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.
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?
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).
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.
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.
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?
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 .
Here is a more broadband design.
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.
