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

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

"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

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