The Utah Amateur Radio Club:
A Wide dynamic range
Field Strength Meter
by Clint Turner, KA7OEI
When direction-finding in close quarters, fancier direction-finding gear (if you have it) can become useless due to very strong reflections. In addition, it may be large, cumbersome, and very conspicuous. At this point one is often very close to the transmitter itself and it's just a matter of figuring out exactly which tree the transmitter is hidden in or which car has the radio with the stuck microphone.
An updated version of this device may be seen on the "Mark
At such close range, two (of several) possible options are an Offset Attenuator and a Field Strength Meter. Each piece of equipment has its own set of advantages/disadvantages:
||Offset Attenuator w/receiver||
(The ability to differentiate one signal from another on a nearby frequency)
|The ability to single out one signal source largely based on the ability of the receiver being used.||Poor selectivity: The field strength meter, without additional filtering, will respond to any nearby. If there is another transmitter nearby on the same band, it may be impossible to provide adequate filtering to reject the "unwanted" transmitter.|
(How much signal it takes to get a usable reading)
|Good Sensitivity: The sensitivity of the detection system is limited mostly by that of the receiver being used and the amount of intrinsic attenuation range available from the offset attenuator circuit.||Sensitivity depends on design: The simplest field-strength meters can detect a transmitter only when it is extremely close (within 10's of feet.)|
(How it responds to signals ranging from weak to very strong)
|Range of metering limited by that of the receiver: Many FM HTs have S-meters that are nearly useless for determining anything about the strength of the signal. Often, they have only 10-15 dB of useful dynamic range, requiring constant adjustment of the attenuator to stay within range.||Capable of wide dynamic range, depending on design and implementation.|
|Ease-of-use:||Frequent readjustment of the attenuator is required to keep
"pegging" the meter and/or to keep it from dropping too low to
Signal strength meter is often difficult to see/use: Most HTs, if they have a signal strength meter at all, use a small number of segments on the display. This display is not only rather hard to see at times, but it usually offers poor resolution in terms of amplitude.
|The operation is rather intuitive: The higher the
closer you are to the transmitter.
If you are building a field strength meter, you can put as large an analog meter as you like. The analog meter doesn't have the problem with a limited number of "segments" as do many digital displays.
Because one can't anticipate beforehand exactly what the situation might be, it's best to have a number of tools in your arsenal - and this may mean that you'll have both an offset attenuator with a receiver and a field strength meter.
"Conventional" field strength meters:
A typical field-strength meter is shown in Figure 2. This is a simple passive (unpowered) circuit in which radio frequency energy is intercepted by the antenna, rectified to DC, and then used to directly drive the meter. The maximum sensitivity of this circuit is based primarily on several factors:
Another not-so-obvious disadvantage is that this meter's useful dynamic range is, at most, only 20 dB or so - with the majority of this range being "crunched" in the bottom half of the meter scale. What this means is that one is required to constantly readjust the sensitivity control to keep the meter reading within range. In practical terms, this is quite a pain as one can experience "peaks and valleys" of signal strength in close proximity of the transmitter well in excess of 10 dB. What's more, with an adjustable potentiometer it can be difficult to tell, between adjustments of the sensitivity control, if one is actually getting closer or farther away from the transmitter as the "reference" point can be lost when one twiddles the sensitivity control.A higher dynamic range field strength meter:
It would be useful to have a field strength meter that not only had better sensitivity, but a means of increasing the dynamic range of what was represented on the meter itself, preferably to 40 dB or more. One such meter is shown in the schematic in Figure 3. The prototype had a useful range of about -45 dBm to +10 dBm, a range of about 55 dB.
Having a wider dynamic range involves added complexity, of course,
this circuit requires some explanation:
A few parts details:
These are likely to be the most critical components. I used a pair of HP 5082-2835 (often referred to "HP-2835") microwave mixer diodes, but other more commonly-available germanium diodes such as the 1N34, 1N60, 1N63, and 1N270 can work as well, but with reduced sensitivity. The microwave-type diodes will easily provide good response from low HF through 70cm while many of the germanium types will start to lose sensitivity on the higher bands. Extremely common diodes such as the 1N914/1N4148 may work, but at reduced sensitivity.LED1, LED2:
It is very important that these two diodes be as closely matched as possible! Precise thermal matching of the more expensive diodes is a bit easier, as their manufacturing tolerances are fairly tightly controlled, whereas this is usually not the case with inexpensive diodes, such as those of the 1N34 variety.
Another possibility is to use an IC that contains several matched diodes such as the CA3019 and CA3039 (and equivalents) but these are getting difficult to find and expensive. Another alternative would be to use some of the newer SMD Schottky RF dual-diodes, but more research would be required to determine suitable devices.
To avoid using a split-polarity power supply, the "bottom" end of D1 and D2 is "lifted" above ground - in this case, by the amount of voltage drop across an LED and for best results a standard Green LED is recommended as it will provide about 2.1 volts of bias. When choosing an LED make certain that you do NOT use an "ultra bright" green variety as these use a different chemistry and require about 4 volts. While these may work OK, this higher voltage will reduce the amount of meter drive capability when the battery starts to weaken: When looking at the specifications, pick an LED with a "Vf" (forward voltage) of about 2.1 volts.
It is also very important that LED1 and LED2 be of the same type with the same voltage drop. The easiest way to assure this is to use two LEDs from the same package.
LED3 is actually present just to protect U1, which has a maximum voltage rating of 7 volts. It, too, should provide a voltage drop of about two volts, but it need not be "matched" to LED's 1 and 2. This LED may be mounted on the front panel as a "power on" indicator. Note that it will glow brighter with increasing meter reading as the circuit draws more current. With the current consumption of the meter's circuitry being only a few milliamps, do not expect it to be very bright. If an LMC6482 is used instead of a MAX492, this LED may be omitted as the '6482 has a 15 volt rating.
These are 100k resistors used for biasing the diodes. For the ultimate in stability, it is recommended that one use 1% surface-mount resistors, all mounted as close to D1 and D2 and each other as possible. If ordinary 5% 1/4 watt resistors are used, make sure that they are of the same type - preferably from the same package. While not absolutely necessary, you may want to sort and find six that are as closely matched to each other as possible.D3, D4:
Why go through all of this trouble? While the major potential source of drift is the diode pair (D1 and D2) it is prudent to minimize the other sources as well. One of the ways to do this, of course, is to use precision components in the first place, but placing them in as close physical proximity to each other as possible (so that they are at the same temperature) also helps.
Finally, another reason why one may consider the use of surface-mount resistors is due to their size: At the higher frequencies, their lower capacitance and inductance (as well as physical size) will help to improve the response - especially above 450 MHz.
These diodes are responsible for the logarithmic response of the meter. While practically any silicon diode will work, one that is slightly oversized for the purpose (like the 1N4001 series) will have less tendency to drift due to thermal heating from the current flowing through it. While only one diode is required for a unidirectional logarithmic response, the second diode makes zeroing of the meter easier, preventing it from going "negative" as easily.The Meter:
While the use of a 1 mA meter is shown, any meter with this (or lower) full-scale sensitivity should work. If a more-sensitive meter is used, R12 and R13 should be appropriately rescaled: R12's job is primarily to protect the meter movement should R13 accidentally be adjusted all of the way to zero ohms. The meter that I used was a 0-15 volt meter from Radio Shack. This meter is simply a 1 milliamp movement supplied with a resistor of approximately 15k for scaling.U1:
While several op-amps were tried, the one of two (that I had on hand) that worked properly was the MAX492 made by Maxim. This op amp is specifically designed for low-voltage operation with rail-to-rail voltage range on both input and output: Most other types of op-amps will simply not work. It is worth mentioning that if a split-polarity supply were used, a common, garden-variety op-amp would probably be usable, but this would require a pair of 9 volt batteries. Its disadvantage is that it has a maximum supply rating of 7 volts and LED3 was used to reduce the voltage slightly to protect it.Rcal1, Rcal2 (R8):
Another suitable and preferred device is the LMC6482 by National Semiconductor. This device is functionally identical to the MAX492 except that it can tolerate a higher supply voltage (15 volts) than the MAX492. Note: I did not have any samples of this device to try when I originally built this field strength meter. When using the LMC6482, eliminate LED3.
Surrounding R7 (the "zeroing" pot) is shown two pots designated R8 and this is typically a 50k trimmer potentiometer and only one of the pots shown will be needed. Depending upon the exact match of D1, D2 and R1-R6, the "zero" point may be either "above" or "below" zero.R16:
When constructing the unit, it is recommended that one temporarily replace both R7 and R8 with a single 50k-100k potentiometer for zeroing. Once it has been verified that the unit operates properly (and when it does, you will note that the zeroing adjustment is quite touchy) R7 and R8 is installed. When this is done, set R7 at mid-rotation and R8 is installed in only one of the noted positions and adjusted for meter zeroing. If meter zeroing cannot be achieved, move R8 to the other position and try again: You should be able to zero the meter (IF it worked when you had the temporary 50k-100k pot installed) with R8 in one of the two positions.
If, during typical operation, you note that a zero offset appears under certain conditions that cannot be "adjusted out" either increase the value of R7
(do not go higher than 5k) or, if the "uncorrectable offset" is always in the same direction, readjust R8 to "slide" the adjustment range of R7 a bit.
This is a "gain" control that is optional. With the logarithmic response of the meter, note that no matter what the setting of R16 might be, the "bottom end" of the meter reading will always represent about the same amount of field strength. What R16 does adjust is the amount of signal required to indicate full-scale. This may be useful if the signal is consistently weak, but it isn't really necessary and may be omitted. If you do chose to include R16, an "audio taper" or "S-taper" version is preferred to reduce the "crunching" of the gain adjustment to one end knob rotation - but if you use a pot with a non-linear taper, make sure that you connect it appropriately to utilize that feature.
The 20 dB attenuator:
A useful addition to this meter is that of a switchable 20 dB attenuator. When getting very close to the transmitter - particularly if it's a fairly powerful one - the signal may constantly "peg" the meter. Being able to throw in a bit of extra attenuation allows one to be able to knock the signal down to something other than full-scale.The audible indicator:
Additionally, if you are getting very near a high-power transmitter, there is the possibility that the detector diode may get burned out if you accidentally get the transmitting and receiving antennas too close to each other.
Note that the "ideal" resistor values are closer to 62 ohms and 240 ohms (instead of 68 ohms and 270 ohms, respectively) but the values shown are more likely to be found in one's resistor drawer and represent about 1 dB of difference from ideal.
It is important to note that a simple attenuator such as this, built onto a subminiature slide switch, will start to degrade badly above 500 MHz. On the unit that I built, I observed a consistent 21 dB of attenuation from 1 MHz through 2 meters, dropping to 18 dB at 70cm and going down to about 11 dB at 1 GHz. This is due to the inductance/capacitance of the resistors being used as well as cross-coupling between sections of the switch.
Note that R17 (as well as R19 through R18) is connected at all times. This provides not only a DC path to ground at all times to prevent static buildup, but it also provides something closer to a 50 ohm termination even when the attenuator is switched out and it reduces the response to extraneous E-fields - See the E-field discussion below.
Another useful addition is that of an audible indicator of field strength and this is done by using U2, a 4046 CMOS PLL (the non-HC version) with VCO. The voltage from pin 7 of U1B is fed into the VCO tuning pin of U2 and as the signal strength goes up, so does the pitch of the tone generated.A few more construction details:
With the components as shown, the pitch of the tone varies from about 1.2 kHz at zero scale to approximately 2.5 kHz at full scale - but these values may be easily adjusted: Increasing the value of R20 or C5 will lower the pitch while resistive scaling of the voltage at pin 9 (using, say, a 10k-100k pot) will reduce the frequency swing. (Another resistor may be added at pin 12 to change the frequency range, but you should refer to the 4046 data sheet for this information.) Note: It is recommended that a mylar or a good quality ceramic capacitor be used for C5 to avoid excessive pitch change with temperature. If you use a ceramic disc type, avoid one marked with a 3-character code beginning with a "Y" or "Z" (e.g. Y5P or Z5U)
The frequency output of the VCO is fed directly into a piezoelectric transducer - chosen for small size and light weight - which is mounted in the enclosure with a hole to the outside world aligned with the hole in its plastic case. This transducer must not be of the sort that beeps merely with the application of supply, but rather it's simply used as an electronic speaker. Note that the frequency range of this transducer is very limited, with very poor response below 1 kHz or so, hence the design range. For mounting the transducer, I simply drilled a hole in the enclosure that was about 1/3 larger than that in the transducer (it is important to avoid obstructing the hole as this can greatly reduce its efficiency) and used silicone to hold it in place. Typical transducers also have tabs, allowing them to be mounted with two small screws on the outside of the case, if that is your preference.
If you choose to use a normal speaker, be careful to place it such that its magnet does not skew the meter movement. Also note that U2 cannot directly drive a speaker: A simple resistor-capacitor-transistor circuit will be required to provide adequate drive. It is also worth noting that piezoelectric sounders such as this are incapable of reproducing audio much below 1 kHz. If the drive signal is lower frequency than this, one primarily hears harmonics of the drive signal as well as a signal based on the "click" of the rising/falling edge of the square wave drive signal.
You will note that the range of tone exceeds that of the meter: Even if the meter is pinned backwards or full scale, you may still be able to hear a pitch change with the differing signal strength. What this means is that if you are not using the meter visually, you could purposely "pin" the meter backwards - or set it upscale - as well as adjust the gain control (if you included it) to set the pitch range of the audio.
This circuit can run directly from the 9 volt battery and does not need to be regulated. Note that the pitch range will change somewhat with battery voltage, but unless you have perfect pitch and calibrate signal strength to a particular musical note this is unimportant. The worst-case current consumption of U2 was measured at 800 microamps at 9 volts and because of this I chose not to add an on-off switch just for the indicator: If there is a reason to mute the tone, a switch may be added, or a piece of tape (or a finger) can be placed over the hole to greatly reduce its loudness.
In Figure 5 (top) the circuit board for the audible indicator is mounted horizontally, just below the meter, while the piezoelectric sounder is on the bottom of the case, against the battery.
Figure 5 (bottom) shows details of the detector board, with the two diodes located directly below the disk ceramic capacitor, C1. While not easily visible, it is just possible to make out C2, a surface-mount 0.01 uF capacitor behind LED1 and C3 is the surface-mount tantalum below it. The sharp-eyed observer may note that schematic calls for C1 and C3 being 100 pF and 10 uF capacitors respectively, but these values are not critical: C1 could be anything from 68 to 220 pF while C3 could be anything from 4.7 to 22 uF. If you don't have a surface-mount capacitor for C2, try to keep the leads as short as possible - especially to the "RF Ground" and this will optimize sensitivity and high frequency response.Constructing the unit:
It is strongly recommended that this meter be housed in a shielded (metal) enclosure to assure that any signals registering on the meter are those that are entering via the antenna connector.
The unit was with the circuits on separate circuit boards, with the "detector board" being a small piece of double-sided glass-epoxy circuit board. Landings were simply cut onto this smaller board to isolate them and the components soldered down. The other board is the meter/amplifier board and this was a small piece of perfboard.
The antenna connection was simply via a BNC connector and short lengths of miniature coaxial cable connect the attenuator and the detector circuit.
If you use the MAX492:
Other than limiting the voltage on U1 to 7 volts (for the MAX492) with LED3, there is little voltage regulation that is required. The circuit should work fine with a battery voltage down to at least 6 volts. Note that the dropping battery voltage will also affect any calibration done to the unit.
If you use the LMC6482:
Because the LMC6482 can easily tolerate through 15 volts, it may run directly from the 9 volt battery with no problems.
If you calibrate the unit in dBm or just want a bit of extra stability, you may wish to include a 5 volt regulator - in which case you would leave out LED3. A standard 78L05 will work but note that it will start to lose regulation when the battery voltage drops below 6.75 volts or so - a voltage at which the battery still has 1/3 of its life left.
If you want the ultimate in low-dropout regulation, a special low-current regulator like the LM2936Z-5.0 (available from Digi-Key and other places.) If you use this regulator, you will need to put a 1-10 uF capacitor on the battery (input) side of the regulator and a good-quality 100 uF capacitor on the output side of the regulator, located very close to it. Failure to add this large output capacitor may result in an oscillating regulator rather than a stable one. The cheaper LM2931Z-5.0 will also work, but it in itself consumes nearly as much current as the rest of the circuit - and this current goes up when the battery gets down below 5.5 volts or so.
Calibration of the unit:
If you have access to a calibrated signal generator, by all means, use it to test the general performance of the unit. On the unit that I constructed (using HP-2835 diodes) I started getting usable readings below -45 dBm with the detector starting to saturate above +10 dBm. In testing, I observed that the unit was flat to within +-6 dB (or better) from 1 MHz to 1 GHz with the attenuator switched out. As mentioned above, the attenuator's accuracy suffers badly above 500 MHz. In testing, the unit also responded to the small amount of leakage from a microwave oven (at approx. 2450 MHz) from well across the room.
If you are going to put calibration marks on the meter, it is best to have installed a low-power 5 volt regulator to eliminate that particular source of drift. When adjusting calibration, it is also best to do it at room temperature as one definite shortcoming of the simple log-amp circuit shown is that it is susceptible to temperature drift (because of D3 and D4) in terms of "dB per meter-unit," with the meter reading high at lower temperatures. (Note that this is not the same as "zero drift": The drift caused by D3 and D4 only affects the magnitude of the readings and not the zeroing.) For this reason, calibration will only be reasonably accurate at the temperature at which the calibration was performed (and within +- 10 degrees F.) If this unit is used simply as a field strength meter (and is not used for absolute measurements) then this calibration drift will not be much of an issue as all you need is a consistent indication of "stronger" and "weaker."
While it is certainly possible to construct a circuit that can
for some of this thermal drift, this was not done in order to preserve
the relative circuit simplicity. Note that some of this
drift" is also due to the fact that the intrinsic sensitivity of the
diodes (D1 and D2) will also change slightly over temperature, but this
effect is less than that of the drifting of D3 and D4.
If you do not have access to a calibrated signal generator, you'll have to do the best you can by adjusting R13 (with R16 - if you use it - to zero ohms) such that you get a full-scale (or nearly full-scale) meter reading when transmitting on an HT a few feet away.
In testing, the unit responded with "reasonable" flatness to about 1 GHz and became somewhat inconsistent in its flatness (with peaks and valleys) at 2.4 GHz. The unit continued to respond up through 12.4 GHz (the frequency limit of the available test gear) but its sensitivity was very erratic, no doubt owing to the fact that the internal wiring and layout is simply inappropriate for such frequencies.
Using the unit:
When first switching on the unit, always disconnect the antenna and zero the meter. Note that the meter zero will drift slightly after the unit is first powered up, and it will also drift slightly with temperature, so it is a good idea to check it periodically. Another thing that you will notice is that, with no signal present, the meter will "wiggle" slightly (the magnitude being of a needle width or less) at the bottom end of the scale. This is to be expected and is caused by the normal (and unavoidable) noise produced by any semiconductor device and is, in fact, the factor that limits the intrinsic sensitivity of this (or any) simple diode-based RF detector.
One of the first things that you'll notice is that the zero drift affects only the bottom-end of the detector range. This is true because of the nature of the log-amp: Weak signals are amplified while stronger ones are seemingly attenuated. Because the overall drift is fairly small, it only shows up in the "low end" of the readings.
With a small rubber duck connected, you'll likely observe a signal strength reading even if you don't happen to have a signal generator or transmitter nearby. Depending on your precise location, you will likely be detecting any nearby broadcast radio/TV stations (or other transmitters - including your own!) Note that because this meter is completely untuned, what you are detecting could be anywhere in the range of AM broadcast through cellular telephone frequencies. It is worth mentioning that, for this reason, you should not be talking on a cell phone while using it!
If you get close enough to the transmitter that the signal is consistently above half-scale to two-thirds scale, it may be time to switch in the attenuator. Experience has shown that, with the attenuator switched in, a nearly full-scale reading indicates that you are probably within an arm's length of the object of your search. In a test, the meter was connected to a 1/4 wave magnetic-mount antenna near the rear of the car roof and a 50 watt 2 meter signal was transmitted into a 5/8 wave mag-mount antenna at the front of the car roof (about 8 feet apart.) With the 20 dB attenuator switched in, the meter was "almost" pegged.
When using this (or any) field strength meter, you may want to have a choice of two different antenna:
While this unit has fairly good sensitivity at Cell, PCS, and wireless LAN frequencies (approx. 800, 1800, and 2400 MHz, respectively) don't expect it consistently register such devices very well. These devices typically use digital modulation schemes and, unless transferring a lot of data (or unless you are talking on it, in the case of a cell phone) their transmitters have a very low duty cycle.
Because this meter does not have peak reading capability, one may
see the meter "jump" briefly - if it moves at all. Additionally,
if you are very near a cell site, the transmit power from a cell phone
may be only a few microwatts as they transmit only as much
as they need. While peak-reading capability could have been added
to this meter, it would have made it more sensitive to things like
LANs and cell phones - possible sources of confusion when looking for a
hidden ham transmitter.
Comment on E-field sensitivity:
You might notice that R17 is shown as always being connected on S2, even when set to the "0 dB" attenuation setting. The reason for this is that without R17 (and, to some extent, R18 and R19 as well) as a swamping resistor, this circuit will act somewhat like an E-field detector, with the antenna input acting on even very high impedance signals such as those that might be seen using a standard 2 meter rubber duck in the presence of a strong field from an AM broadcast station or an HF transmitter.
If this connection were not made, one would likely notice more than a 20 dB drop in signal when the pad was switched in as those signals off-resonance (e.g. not 50 ohms) would be heavily swamped.
If you do not include the 20 dB attenuator, you may want to include a similar swamping resistor of 47-100 ohms (value not critical) to prevent an excessive E-field response. If, in fact, you don't mind this response, you may leave it out. Note that by not having something connected to the input that terminates your sense antenna in something resembling 50 ohms, you may not be taking advantage of your antenna's resonance and its bandwidth-limiting response and increase the likelihood of your detector "seeing" signals that are far off-frequency from your sense antenna's resonance.
There is little doubt that this field strength meter could be improved. A few possibilities:
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Note: Neither the author or UARC officially endorse any vendors or projects mentioned above. The level and satisfaction of performance of any of the above circuits is largely based on the skill and experience of the operator. Your mileage may vary.
This page originally created in 2004-2005,
updated on 20110531