Frequency counter doesn't detect RF field from sine wave?  - Page 1
Forum section: electronics / rf-microwave

author: Psburg
author_rank: Newbie
timestamp: July 26, 2020, 08:47:13 am
content: Just playing with a simple sine wave generator, running it through a small radio coil, my handheld frequency counter doesn't detect anything, but when I switch to square wave it picks up the frequency fine, why is this?

author: Miti
author_rank: Super Contributor
timestamp: July 26, 2020, 11:35:56 am
content: What frequency, what counter, what coil? Does it show the correct frequency with square wave?

author: janoc
author_rank: Super Contributor
timestamp: July 26, 2020, 12:08:20 pm
content: Quote from: Psburg on July 26, 2020, 08:47:13 amHarmonics from the square wave.You haven't given enough information but I guess your sine wave generator frequency is below the minimum frequency of the counter, so you don't see anything. However, when you switch the generator to square wave (on the same frequency), the harmonics from the square wave (look up Fourier analysis) are at much higher frequencies and are detectable by your counter. The counter is likely not showing the fundamental frequency of the generator but one of the harmonics in such case.

author: radiolistener
author_rank: Super Contributor
timestamp: July 26, 2020, 01:54:59 pm
content: Quote from: Psburg on July 26, 2020, 08:47:13 amsince you didn't provided details (frequency value and inductor value) it's hard to say what you're doing...Since inductor has high impedance for high frequency, most of all in your case it just filter sine wave, so the sine amplitude is not enough for your counter. Square wave has high frequency harmonics, which may leak through capacitance of your wires and between inductor turns, so this is the reason why your counter can see these harmonics.

author: Psburg
author_rank: Newbie
timestamp: July 26, 2020, 04:49:49 pm
content: Sorry for the lack of detail, using any frequency, the counter reads accurately what the generator is set to (on square wave), say 44khz, well within the range of the counter, and adjusting amplitude does not help, going up to 12-20v. The coil is just small (~200turns, half inch diam, 0.2m wire), but have tried several coils, and it is the same thing.This is the counter I'm usinghttps://www.ebay.ca/itm/Portable-Frequency-Counter-10Hz-2-6GHz-NDK-TCXO-for-Kenwood-Icom/291738532023,I've tried the different gate times.

author: Miti
author_rank: Super Contributor
timestamp: July 26, 2020, 05:45:55 pm
content: Connect that coil series with a capacitor and use one of those online calculators to tune the LC circuit to the frequency of interest and see what happens.

author: janoc
author_rank: Super Contributor
timestamp: July 26, 2020, 05:46:22 pm
content: Quote from: Psburg on July 26, 2020, 04:49:49 pmUhh and you are expecting a meaningful coupling from a coil into the rubber duck antenna of that counter at44kHz??That's waaay too low frequency, that antenna there is won't pick up anything meaningful there. That "rubber duck" antenna is probably completely useless below 100MHz or so ... That your counter spec says "10Hz-2.6GHz"does not meanthat the bundled antenna covers that range too. You are expected to either connect the device to whatever you are measuring directly (coaxial cable) or provide your own probe/antenna suitable for the frequency range you are measuring.

author: janoc
author_rank: Super Contributor
timestamp: July 26, 2020, 05:48:52 pm
content: Quote from: Miti on July 26, 2020, 05:45:55 pmAt 44kHz with that coil that would be one heck of a capacitor ...

author: Miti
author_rank: Super Contributor
timestamp: July 26, 2020, 09:17:41 pm
content: Quote from: janoc on July 26, 2020, 05:48:52 pmA rough calculation with 200t, 15mm length, 12.7mm diameter, I get 307.1uH. 307uH with 42.6nF, I get a resonant frequency of 44KHz. Am I missing something?But I agree, at 44KHz the OP should use a coax, watching carefully the input level.Some time ago I played with a ferrite antenna and a series capacitor to make a DCF77 transmitter at 77.5KHz using a FeelTech FY6600 generator, and it worked like a charm across the room in my basement lab, but I had to tune it. Without tuning, I got nothing. The carrier amplitude was set to 5V if I remember correctly. I can see the carrier on the spectrum analyzer with a big patch antenna.Edit: Lool, at 44 KHz a twisted pair is good enough.

author: janoc
author_rank: Super Contributor
timestamp: July 26, 2020, 09:43:51 pm
content: Quote from: Miti on July 26, 2020, 09:17:41 pmOK, good point. I didn't calculate it through. However, what exactly are you expecting to happen from making this thing resonate?That rubber duck antenna he is using will still suck the same at such low frequency.

author: Miti
author_rank: Super Contributor
timestamp: July 26, 2020, 09:55:15 pm
content: Quote from: janoc on July 26, 2020, 09:43:51 pmYou're right, I don't expect to measure anything. Please see above, I modified my post.