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Home Brew Field Strength Meter

wirejock

Well Known Member
I love home made tools. My Mentor, Dave recently ignited my passion for electronics. I have been an amateur radio operator for a long time. KI0TK. I also was a radio technician quite a few years. I fixed my 2m/70cm handie talkie and Soortys gave me $25 off a new SP400. My old SP200 was T.U.
Anyway.
This is a pretty handy little box. Field Strength Meter
The meter is used to scan for RF fields. You can scan your radio/antenna system to see the radiation pattern, check for EMI leaks in the airplane or house. Lots of uses. Looking for RF noise is primary purpose. Noise like LED switching or coax leaks can be found pretty quickly.
I had one long ago and lost it. Super useful and very easy to assemble with pieces of wire. I used pieces of Cat 5 data wire I had laying around. It solders easy.

I offered to our Colorado builders but no one replied.
I have enough of the diodes, capacitors and potentiometers for four more units plus enough diodes and capacitors for quite a few meters.
All you need is a 50uA (micro amp) meter movement and a project box. Maybe $15 for both. The other parts (2-diodes, 1-capacitor, 1-potentiometer) free to anyone interested. I'll send by snail mail. You buy the meter and box. If additional builders want the diodes and caps, you can buy the 100K pot pretty cheap. I may throw in a BNC jack if there are any in my hobby box.

This company has some good deals. I got the meter from them. They also have project boxes and pots. Make sure the box is deep enough.
www.jpmsupply.com/

You will need some sort of antenna jack. I had a BNC but almost any jack will work. PL-259 female & male are handy because a piece of #12 Romex fits right in the pin hole. Solder it and fill the cavity with epoxy to keep it insulated. You can look up antenna calculators on the internet and cut the Romex to whatever length needed. You might get really cool and use a telescoping antenna. Just extend it to the resonant length. I found these pretty cheap.
https://www.amazon.com/dp/B07H9PXZX...abc_CPCW6KAJN11FQXQGT2QK?_encoding=UTF8&psc=1

I added a piece of flattened copper pipe bolted to the bottom of the box as a ground because it was easy to solder a wire to it.

The simple schematic and photo are below.
If anyone wants to build one I'll send out the leftover diode, cap and pot.
Field Strength Meter Schematic.jpg
20211105_163734.jpg
 
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Hi Larry,

Any way you can explain the circuit?

I don't understand the 470pF capacitor. It'll attenuate or ground any frequency higher than ?

First diode clamps negative part of waveform to - 0.3V
Second diode rectifies the positive part of the waveform and I guess the meter averages it (attenuated by the pot), where one would typically have a cap to average it to a DC signal.
Any signal with peak voltage less than 0.3 V won't register on the meter?

Finn
 
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Meter

Hi Larry,

Any way you can explain the circuit?

I don't understand the 470pF capacitor. It'll attenuate or ground any frequency higher than ?

First diode clamps negative part of waveform to - 0.3V
Second diode rectifies the positive part of the waveform and I guess the meter averages it (attenuated by the pot), where one would typically have a cap to average it to a DC signal.
Any signal with peak voltage less than 0.3 V won't register on the meter?

Finn

You're way past me. My understanding is the cap smooths out the signal, but if it attenuates above a certain freq, that will need to be changed. I think it's in the correct place.
Agree on the first diode but you got me on the voltage.
Second diode rectifies.
I don't know about the voltage. I should give credit to the author but it's been too long. I just transposed an old pencil schematic.
I just wanted another field strength meter and this one was easy. When I'm working on a radio and antenna setup, first question, is anything transmitting? This meter is quicker than hooking up a watt meter and dummy load.
It works really well with my 5W handie talkie on 2M. Pot has to be turned up to attenuate or it pegs. I'll have to try 70cm and a tuned antenna to see what happens. Also worked well at aviation Vhf freq.
I have an attenuation box for high power transmitters. It has several stages with switching and BNC in and out. Another home brew I made way back when I hunted stuck transmitters. A 30+ watt signal is hard to localize in a parking lot full of radios without attenuation. When I got close, touching the heat sinks confirmed the target.
 
The diodes rectify the RF so positive and negative goes to the correct sides of the DC microammeter, but that signal is pulsed. The 470pF capacitor is to "smoothen" out the pulses so the ammeter "sees" it more as a "DC" current and not a pulsing AC. The rheostat is to adjust the sensitivity based on how close/far you are from the transmitter. Obviously this circuit only works for a certain range of RF frequencies since above a certain frequency the capacitor will present a "short" and the meter will not have any signal to work with. :cool:
 
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The diodes rectify the RF so positive and negative goes to the correct sides of the DC microammeter, but that signal is pulsed. The 470pF capacitor is to "smoothen" out the pulses so the ammeter "sees" it more as a "DC" current and not a pulsing AC. The rheostat is to adjust the sensitivity based on how close/far you are from the transmitter. Obviously this circuit only works for a certain range of RF frequencies since above a certain frequency the capacitor will present a "short" and the meter will not have any signal to work with. :cool:


"The 470pF capacitor is to "smoothen" out the pulses so the ammeter "sees" it more as a "DC" current and not a pulsing AC."

Then it's in the wrong position and should be after the second diode.

Finn
 
"The 470pF capacitor is to "smoothen" out the pulses so the ammeter "sees" it more as a "DC" current and not a pulsing AC."

Then it's in the wrong position and should be after the second diode.

Finn
I agree. It should be between the diodes and the rheostat. Didn't catch that. :cool:
 
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Show Off

You smart guys are showing off!! It’s like when I’m cruising in my 7A and I see jets in the cons. You are wayyyy over my head!

I love how smart guys are on this forum. If you just read through randomly, which I often do, you find conversations like this one are not unusual. There are a lot of wicked smart people on this forum and, even though I don’t understand much of it, I love reading it!

I spent a lot of years flying fairly high performance airplanes but when they broke I just tossed the crew chief the keys and said, “she’s broke”. Now I find I fly my girl with a lot more love because I have so little idea how the beeps and squeaks of the airplane really work. You geniuses (and trust me, that is a term of endearment) teach me something new every day. Thank you!
 
Wild theory

I agree. It should be between the diode and the rheostat. Didn't catch that. :cool:

Actually, I think the purpose of the capacitor is to work with the first diode to shift the DC level of the input up (by negative peak of the waveform less 0.3V). The second diode then adds the positive peak of the waveform (less 0.3V) to that.

Actual averaging (smoothing) is done by the inertia of the meter needle.

(1N34A is a germanium diode which typically has a 0.3 voltage drop over it when conducting.)

So meter should begin to register with less than the 0.3V of a peak waveform which you would expect from a single rectifier.

Of course the capacitor will attenuate the input some. Not sure how to calculate that. Something on the order of 30 to 75 ohms into 470pF at whatever frequency is being measured?

Anyway, that's my theory. Otherwise I don't see a reason for the first diode and capacitor.

Anyway, I'm most likely wrong.

Finn
 
Here is a similar circuit: http://www.circuitdiagram.org/images/field-strength-meter-circuit-schematic.gif

The second diode is there to protect the detector diode against large negative voltages in high field strengths.

V
That circuit is what you would have if you move the capacitor in the original drawing between the diodes and the rheostat, but with a little more protection and filtering.

RF Meter.jpg

Remember, you still have to provide the microammeter with the negative part of the rectified signal. Without both the positive and negative there is no current flow thru the microammeter and it won't indicate anything. ;)
 
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Schematic

Doh! My error. You guys are absolutely correct. I built it right but drew the schematic wrong.
Galin's drawing is correct.
 
EDITED FOR CLARITY. APOLOGIES TO @wirejock

I applaud the makers an tinkerers that build their own gear. I know it’s expected in the Ham community. And, Homebuilt electronics is fun.

If you’re like me though, building an airplane seems to take up all the creativity bandwidth. I was testing USB chargers after a recent thread about using non certified USB in the cockpit. In gathering up the parts to make my “field analyzer”, I discovered the TinySA spectrum analyzer that costs $85 and works pretty slick for several things. It’s a great addition if you’re looking for test equipment on the cheap, but don’t have time to build.

If you’re interested, I recommend you check them out and understand them a bit before plunking your money down and ending up with a TinySA imposter (like some of the ones for sale on eBay) that doesn’t work.

https://www.tinysa.org/wiki/
 
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Meter

I was testing USB chargers after a recent thread about using non certified USB in the cockpit. In gathering up the parts to make my “field analyzer”, I discovered the TinySA spectrum analyzer that costs $85 and works pretty slick for several things.

If you’re interested, I recommend you check them out and understand them a bit before plunking your money down and ending up with a knock-off version that doesn’t work.

https://www.tinysa.org/wiki/

Ouch. I'm not suggesting anyone throw money away. In fact, I'm offering the small parts for free. It's simply a Field Strength Meter project for fun.
 
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Ouch. I'm not suggesting anyone throw money away. In fact, I'm offering the small parts for free. It's simply a Field Strength Meter project for fun.

Sorry. This was not meant to slam your DIY field strength meter. I’ve gone back and edited my original post.

What I meant by “knock off version that doesn’t work” was the TinySA imposters for sale on eBay.

In fact, the only reason I posted at all is that I though some folks might want a gadget without building one.

I sincerely apologize if I offended you.
 
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No worries

Sorry. This was not meant to slam your DIY field strength meter.

What I meant by “knock off version that doesn’t work” was the TinySA imposters for sale on eBay.

In fact, the only reason I posted at all is that I though some folks might want a gadget without building one.

I sincerely apologize if I offended you.

No worries.
Interesting history. My Spectrum Analyser on my bench 30 years ago cost $15,000! :D
 
No worries.
Interesting history. My Spectrum Analyser on my bench 30 years ago cost $15,000! :D

The TinySA is pretty light weight. I don’t think it’s gonna challenge your $15k bench gear.

The gear I had in the Navy was pretty neat, if a bit antiquated. Way out of my price range though.
 
The TinySA is pretty light weight. I don’t think it’s gonna challenge your $15k bench gear.

The gear I had in the Navy was pretty neat, if a bit antiquated. Way out of my price range though.

Folks. Don't bother with this home brew project.
That little TinySA box is pretty amazing. I watched the videos and ordered one. Can't wait to play.
Consider my arrogant self humbled. My apologies Ken.
 
I love home made tools. My Mentor, Dave recently ignited my passion for electronics. I have been an amateur radio operator for a long time. KI0TK. I also was a radio technician quite a few years. I fixed my 2m/70cm handie talkie and Soortys gave me $25 off a new SP400. My old SP200 was T.U.
Anyway.
This is a pretty handy little box. Field Strength Meter
The meter is used to scan for RF fields. You can scan your radio/antenna system to see the radiation pattern, check for EMI leaks in the airplane or house. Lots of uses. Looking for RF noise is primary purpose. Noise like LED switching or coax leaks can be found pretty quickly.
I had one long ago and lost it. Super useful and very easy to assemble with pieces of wire. I used pieces of Cat 5 data wire I had laying around. It solders easy.

I offered to our Colorado builders but no one replied.
I have enough of the diodes, capacitors and potentiometers for four more units plus enough diodes and capacitors for quite a few meters.
All you need is a 50uA (micro amp) meter movement and a project box. Maybe $15 for both. The other parts (2-diodes, 1-capacitor, 1-potentiometer) free to anyone interested. I'll send by snail mail. You buy the meter and box. If additional builders want the diodes and caps, you can buy the 100K pot pretty cheap. I may throw in a BNC jack if there are any in my hobby box.

This company has some good deals. I got the meter from them. They also have project boxes and pots. Make sure the box is deep enough.
www.jpmsupply.com/

You will need some sort of antenna jack. I had a BNC but almost any jack will work. PL-259 female & male are handy because a piece of #12 Romex fits right in the pin hole. Solder it and fill the cavity with epoxy to keep it insulated. You can look up antenna calculators on the internet and cut the Romex to whatever length needed. You might get really cool and use a telescoping antenna. Just extend it to the resonant length. I found these pretty cheap.
https://www.amazon.com/dp/B07H9PXZX...abc_CPCW6KAJN11FQXQGT2QK?_encoding=UTF8&psc=1

I added a piece of flattened copper pipe bolted to the bottom of the box as a ground because it was easy to solder a wire to it.

The simple schematic and photo are below.
If anyone wants to build one I'll send out the leftover diode, cap and pot.
View attachment 17872
View attachment 17873

Can't you build and sell to us guys that know nothing about this stuff?
 
In my early teens (late 60’s), I built an RC radio and receiver from a kit. When it didn’t work as expected, I ended up building a signal strength meter from a schematic that was either the same or very similar to this one. It worked but the $15 for the uAmmeter was just about outside my affordability.
I seem to recall the cap value had to be selected based on the frequency you intended to use it for.

However, the cheap SA’s today are much,much more useful.
 
Folks. Don't bother with this home brew project.
That little TinySA box is pretty amazing. I watched the videos and ordered one. Can't wait to play.
Consider my arrogant self humbled. My apologies Ken.

If you get a chance, check out the NanoVNA by the same guys.

My MFJ antenna analyzer never even comes out of the box anymore.
 
I have a MFJ-801 RF Field Strength Meter. I think it was $40. I sure this DIY meter works as well or even better. Good work.

Some of the fancy RF Field Strength Meters are cool, but can't justify buying one... Their is a whole market of 3 Axes EMF meters. RF is all around us now, cell phones, wifi, blue tooth, lights, power supplies, utility power meter, commercial TV and Radio, WX Radar, microwave links, microwave ovens, power lines. I use a SDR (software defined Radio) with a spectrum analyzer and a laptop to hunt out RF noise. It is amazing (sad) how much the RF noise floor has gone up in the last decades. Looking at the spectrum some of it is blanked out with noise or transmitters.
 
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