Ofcom's recent reports are a great read, and it looks like they are
taking a sane approach at this point, differently to the fervour for BPL
expressed by Powell when at the FCC.

I better stop at that, I am getting OT!

Owen

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

Looks pretty decent, until the very end. Antenna Factor (AF) is the ratio
of the field strength voltage to the output VOLTAGE, not power, although
you did get the numbers right.

I have looked at it and I can't see that I said "power" in relation to
Antenna Factor. Perhaps I am blind. (You didn't confuse the units dB/m (dB
per meter) with dBm (db wrt 1mW) did you?)

Owen


Yep, that's exactly what I did. Maybe I was looking at your units too fast
and didn't see that "/" in there.

--
Ed
WB6WSN
El Cajon, CA USA

"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?

--
Ed
WB6WSN
El Cajon, CA USA