Field Strength Meter - Revisited

[John Smith]

I built a field strength meter using Analog's design which was based on
Paul's requirements. Mine was built for about 73 MHz (the US model aircraft
frequency). I did not have a uA meter, so I just left it out and attached a
digital VOM at those points. I used only one grounded-base RF amplifier
before the meter detector. My range was about 40 feet, but I had not
adjusted the RF amp's coil for maximum indication. I can't find any
reference to the power output specification for my Futaba 8F/U R/C
transmitter, but I think it may be 2 milliwatts.

- Don't count on a known, fixed antenna impedance. It varies widely as the
meter is moved from place to place, according to my MFJ Antenna Analyzer.
Probably best to sit the FSM down somewhere and walk the transmitter around.
In any case, I don't think the FSM should be designed so that it is
dependent on some particular value of antenna terminal impedance. Perhaps a
resistance range from 30 to 100 ohms and perhaps the same for reactance.

- Paul, you are probably underestimating the signal strength your FSM will
be getting. I estimate you will have .01 volts at your meter 100 meters
distant if your transmitter produces 1/2 Watt. Yes, that is with lossless
isotropic radiators in space, no path loss, etc. However, there is some
compensating radiation directivity (which an isotropic radiator would not
have) if you are using a 1/2 wave vertical radiator on earth. So I call it a
wash until more measurements are made.

I had a lot of fun doing this and I think I learned a lot. I thank Paul for
getting this thread started and I thank Analog, Phantom, and all the other
contributors to the threads involving this project.

John

 

[Paul Burridge]

I built a field strength meter using Analog's design which was based on
Paul's requirements. Mine was built for about 73 MHz (the US model aircraft
frequency). I did not have a uA meter, so I just left it out and attached a
digital VOM at those points. I used only one grounded-base RF amplifier
before the meter detector. My range was about 40 feet, but I had not
adjusted the RF amp's coil for maximum indication. I can't find any
reference to the power output specification for my Futaba 8F/U R/C
transmitter, but I think it may be 2 milliwatts.

I'm going to start building it today. Owing to the split supply
analogue's used I'm going to have to obatin a -5V reg. first but have
to go by Maplin later today anway. I think I've got everything else
'in stock' as it were.

Analogue's design as it stands has no selectivity, so what are you
doing about the front-end where he's currently showing a signal source
on the schematic? Are you using some sort of tunable variation or just
banking on the signal you're trying to measure overriding everything
else?

[...]

- Paul, you are probably underestimating the signal strength your FSM will
be getting. I estimate you will have .01 volts at your meter 100 meters
distant if your transmitter produces 1/2 Watt. Yes, that is with lossless
isotropic radiators in space, no path loss, etc. However, there is some
compensating radiation directivity (which an isotropic radiator would not
have) if you are using a 1/2 wave vertical radiator on earth. So I call it a
wash until more measurements are made.

You may well be right. My initial stab in the dark figure was supposed
to be 'worst case' anyway; to be on the safe side.

 

[James Meyer]

Analogue's design as it stands has no selectivity,

"No selectivity"? Have you simulated the AC response of his circuit?
It's tuned twice for a very reasonable amount of selectivity. In fact, if you
don't make the two inductors adjustable and tune them for maximum response, you
might think it isn't working.

Jim

 

[John Smith]

I'm going to start building it today. Owing to the split supply
analogue's used I'm going to have to obatin a -5V reg. first but have
to go by Maplin later today anway. I think I've got everything else
'in stock' as it were.

I used two battery holders obtained from Radio Shack which would hold two AA
size batteries each.

Analogue's design as it stands has no selectivity, so what are you
doing about the front-end where he's currently showing a signal source
on the schematic? Are you using some sort of tunable variation or just
banking on the signal you're trying to measure overriding everything
else?

See Jim's response. In my case, the coil of the first stage is part of a
resonant circuit just as are the coils and capacitors associated with the
first two stages of Analog's original design. I am mostly counting on the
proximity of the nearby signal, but I will eventually adjust the coil for
maximum sensitivity which will give some selectivity. As long as you adjust
the tuning for sensitivity, I don't think you need to worry about
selectivity. There probably won't be many strong emitters nearby. If there
are, it's a whole new ballgame anyway.

[...]

- Paul, you are probably underestimating the signal strength your FSM will
be getting. I estimate you will have .01 volts at your meter 100 meters
distant if your transmitter produces 1/2 Watt. Yes, that is with lossless
isotropic radiators in space, no path loss, etc. However, there is some
compensating radiation directivity (which an isotropic radiator would not
have) if you are using a 1/2 wave vertical radiator on earth. So I call it a
wash until more measurements are made.

You may well be right. My initial stab in the dark figure was supposed
to be 'worst case' anyway; to be on the safe side.

I couldn't argue the point earlier. But, since then I have studied the PDF
referenced in one of your threads on loop antennas and studied Antennas by
Kraus and then made some estimates. If you intend to have the meter close
enough to see the meter's pointer from the transmitter, I can guarantee your
circuit has far more gain than you need. In any case, it is definately a
design for worst case.

I decided to use a telescoping antenna so that I could reduce sensitivity by
simply collapsing the antenna. Also, my coil is made from about 8 turns of
#22 AWG wire wound on a pencil. I can adjust the tuning by simply spreading
the turns. The whole circuit is soldered to a piece of Cu/FR4 about 2.5 sq
mm. I'll post a picture to abse upon request (but not before Thursday
night).

Anyway, good luck. I'm anxious to learn how well yours works.

John

 

[Paul Burridge]

"No selectivity"? Have you simulated the AC response of his circuit?

D'oh! I completely forgot to run that check. Yeah, it looks like L1
alone would be sufficiently tunable for my purpose at least. Sheesh.
I'm afraid my lack of familiarity with this type of configuration has
caused me some confusion. :-(

 

[Paul Burridge]

I used two battery holders obtained from Radio Shack which would hold two AA
size batteries each.

What the hell. I'll do likewise. It's the kind of thing you don't
really want 200 yards of supply cable trailling from anyway. Kind of
curbs the useability of it in 'far-field' measurement.

See Jim's response. In my case, the coil of the first stage is part of a
resonant circuit just as are the coils and capacitors associated with the
first two stages of Analog's original design. I am mostly counting on the
proximity of the nearby signal, but I will eventually adjust the coil for
maximum sensitivity which will give some selectivity. As long as you adjust
the tuning for sensitivity, I don't think you need to worry about
selectivity. There probably won't be many strong emitters nearby. If there
are, it's a whole new ballgame anyway.

I couldn't argue the point earlier. But, since then I have studied the PDF
referenced in one of your threads on loop antennas and studied Antennas by
Kraus and then made some estimates. If you intend to have the meter close
enough to see the meter's pointer from the transmitter, I can guarantee your
circuit has far more gain than you need. In any case, it is definately a
design for worst case.

I decided to use a telescoping antenna so that I could reduce sensitivity by
simply collapsing the antenna. Also, my coil is made from about 8 turns of
#22 AWG wire wound on a pencil. I can adjust the tuning by simply spreading
the turns. The whole circuit is soldered to a piece of Cu/FR4 about 2.5 sq
mm. I'll post a picture to abse upon request (but not before Thursday
night).

Anyway, good luck. I'm anxious to learn how well yours works.

It'll be interesting to compare. Did you say you were only going to
use one c/b stage instead of the suggested 2 in analogue's creation?

 

[John Smith]

It'll be interesting to compare. Did you say you were only going to
use one c/b stage instead of the suggested 2 in analogue's creation?
--

Yes. It currently has only one stage and it seems to have plenty of
sensitivity for my purposes. I only want about 10 meters or a bit more.

 

[Paul Burridge]

Yes. It currently has only one stage and it seems to have plenty of
sensitivity for my purposes. I only want about 10 meters or a bit more.

Okay... Well I'm looking forward to seeing the picture(s) when you've
finished. I'm not as quick as most folk when it comes to assembling
these things so it'll take me prolly until the middle of next week!