inexpensive broadcast fm field strength meter?

[aandrew]

Continuing my experiments with low power, unlicensed broadcast FM, I'd like to get an idea of just how much power I'm actually putting into the air.

I'm using an si4713 for the transmitter. It claims it can push out 115dBuV (120dBuV if I'm not interested in voltage stability) and just have a 120nH high-Q (thanks BrianHG!) inductor as shunt, feeding 6cm of wire. (I've also tried longer lengths, but this seems to work the best.)

When the transmitter board is powered off of a wall wart, I get really good performance. Maybe a hundred feet, which is pretty snazzy.

When powered off of a regular old 18650 lithium cell, I can't even get 10 feet. I *suspect* that it's got to do with a lack of solid ground, and will be experimenting with a dipole antenna (basically two 6cm lengths of wire, one grounded, one the driven element, in a kind of "T" shape).

That's all backstory. What I'm really trying to do is figure out how to measure the field strength of the transmitter+antenna. I've found a little circuit (http://lowpowerradio.blogspot.com/2010/12/field-strength-meter-for-fm.html) which will probably get me in the ballpark but I am not sure how to "tune" it to ensure it's MY transmitter I'm measuring and not, for example, the local college FM radio station.

Is it "just" a matter of inserting an appropriate filter in front of the meter so only the frequency I'm interested in makes it through? It seems that that would be difficult given how narrow FM is and how closely the stations are packed. I am not sure if I can equate the dBuV that the transmitter chip is creating with an actual field strength in uV/m measured 3m from the transmitter with a "standard" antenna (whatever that means).

Any tips/ideas/guidelines/interesting stories are very much welcome.

[ogden]

Field strength measurements is hard way because specific receiver and antenna, both calibrated, needed. Better just measure transmitter output power going to antenna, then use math. To be on safe side you assume ideal antenna and will be sure that you are within regulations. Anyway this is what you want to ensure, right? This will solve your problem of college FM radio station interference as well. U need either spectrum analyzer or power meter.

[MasterT]

Get RDA5807 for less than $1 from ali, it covers 50-115 MHz and has RSSI indicator.

[aandrew]

Get RDA5807 for less than $1 from ali, it covers 50-115 MHz and has RSSI indicator.

The si4713 can do this too; that's not what I'm after. I want to measure the signal strength it is *transmitting* not receiving.

[aandrew]

Field strength measurements is hard way because specific receiver and antenna, both calibrated, needed. Better just measure transmitter output power going to antenna, then use math. To be on safe side you assume ideal antenna and will be sure that you are within regulations. Anyway this is what you want to ensure, right? This will solve your problem of college FM radio station interference as well. U need either spectrum analyzer or power meter.

Interesting; Could you help me narrow down the search terms when I'm trying to figure out the math to use? The output of the transmitter is matched with an internal (to the IC) varicap and works against a 120nH shunt inductance that I place nearby. Do I assume the impedance of the antenna is 377 ohms?

[ogden]

Interesting; Could you help me narrow down the search terms when I'm trying to figure out the math to use?

You shall tart with reading: https://www.silabs.com/documents/public/application-notes/AN383.pdf

Wait.. Further looking into chip, I noticed that documentation (AN332) about TX_TUNE_POWER register states: "Sets the tune power in dBµV in 1 dB steps. The valid range is from 88–115 dBµV. Power may be set as high as 120 dBµV; however, voltage accuracy is not guaranteed.". So I don't see why you shall even bother to measure output power or for gods sake field strength.

The output of the transmitter is matched with an internal (to the IC) varicap and works against a 120nH shunt inductance that I place nearby. Do I assume the impedance of the antenna is 377 ohms?

If manufacturer is not offering impedance matching circuit for 50 Ohm cable for measurements, it is not worth to try develop it yourself. You can check FCC documents (if any) - maybe in test papers are said how to connect particular IC to test equipment, but I doubt it. In your place I would drop idea of measuring anything and just follow TX_TUNE_POWER programming docs.

[HB9EVI]

I was used to different datasheets from Si, this one is quite disappointing. Just wondering, why they cannot specify the output power in dBm into a 50 Ohm load.

If you really bother to have a clue about the power of a signal, you have to measure on the senders output and calculate with the antenna properties the estimated radiated power - like you already got advised.
A field strenght meter won't help much, since your power is likely too small and the output power of nearby fm-stations too high.

A simple rf-powermeter you can build with an AD8307 - it does fine on VHF and is readily available

and btw. a dipole for the FM center frequency would be about 2x 1,5m