The Utah Amateur Radio Club:

 Highly sensitive and selective
Field Strength Meter system

by Clint Turner, KA7OEI

The field strength meter shown connected to the (modified) HT.
Figure 1:  The Field Strength Meter (Mark II) and interconnect cable with the modified Icom IC-2A/T
Click on image for a larger version.


A field strength meter is a very handy tool for locating a transmitter.  A sensitive field strength meter by itself has some limitations, however:  It will respond to practically any RF signal that enters its input.  This property has the effect of limiting the effective sensitivity of the field strength meter, as any nearby RF source (or even ones far away, if the meter is sensitive enough...) will effectively mask the desired signal.

This property can be mitigated somewhat by preceding the input with a simple tuned RF stage and, in most cases, this is adequate.  A simple tuned circuit does have its limitations, however:

An obvious approach is to use a receiver.  While many receivers have "S-meters" on them, very few of them have meters that are truly useful over a very wide dynamic range, most firmly "pegging" even on relatively weak signals.  While an adjustable attenuator (such as a step attenuator or offset attenuator) may be used, the range of the radio's S-meter itself may be so limited that it is difficult to manage the observation of the meter and adjusting the signal level to maintain an "on-scale" reading.

Another possibility is to modify an existing receiver and interface it with something like the Wide Dynamic Range Field Strength Meter - and that is what is discussed here.

Picking a receiver:

When I decided to take this approach, I began looking for a 2 meter (the primary band of interest) receiver with these properties:

Based on a combination of past familiarity with various 2 meter HTs and looking at prices on Ebay, at least three possibilities sprang to mind:

Close-up view showing the buffer circuit mounted atop IC1
Another close-up view showing the buffer circuit mounted atop IC1
Figure 2 (top) and Figure 3 (bottom):  Two different view of the JFET buffer circuit tacked atop IC1.
Click on either image for a larger version.

Each of these radios is a plain, thumbwheel-switch tuned synthesized, plain-vanilla radio.  As you might have guessed, I chose the Icom (it is also the most common) and obtained one on Ebay for about $40 (including accessories) and another $24 bought an IC-8, an 8-cell alkaline battery holder (from Batteries America) that is normally populated with 2.5 amp-hour NiMH cells.  I will often use this radio as a "listen around the house" radio since it will run for days and days!

Modifying the IC-2A/T (and circuit descriptions):

This radio is the largest of those mentioned above and has a reasonable amount of extra room inside its case for the addition of the few small circuits needed to complete the modification.  When done, this modification does not, in any way, affect otherwise normal operation of the receiver:  It can still be used as it was intended!

An added IF buffer amplifier (see figure 7, below):

This radio uses the Motorola MC3357 (or an equivalent such as the MP5071) as the IF/demodulator.  This chip takes the 10.7 MHz IF from the front-end mixer and 1st IF amplifier stages and converts it to a lower IF (455 kHz) for further filtering and limiting and it is then demodulated using a quarature detector.  Unfortunately, the MC3357 lacks an RSSI (Receive Signal Strength Indicator) circuit - which partly explains why this radio doesn't have an S-meter, anyway.  Since we were planning to feed a sample of the IF from this receiver into our field strength meter, anyway, this isn't too much of a problem.

We actually have a choice to two different IFs:  10.7 MHz and 455 kHz.  At first glance, the 455 kHz might seem to be a better choice as it has already been amplified and it is at a lower frequency - but there's a problem:  It compresses easily.  Monitoring the 455 kHz line, one can easily "see" signals in the microvolt range, but by the time you get a signal that's in the -60 dBm range or so, this signal path is already starting to go into compression.  (-60 dBm is about the strength that one gets from a 100 watt transmitter that is clear line-of-sight at about 20 miles distant, using unity-gain antennas on each end.)

The other choice is to tap the signal at the 10.7 MHz point, before it goes into the MC3357.  This signal, not having been amplified as much as the 455 kHz signal, does not begin to saturate until the input reaches about -40 dBm or so, reaching full saturation by about -35 dBm.  One point of concern here was the fact that at this point, the signal has less filtering than the 455 kHz, with the latter going through a "sharper" bandpass filter.  As it turns out, while the filtering at 10.7 MHz is a bit broader, the 4 poles of crystal filter do attenuate  a signal 20 kHz away by at least 30 dB - so unless there's another very strong signal on this adjacent channel, it's not likely that there will be a problem.  As it turns out, the slightly "broader" response of the 10.7 MHz crystal filters is conducive to "offset tuning" - that is, deliberately tuning the radio off-frequency to reduce the signal level reading!

To be able to tap this signal without otherwise affecting the performance of the receive requires a simple buffer amplifier and a JFET source-follower does the job nicely.  Consisting of only 6 components (two resistors, three capacitors and an MPF102 JFET) this circuit is simply tack-soldered directly onto the MC3357 as shown in figures 2 and 3.  This circuit very effectively isolates the (more or less) 50 ohm output load of the field strength meter from the high-impedance input to the MC3357, and it does so while only drawing about 700 microamps which is only 3-4% of the radio's total current when it is squelched.

As can be seen from the pictures (figures 2 and 3) all of the required connections were made directly to the pins of the IC itself, with the 330 pF input capacitor connecting directly to pin 16, supply voltage being pulled from pin 4, and pins 12 and/or 15 being used to get the ground connection.  A word of warning:  Care should be taken when soldering directly to the pins of this (or any) IC to avoid damage.  It is a good idea to scrape the pin clean of oxide and use a hot soldering iron so that the connection can be made very quickly.  Excess heat and/or force on the pin can destroy the IC!  It's not that this particular IC is fragile, but this is care that should be taken.  As can be seen from the picture and schematic, there is also a required inductor (value not critical - use anything from 10 to 100 uH) that is mounted at the microphone connector.
(tope) - Showing the general layout and cable routing of the modifications.
(bottom) - Close-up of the connection of the I.F. carrying coax to the mic connector.
Figure 4 (top):  Overview of the inside of the modified IC-2A/T showing the connection to the buffer circuit and cable routing.
Figure 5 (bottom):  Close-up of the connection of the coax to the microphone connector.  The 10-100 uH blocking choke is the green resistor-like component under the orange wire.
Click on image for a larger version.

Getting the IF signal outside the radio:

The next challenge was getting our sampled 10.7 MHz IF energy out of the radio's case.  While it may be possible to install another connector on the radio somewhere, it's easiest to use an existing connector - such as the microphone jack.

One of the goals of these modifications was to retain complete function of the radio as if it were a stock radio, so I wanted to be sure that the microphone jack would still work as designed, so I needed to multiplex both the microphone audio (and keying) and the IF onto the tip of the microphone connector.  Because of the very large difference in frequencies (audio versus 10.7 MHz) it is very easy to separate the two using capacitors and an inductor:  The 10.7 MHz IF signal is passed directly to the connector with the series capacitor while the 10.7 MHz IF signal is blocked from the microphone line with a small choke.

The buffered IF is conducted to the microphone jack using some small coaxial cable:  RG-174 type will work, but I found some slightly smaller coax in a junked VCR.  To make the connections, the two screws on the side of the HT's frame were removed, allowing it to "hinge" open, giving easy access to the microphone connector.  The existing microphone wire was removed and the choke was placed in series, with the combination insulated with some heat-shrinkable tubing.  The coax from the buffer amp was then connected directly to the "tip" of the microphone connector.  One possible coax routing is shown in Figure 4 but note that this routing prevents the two halves of the chassis from being opened in the future, unless it is disconnected from one end.  If this bothers you, a longer cable can be routed so that it follows along the hinge and then over to the buffer circuit.  Note:  It is important to use shielded cable for this connection as the cable is likely to be routed past the components earlier in the IF strip and instability could result if there is coupling.

Interfacing with the Field Strength meter:

Using RG-174 type coaxial cable, an adaptor/interface cable was constructed with a 2.5mm connector on one end and a BNC on the other.  One important point is that a small series capacitor (0.001uF) is required in this line somewhere as a DC block on the microphone connector:  The IC-2A/T (like most HTs) detects a "key down" condition on the microphone by detecting a current flow on the microphone line and this series capacitor prevents current from flowing through the 50 ohm input termination on the field strength meter and "keying" the radio.

Dealing with L.O. leakage:

As soon as it was constructed, I observed that even with no signal, the field strength meter showed a weak signal (about -60 to -65 dBm) present whenever the receiver was turned on, effectively reducing sensitivity by 20-25 dB.  As I suspected, I determined that this signal was coming from two places:

The magnitude of these signals was about the same, roughly -65 dBm or so.  Now the VHF local oscillator would be very easy to get rid of:  A very simple lowpass filter (consisting of a single capacitor and inductor) would adequately suppress it, but the 10.245 MHz signal poses a problem as it is too close to 10.7 MHz to be easily attenuated enough by a very simple L/C filter without affecting it.

Fortunately, with the IF being 10.7 MHz, we have another (cheap!) option:  A 10.7 MHz ceramic IF filter.  These filters are ubiquitous, being used in nearly every FM broadcast receiver made in the past 20 years, so if you have a junked FM broadcast receiver kicking around, you'll likely have one or more of these in them.  Even if you don't have junk with a ceramic filter in it, they are relatively cheap ($1-$2) and readily available from many mail-order outlets.  This filter is shown in the upper-right corner of Figure 7, below.

Typically, these filters have a bandpass that is between 150 kHz and 300 kHz wide (depending on the application) at their -6 dB points and will easily attenuate the 10.245 MHz local oscillator signal by at least 30 dB.  With this bandwidth, it is possible to use a 10.7 MHz filter (which, themselves, vary in exact center frequency) for some of the "close - but not exact" IF's that one can often find near 10.7 MHz like 10.695 or 10.75 MHz.  The only "gotcha" with these ceramic filters is that their input/output impedances are typically in the 250-350 ohm area and require a (very simple) matching network (an inductor and capacitor) on the input and output to interface them with a 50 ohm system.  The values used for matching are not critical and the inductor could be anything from 1.5 to 2.2 uH without much impact of performance (other than a very slight change in insertion loss.)
Image showing the inline bandpass filter.
Figure 6:  A close-up of the interconnect cable and the 10.7 MHz bandpass filter.  The circuit was constructed in a small enclosure made of circuit board material and a piece of #12 wire was soldered to it, providing an anchor point for the strain reliefs on the coax.
Click on image for a larger version.

While this filter could have been crammed into the radio, I was concerned that the L.O. leakage might find its way into the connector somehow.  Instead, this circuit was constructed "dead bug" on a small scrap of circuit board material with sides, "potted" in thermoset ("hot melt") glue and it can covered with electrical tape, heat shrink tubing or "plastic dip" compound, with the entire circuit installed in the middle of the coax line (making a "lump.")  Alternatively, this filter could have been installed within the field strength meter itself, either on its own connector or sharing the main connector and being switchable in/out of the circuit.

With this additional filtering, the L.O. leakage is reduced to a level below the detection threshold of the field strength meter, allowing sub-microvolt signals to be detected by the meter/radio combination.

Operation and use:

When using this system, I simply clip the receiver to my belt and adjust it so that I can listen to what is going on.

There's approximately 30 dB of processing gain from the antenna to the 10.7 MHz IF output - that is, a -100 dBm signal on the antenna on 2 meters will show up as a -70 dBm signal at 10.7 MHz.  What this means is that sub-microvolt signals are just detectable at the bottom end of the range of the Field Strength meter.

The major advantage of using the HT as tunable "front end" of the field strength meter means that the meter has greatly enhance seletability and sensitivity - but this is not without cost:  As noted before, this detection system will begin to saturate at about -40 dBm, fully saturating above -35 dBm - which is a "moderately strong" signal.  In "hidden-T" terms, it will "peg" when within a hundred feet or so of a 100 mW transmitter with a mediocre antenna.

When the signals become this strong, you can do one of several things:

If you want to be really fancy, you can build the 10.7 MHz bandpass filter and add switches to the field strength meter so that you can switch the 20 dB of attenuation in and out as well as routing the signal either to the receiver, or to the field strength meter (using a resistive or hybrid splitter to make sure that the receiver gets some signal from the antenna even when the field strength meter is connected to the antenna.)
Schematic of the field strength meter.
Figure 7:  Schematic of the buffer circuit and 10.7 MHz filter (upper right.)  Added components are within the dashed lines.
Click on image for a larger version.

Additional comments:

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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 created in 2005 and updated on 20110531