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  • Measuring an electric field

    skipper

    Member

    Updated: Oct 25, 2024
    Views: 1.5K
    Riddle me this:

    If you have a battery-powered radio, can you think of a way to use it to get a qualitative measurement of local field strength of RF?

    I would say all you need to do is tune the radio to different stations and try to adjust the volume to the same "strength" each time. So you can use an ordinary portable radio to record the appropriate volume setting for a number of local and remote stations to get a qualitative measurement of the sum of their RF outputs.

    Is this like finding an RMS background field strength, and would using a longer or shorter aerial mean a kind of calibration of the radio, in that case? And would using a signal meter that displays true RMS be like using an all-band radio that tunes every station at once, at different sound volumes?
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  • durga ch

    Member • Oct 14, 2010

    is it measuring the received power or frequency?
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  • skipper

    Member • Oct 14, 2010

    durga
    is it measuring the received power or frequency?
    By "it" I assume you mean the radio?
    The power depends on the strength of the field, so a radio will output sound at a volume proportional to the field strength. The radio "measures" frequency via the tuning circuit, but a signal meter measures all frequencies at once.

    I was informed by someone who seems to believe they know what they're talking about, that using a battery as a reference is absurd, or that the length of a receiving antenna as a calibration of field strength is also absurd. I'm just analysing this "absurdity" and I think they don't know as much as they are claiming.
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  • skipper

    Member • Oct 14, 2010

    Can you think of a way to use a multimeter as a signal meter that measures field strength? If you attach an antenna (aerial) to one of the inputs, and select AC voltage, is the MM an effective signal meter apart from being uncalibrated? Even so, does it give you a way to measure relative field strength qualitatively, and does the quality depend on the length of the antenna? What is the voltage reference if the meter isn't grounded?

    Does using a MM this way correspond to using an all-band radio receiver, since battery-powered radios aren't grounded either--they have an internal "floating" ground?
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  • durga ch

    Member • Oct 14, 2010

    as what i get , radio volume will be the internal control of amplifier right? how does it being maintained constant at various frequencies/power, convey the strength of the signal received?
    as far as length of field antenna is concerned, theoretically yes, the received power does vary according to the received height.
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  • skipper

    Member • Oct 14, 2010

    radio volume will be the internal control of amplifier right? how does it being maintained constant at various frequencies/power, convey the strength of the signal received?
    Well, if you adjust the volume for a number of different broadcast signals, using a battery-powered radio, so that in each case they output the same sound level, then the volume setting is a measure of the strength of the signal at that frequency. The only practical difference is that a radio is tuned to a specific frequency at a time, a signal meter is "tuned" to every radio frequency at once.

    So, if you add together the individual volume settings for a number of stations (let's say there are a potentially infinite number) given you have a way to set the volume to the same "qualitative" output level as you do this (although you should probably use a decibel meter to ensure the output volume is the same in each case, rather than just your sense of hearing), the sum is like an RMS determination of signal strength (AC to DC conversion). The DC part of the RMS for a radio is the volume setting, in that case.
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  • skipper

    Member • Oct 15, 2010

    Well, I was kind of hoping someone would raise an objection to using a radio receiver like this.
    Determining the total background RF at a particular location using a radio means doing a lot more work, for one. Another objection is that the radio can only measure broadcast signals that "carry" an embedded audio signal, so you can only guess what the strength of received non-audio RF is, when the radio is tuned between stations.

    But suppose you decide to modify the radio receiver, and replace the tuner circuitry with an all-band circuit, so you can "listen" to all frequencies at the same time, including frequencies that aren't from a broadcast station. Then you replace the amplifier with a decibel meter or an RMS meter (or both). Essentially you have modified the radio so it's now a signal meter which samples all RF signals locally and outputs the result. Alternatively you can just connect an aerial to the positive input of a multimeter that can measure AC RMS voltage, so the negative input is then just the negative battery terminal. So that a multimeter with AC RMS capability can be used to measure relative field strength.

    Actually using a DMM like this (or an oscilloscope) is suggested by a physics teacher as a way to measure the field around a plasma ball. It works too, in fact you can use a DMM like this anywhere, to get a qualitative measurement of local RF, although since the battery is the voltage reference, calibrating the signal strength will mean having a source of RF at a known power output, and possibly using different length aerials. But calibration isn't actually required because you just have to measure the field at different locations--you could drive around in a car for instance.

    I don't think there's anything absurd about using a "floating" voltage reference this way, or about calibrating the signal strength against the length of an aerial.
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  • Ashraf HZ

    Member • Oct 15, 2010

    Grr.. too much text! need diagrams 😛

    One problem is, what kind of antenna would you want to use? You'll need to consider their gain. Wide band antennas can have different gains with varying frequencies, which will distort your readings. Another issue is the directivity, point the antenna in different directions and you will yield different results too, especially when taking polarization into account.

    We could just fix some parameters in the beginning, like which FM carrier frequency we want to measure the strength for, where the radio tower is, and an open area so that there are no multipath propagation (resulting from reflection off things like buildings, etc). We can get some decent results from that set up and probably calibrate the meter at the same time using the Friis Transmission equation. Even better, if you have access to an RF signal generator, you can do all this in the lab.
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  • reachrkata

    Member • Oct 16, 2010

    skipper
    Well, if you adjust the volume for a number of different broadcast signals, using a battery-powered radio, so that in each case they output the same sound level, then the volume setting is a measure of the strength of the signal at that frequency. The only practical difference is that a radio is tuned to a specific frequency at a time, a signal meter is "tuned" to every radio frequency at once.
    There are constraints which will make it impossible to use a normal radio to measure bradcast filed strength.

    1) Usually radios are designed in such a way that after a particular signal strength, the radio would give the same audio level even though the broadcast signal strength may further increase.

    2) The audio level also depends on frequency deviation (for FM) or modulation depth (for AM). For example, if there are 2 FM station transmitting same RF strength, with station 1 having 30kHz deviation and station 2 having 50kHz deviation, station 2 will be heard louder than station 1. even though the signal strength is the same.

    -Karthik
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  • skipper

    Member • Oct 16, 2010

    1) You can adjust the audio level with the volume control. As stated earlier this gives a qualitative measurement of the field strength at that frequency.

    2) I realise there are practical difficulties because of the way a tuner circuit "dumps" sideband frequencies, etc. The idea is to start with an ordinary radio and modify it so you get a signal meter. For instance, removing the tuner and connecting the aerial input directly to the amplifier will give you a more general measurement. If you replace the amplifier with a circuit that converts AC to DC voltage as RMS, you have the same thing as a DMM set to AC voltage, with an aerial attached to say, the +ve lead. Connecting the -ve lead to ground won't give you the "floating" value of RMS you're after.

    3) I realise there are several other practical difficulties with using an unmodified FM/AM receiver which tunes specific RF bands, but the point is to illustrate what RF is and what signal meters and radios do with it. If there is any doubt about using a DMM as a signal meter, try connecting an aerial (a length of wire) to one and pointing it at your computer screen, then moving it towards and away from the screen. Then repeat the procedure with the other lead grounded.
    Hell, try just switching a DMM on and setting it to AC voltage, you should see a value displayed. What is the DMM measuring if the leads aren't connected?
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