Sorry for the slow response.

The voltage at the terminals of a short, lossless dipole of length l
which is perfectly terminated with the complex conjugate of its
feedpoint impedance is E * l / 2 where E is the impinging field in volts
per meter. Any mismatch lowers this value -- it's one of those valid and
useful applications of the concept of "mismatch loss", which can be used
to quantify the reduction when mismatched. It's one of the contributions
to the extensive variation with frequency of the "antenna factor"(*) of
test dipoles used in EMI work. (And careful measurement of this factor
is a large fraction of what you're paying for when you purchase one.)
Doesn't matter how wonderful your field strength meter is unless you
have a very good handle on the "antenna factor" of the test antenna.

In simple terms, the factor of 1/2 is the ratio of the average current
to the feedpoint current. The average current along a short dipole is
about 1/2 the current at the feedpoint. You could get a factor of 1 if
you used a short dipole with large end hats to make the current
distribution uniform. Kraus addresses this case in detail in _Antennas_.

And no, I don't mean "antenna to free space" match. I mean the match
between the antenna feedpoint as a source and the detector as a load.

(*) Ratio of output voltage when terminated in 50 ohms to the impinging
field in volts/meter.

Roy Lewallen, W7EL

Active8 wrote:
On Sat, 13 Dec 2003 17:39:27 -0800, said...

It's too bad it isn't that simple.

A 1V/m field doesn't result in one volt at the feedpoint of a perfectly
matched one meter dipole or monopole, and the value it does induce
depends on the quality of the impedance match


you mean antenna to free space, right?


as well as the fraction of
a wavelength the one meter antenna length represents.


amplify, very please. por favor. Refresh my ram.


| E(uV/m) |
V (dBmV) = 20log | --------- / 1000 |
| 0.021f(MHz) |

plus correction for distance (regulations for limits are for
specific measuring distances), etc. I'll mull the above eq over.
Gotta figure out where the .021 came from, but not now. My eyes are
getting fatigued from this 'puter.


And, if one volt
does appear at the feedpoint, it's very unlikely that a simple circuit
will measure it as one volt.


It would have to be calibrated to compensate for the circuit. Maybe
that's why it's called a "relative" field strength meter. Relative
to another signal or no signal


Probably best to stick with your $1.5 kilobuck meter if you really want
to measure field strength.


I don't. He does At least not tonight. But my SLM *will* measure
field strenth using a cheap ass dipole cut to the frequency of
interest with or without an external preamp and do it to the
satisfaction of the FCC, assuming it's calibrated. I even have a
near-field probe, not so cheap. It beats guess work.

I wouldn't expect his sniffer to be real accurate but he did ask
for guesstimates. Started off as "around 4 feet" for a half watter
now we're at 100mW - prob his reference Tx.

BRs,
Mike



Roy Lewallen, W7EL

Active8 wrote:

On 13 Dec 2003 03:50:41 -0800, said...


What you are descrbing is a "signal sniffer", not a signal strength meter.


Who are you replying to? Paul did not say Signal Stength Meter, but
the guy you replied to kinda hints at it when he mentions power
level.

I would have to say that my 1GHz Signal Level Meter, which cost
$1500 would be the better than a relative field strength meter, but
if he measures say, 1V with his Tx off and 2V with it on, then
that's 1V and if his Rx antenna is a 1m dipole, that's 1V/m.

Mike