Ham Radio - DIY test gear, build a simple passive Field Strength Meter.

[@corywelch2383]

Hey Kevin, thanks for the incentive to add a simple field strength meter to my home brew MagLoop. Works really well for tweaking the most from the MagLoop. Also tried your suggestion for compressing the smaller loop to get more surface area with the outer loop. This does improve the ability to tune for resonance on 10M through 40M. Gets a lot of intrigued looks during portable outings. MagLoops are great! Everybody should build at least one and your videos make it easy! Great job on all the MagLoop designs you have demonstrated. 73 de N4UVR

[@loughkb]

Thanks for the kind words Cory. Yeah, everyone should build a magloop! They're always surprising antennas.

[@chriswesney]

The sound of a dry marker being pressed into paper like that gives me the heebie-jeebies...It's like nails on a chalk board but worse. Am I the only weirdo who feels that way? Thanks for the video!

[@nsknyc]

Late for a reply, but yessss. I can't stand the sound of markers or chalk.

[@johndk9jc354]

Thank you. Just made one and it perfectly works. Had no luck with a 50mA meter (had it already), but a Technics db-meter out of an old tape deck worked. 73s from Germany

[@Devin82m]

VERY cool! You need to make more videos like this building your own test gear. Thanks!

[@clintonhunter6821]

Thank you, just finished mine, works great!

[@acestudioscouk-Ace-G0ACE]

Thanks for this video, it's just what I need and shall build one...found all the bits in the junk box too!

[@julianopificius6910]

3:26: lower forward voltage drop, not lower forward resistance. Using germanium diodes instead of solicon improves sensitivity & linearity at low signals strength levels.

[@seniorchief48]

Great video. What is the value of the two diodes that you are using?

[@loughkb]

Oh any generic signal diode will work. 1N914 if memory serves is a common one.

[@ablaauw70]

Hi Kevin, I used the 1N4148 works fine, costs nothing but I only used the stuff I had. I didn't have the capacitors in those values ​​and used other ones. Afterwards you can just leave them all out, it works fine for me. What is the function of the capacitors actually?? Greetings, Andre. PD3A 73'

[@loughkb]

The capacitors smooth out the rectified RF and build a very small amount of charge that helps to smooth out and bolster the operation of the meter. I've always seen better deflection using them than without.

[@theodoresears4679]

I got an idea to use this in line external RX field strength meter on my Cb walkie talkie. My little radio never had a way to see the field strength on receive.

[@BrekMartin]

Hello, What’s the purpose of the cap? and why the particular value you used? What happens if you remove it??

[@thomassappenfield8843]

will an amp meter work. only meter i could find after a short search. i plan on using this during an fox hunt. will it work for this purpose. thanks in advance for answering

[@loughkb]

The meter movement has to be clear of any shunt like you might find installed in an amp meter. It wouldn't work for fox hunting. The fox is usually a very low power, milliwatt range, and wouldn't produce a field strong enough to deflect the meter. You'd need some kind of broad band RF amplifier in the circuit.

[@ElianMiculob]

Hi Kevin. May I know what is the value of the diodes used?

[@loughkb]

Just about any germanium diodes will work fine.

[@mauricecraft890]

When do you use the ground binding post? You did not use it when you tested the meter.

[@loughkb]

I have touched it a few times to increase sensitivity. I also used it once with the meter connected to my main HF antenna. By grounding, I was getting a strong deflection on the meter from local broadcast stations. Just as an experiment. It's not really needed, I just like having options.

[@rayhagan1339]

Terrific video! I'm new to ham radio and just went to a meeting about fox hunting. Going to build a Yagi antenna and want to build this meter to read the signal strength, instead of the S-meter on my HT radio. What size is the box you're using? I just ordered a 0-50 µa/2kΩ panel meter for the project. Looking forward to building this. I think it's just what I need. Thanx!

[@loughkb]

I doubt you'd have enough signal strength to move the meter without an RF amplifier built in. The transmitted power of a fox is usually very low, just a few milliwatts.

[@Mike-zw3zs]

Simple and reliable. Tested 3.5...440MHz. EX8BS 73 TU E E

[@richardday2893]

What length antenna for 935mhz please?

[@loughkb]

A standard formulae that gets you close in feet is 468 divided by the frequency in MHz.

[@ahmetozdemir7173]

Hello, it was a nice video. Thanks for information. Can you give information about a subject? What is the frequency range that the circuit can detect? my second question; Is the shape of the antenna you use in the circuit important? So, is it possible to convert it to a larger, square or circular 30 cm antenna? My third question is; Wouldn't it make more sense to use a directional antenna in the circuit? That is, how can an antenna be added to the circuit that is aimed at the frequency source of rf and to measure the strength of the signal coming from that direction only. Thank you in advance for your answers.

[@loughkb]

There's no tuned circuit in this, it should be sensitive to a very broad range. Antennas, do whatever you like. It's going to indicate whatever strong source the antenna picks up.

[@ahmetozdemir7173]

@@loughkb Ok. Thanks for the answer. Let me forgive my ignorance, but I have to ask; Isn't the power of the antenna also related to the amplitude of the em waves radiating from the antenna? So, the closer you are to the antenna, the larger amplitude em waves can be measured to the rf power meter device. So I think it is also possible to measure the amplitude, that is, the power, of the waves that have gone farther than the antenna. So, can it be said that the rf power meter can measure the strength of the rf signal that can be detected by any radio, even if it is on yada onbeş meters away? Thanks

[@loughkb]

@@ahmetozdemir7173 It's a square law. The energy leaving a transmitting antenna is dispersing outwards. The further you get away, the less energy you can measure in the same amount of space. For every doubling of distance, you will have 1/4th the energy present.

[@ahmetozdemir7173]

@@loughkb Thank you so much for answere.

[@clg2c84]

Can you use an LED instead of a meter? Kind of like an ALEXTUNE?

[@loughkb]

You would need a power source and a driver stage for the LED, like a transistor. You also wouldn't be able to see very fine changes in brightness as well as you can see small movements of the meter.

[@barretto4152]

Could you swap the meter out for a led with this circuit and just tune for maximum brightness when tuning your magloop?

[@loughkb]

No. There wouldn't be enough current to.light the led.

[@barretto4152]

I just threw the circuit together with an LED and guess what it worked now I just have to stop decide which method to use for my Loop tuner. I don't know if it's just me but it seems like the LEDs have some kind of point of Maximum brightness to wear if you have it set to sensitive you'll have a hard time telling the difference between say a 3:1 or 1:1. Can anyone confirm that or is it just me.?

[@barretto4152]

I had forgot to mention using it as a loop tuneing indacatorI had it right next to the mag loop I don't think you could get it more than a couple feet away before the LED goes out so I don't think it would work for a normal field strength meter

[@loughkb]

@@barretto4152 OK, I hadn't thought about the strength of the field around a magloop. I guess yes there would be enough current there. I was thinking about the other ways I use the meter. I'd still use a meter though. You get a much better indication of small changes where the meter moves a bit but telling the difference in brightness of an LED with such small changes would be hard.

[@barretto4152]

, I agree with you Kevin after experimenting I am going to stick with the meter it gives you more confidence in your reading and allows you to see in front of and behind where you're at. It also lets you take exact measurements. With the meter I'm able to watch how the efficiency or lack of deflection falls off as you go lower in frequency with the same size Loop. Thanks Kevin for sharing the circuit.

[@angeldelvax7219]

I built one today, with parts I had laying about. Used two 170pF caps parallel instead of one 450pF, and a 10k potmeter. Thing is, I don't get a reading! I built it with a BNC just before the potmeter to be able to connect an external meter, and it measures just fine. it's extremely sensitive in the 70cm band, But no movement in the panel meter. Any suggestions?

[@angeldelvax7219]

never mind, found the problem... the panel meter died :(

[@plutarco8556]

Condensator 470 pf?

[@loughkb]

That would work fine.

[@manonegra366]

did one works great thanks

[@NamasenITN]

Hi Kevin, beginner here. I have a basic question: I understand that the specific diodes configuration you used is nothing more than a full-wave rectifier. Are you however sure it works? Usually for a full-wave rectifier, people use 4 diodes arranged as in a bridge. I simulated the circuit with LTSpice (using a voltage generator instead of the antenna) and I have seen it does not operate as a full-wave rectifier but only as half-wave. In the MHz range, the RC value you have chosen is of course smoothing the oscillations pretty well. Thank you for your answer (and patience).

[@loughkb]

Oh it works. It's a very old classic design. There's enough energy present that many earlier field strength meters only used one diode. Look at a crystal radio design with a single diode. The same basic thing is happening here, except that instead of driving an earphone, the energy is driving a meter movement.

[@MrTerrymiff]

Just looking at it I would say that the diode from the antenna to earth is doing nothing. The diode going to the RC tuning circuit is giving half-wave rectification when the incoming signal goes positive.

[@julianopificius6910]

It isn't a full wave rectifier, the circuit is called a "voltage doubler"; here's how it works. The first, and absolutely the most important thing to understand is that the coupling of the signal voltage is capacitive, because there is (obviously) no d.c. connection to the transmitter. You should therefore imagine for this description that the induced voltage is coupled from the antenna to the horizontal diode through a virtual capacitor, which blocks dc. The signal induced into the antenna relative to the more negative side of the circuit (the one marked as ground in the schematic) is alternating, so it passes through the virtual capacitor into our circuit, but the diodes play some magic, and I'll explain how... Now, as the signal voltage goes positive with respect to the "ground" side, signal current flows through the forward biased horizontal diode and charges the real capacitor. The vertical diode is reverse biased, so no current flows through it. Some current begins to flow through the meter, as you would expect. Now this is where the magic begins - the first of a one-two punch, if you will... When the induced signal voltage goes back down to zero and goes negative with respect to the ground side of our circuit, that current flows through the now forward biased vertical diode. The right hand side of the virtual capacitor is held at relative ground by that diode. Also, because the horizontal diode is now reverse biased, the real capacitor does not discharge through it, and holds most of its charge. Some discharge does continue to take place through the meter, of course. So, as the vertical diode holds the right side of the virtual capacitor at relative ground, the negative going signal voltage on the left side of the virtual capacitor charges the virtual capacitor negatively. Now, here's where the magic trick is completed... As the signal voltage comes back up from its maximum negative value to zero, the voltage on the right side of the capacitor is "pushed" up by the increasing voltage on the left side (remember that current has to flow out of a capacitor to discharge it -i.e. for the voltage across it to drop). So the new positive half cycle voltage of the signal the is ADDED to the existing charge on the virtual capacitor, so the voltage on the right side rises ABOVE the value of the incoming signal voltage. Current flows through the now positively charged horizontal diode to further charge the real capacitor. If there was no current drawn out by the meter, the voltage would be twice the peak voltage of the signal voltage, hence the term voltage doubler. In reality, because of the virtual and real component values it will take several cycles to fully charge the real capacity, but it isn't long. I hope this description was helpful.

[@julianopificius6910]

@@loughkb right; the vertical diode turns the single wave rectifier into a voltage doubler, as I describe below.

[@seyedmarashi]

please leave Diodes Part# to have same circuit here and there!

[@loughkb]

Any germanium diodes will work fine. Not critical.

[@davedago3241]

Hello from a bright but cold Wales brilliant presentation going build this afternoon will use to check my end fed project do you think it will show rf patterns from various ununs or baluns great stuff Dave C G0NVF 73

[@loughkb]

Hello Dave, It might. I bent the wire antenna into a circle and the meter was deflecting from the EM field around a compact florescent light bulb. The reason I used a binding post, was I intend to make a few different pickup wires and be able to just plug them in using banana plugs. So, make a smaller pickup loop and move it around the outside of the balun, you will probably see the peaks and valley's of the local field.

[@davidclifford2804]

many thanks more playtime needed DaveC 73

[@Dennis35793]

Thanks,

[@alvinfreeman8838]

Where is the power source?

[@loughkb]

I can't tell from the thumbnail on my phone app, but if this is the video on building a field strength meter, it's the rectified rf picked up from the transmitter

[@pedrolarenas]

Cool meter I will made one

[@R-E-D-E-M-P-T-I-O-N]

i know this is a 5 year old video was hoping someone would know what the specs of diodes. this is my first project. he gives values for everything but the diodes. thanks in advance

[@loughkb]

Any geranium diodes would be good.

[@R-E-D-E-M-P-T-I-O-N]

@@loughkb one other question when i typed in 470 pf what came up on digikey was one that was a 6800 pf ??? maybe a part number or something, remember this is my first project for ham radio -dipole which isnt much

[@loughkb]

www.digikey.com/en/products/detail/kemet/C317C471J1G5TA7301/6562564?General&PMax%20Shopping_Product_Zombie%20SKUs&General&Cj0KCQjw-pyqBhDmARIsAKd9XIM_-lgCzNb__ipoAWhY46f-UemcH2wayc2wWsXLS1I_rg_8RQIr6ucaAotHEALw_wcB&gad_source=1&gclid=Cj0KCQjw-pyqBhDmARIsAKd9XIM_-lgCzNb__ipoAWhY46f-UemcH2wayc2wWsXLS1I_rg_8RQIr6ucaAotHEALw_wcB

[@R-E-D-E-M-P-T-I-O-N]

what size caps do i need? you said 470, but they go from 1000 pf and up. are they ceramic and what exactly pf does the cap need to be@@loughkb

[@loughkb]

@@R-E-D-E-M-P-T-I-O-N they're not that critical. They just smooth the RF to dc. you could put .01 uf disc caps on there and they'd work

[@leos9865]

Very cool

[@manonegra366]

I use two zene diode and 1 cap 472 and a old yaesu meter and a variable from a yaesu old radio hahaha works for me.