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I'm trying to measure the relative signal strength received by a dipole antenna for orientation purposes. I'd like to do so using an ordinary multi-meter.

I don't know much about RF circuits, but this is the circuit I came up with.

enter image description here

I was planning on measuring the resistance across the banana connectors to represent the relative inverse of signal strength.

Would this work? If so, which diodes and transistor should I use? If not, which circuit is best?

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closed as unclear what you're asking by Andy aka, ThreePhaseEel, Voltage Spike, nidhin, DoxyLover Feb 24 '17 at 1:54

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ What frequency, what power is the transmitter and how far away from it is the antenna? \$\endgroup\$ – Bruce Abbott Feb 22 '17 at 8:23
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    \$\begingroup\$ Look up 'biassed schottky diode detector', that bridge rectifier stands not a snowball in Hades' chance of detecting RF \$\endgroup\$ – Neil_UK Feb 22 '17 at 9:21
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    \$\begingroup\$ Look up the MAX2015. It and other chips like it can do what you want. The MAX2015 is good from somewhere below 100MHz up to around 3GHz. Buid as described in the datasheet, read the voltage on the output with your voltmeter, do a bit of calculation, and there you have the signal strength in dBm. The Max part is just an example. With that, you can find parts in the needed frequency range and sensitivity. \$\endgroup\$ – JRE Feb 22 '17 at 10:34
  • \$\begingroup\$ A dipole has two connections that are balanced. I have no idea what your diagram is meant to be. \$\endgroup\$ – Andy aka Feb 22 '17 at 11:44
  • \$\begingroup\$ @Andyaka One pole would be connected to ANT1 and the other to ground? Maybe I just labeled them wrong. \$\endgroup\$ – user2005848 Feb 22 '17 at 16:44
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You need a RF power detector that suits your target frequency and power ranges.

You may also need some kind of band-pass filter if you want to detect a signal whose bandwidth is much narrower than the dipole antenna bandwidth and there are other interfering signals or adjacent channels that could be picked up by the antenna.

If RF is not your field of expertise, I'll recommend using COTS modules because RF is very, very tricky. However, they may be an expensive solution for your application.

Mini-Circuits RF power detectors run at $90 approx, but have huge bandwith and dynamic range. You may find lower priced, less capable, RF power detectors that may still comply with your frequency/power/precision needs. If yours is a DIY project, you could turn to AliExpress and the like to find much cheaper options, but keep in mind that you'll get what you pay for (still, it may be good enough for your needs).

You may want to take a look at this superb hack: a guy called Tim Heavens used one of Mini-Circuits RF power detectors, an Arduino, a LCD and a case and built a RF power meter.

EDIT:

You may also need to match the impedance of the antenna to that of the modules down the RF chain (usually 50 Ohm).

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Your rectification mechanism needs to be matched to the frequencies you intend to detect. If you take a random full bridge rectifier you are likely not to see anything at all, as the RF waves will bounce off it. Look up literature on RF rectifiers for assistance here.

Furthermore, as RF signal strength is usually very low any considerable distance away from the source, you will be unlikely to ever see enough voltage over the rectifier unless you are practically standing beside the RF source to activate that Q1 base-emitter junction. Without a capacitor after the recitifer you would also see a pulsed waveform there, which is likely at a frequency Q1 cannot handle unless you pick it very carefully.

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  • \$\begingroup\$ Interesting you say that because it will be used standing right beside the RF source. I just need a device to help find the optimal orientation between two antennas before putting them to use. Maybe the source wont be strong enough though? \$\endgroup\$ – user2005848 Feb 22 '17 at 16:48
  • \$\begingroup\$ It would depend on the source strength, your antenna parameters and how well it is matched. But given that your rectifier is matched and with the addition of a capacitor to get a constant voltage accross the BE junction and a small base resistor so not to drain the energy instantly through the BE diode then it might work for detection purposes. The circuit doesnt really provide you a relative scale as designed, but more on/off but you may know that already. A collector resistor would change that \$\endgroup\$ – Stonie Feb 23 '17 at 10:03
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Here is a peak-detector from another answer; the response is LINEAR with input voltage. Installing a diode across the topleft resistor provides a LOG response.

enter image description here

There is no Miller Effect, in that V_collector does not move quickly. Expect to get common-base performance from your transistor, close to 300MHz with branded 2N3904. Or use a genuine RF device. The S11 will be poor. Do you care?

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Your diagram shows a bridge rectifier .Most of the time they are designed for mains and therefor are far too slow.user 70614 has covered this .The bridge circuit has 2 diode drops so it will have poorer weak signal performance .Ge diodes have very low drop and are normally found in glass packages and have low capacitance.I used Ge diodes in a mini production run of I think about 25 I think 25 years ago .The field strength meter worked very well .It was used by Auto electricians for testing RF remotes .A skottkey diode should work fine we did some tests and found that low capacitance diodes worked as well as Ge .High capacitance diodes performed badly .We did not need any more field strength meters so none were produced with low capacitance shottkey diodes .I used a half wave voltage doubling detector and run its output into some case mounted red and black bananna jacks that plugged into a standard DVM .The DVMs that I looked at all had the same plug spacing .So the FSM plugged directly into the DVM sitting on top .Normally the DVM was set on the 300mV range and it worked fine for the job .Placing the DVM on ohms gave even more sensitivity because the DVM was providing current to prebias the diodes .

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Here is a more broadband design.

schematic

simulate this circuit – Schematic created using CircuitLab

By the way, injecting into the EMITTER is an approved method to boost the performance of Trans Impedance Amplifiers.

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