RF Field Strength Meter

Relative Field Strength Meter

Fox    What is it? A radio frequency (RF) field strength meter is like an S-meter for a big chunk of the electromagnetic spectrum. The project described in the following paragraphs is an uncalibrated meter. Readings are relative and the display range is adjustable. In theory (and in advertisements) it is possible to measure antenna radiation patterns using a relative field strength meter, and to compare such observations to design-based expectations. This exercise might work if there were no trees, houses, fences, etc. in the way. Otherwise, it becomes rather difficult to carry out. Another different sort of application called ‘fox hunting’ was featured in a recent QST article.1 The suggestion was that once the hunter (using a radio direction finder) comes near enough to the fox (a hidden radio transmitter), a field strength meter can be used to zero-in on the fox’s exact location. —Perhaps so. 

    RF Exposure: This topic is an odd mixture of hocus pocus (tinfoil hats, cellphone stickers, etc.) and genuine scientific concern or interest. In regard to the latter, the US Federal Communications Commission (FCC) has recently revised its rules for how amateur radio stations should evaluate RF-exposure (ARRL FAQ). To be clear, the long standing safe exposure limits themselves have not changed, just the requirement as to how US licensed ham stations must evaluate compliance.

    When they are required, RF-exposure evaluations generally consist of making calculations from parameters such as transmit power, frequency of transmission, transmit duty cycle, and antenna gain. These calculations are made easier by the use of an RF Exposure Calculator. Direct measurement of RF fields is not practical for this purpose, at least not with the type of relative field strength meter described on this page. Even with a laboratory-grade calibrated instrument, direct assessment of RF exposure would be a formidable challenge (ARRL summary).

Field Strength Meter Schematic

    RF log amplifier: A remark by a fellow ham about the possibility of direct measurement as a means of compliance with the FCC RF-Exposure rule change sparked my interest in RF field strength meters.2 I had previously experimented with an Analog Devices AD8318 RF log amplifier chip in an unrelated context. Frequency bands that are designated for amateur radio operation range across the electromagnetic spectrum from long-wave to microwave, all the way to visible light. However, the most popular bands are located in the
approximately 1 to 30 MHz (High Frequency or HF) part of the spectrum. This makes the Analog Devices AD8307 (100 KHz to 500 MHz) an appropriate choice for the detector part of a general purpose relative field strength meter.

    The AD8307 features in several amateur construction circuits, including one retired kit from QRPGuys, as well as the aforementioned QST project. The present project
(schematic above) was chiefly based on the QRPGuys design (schematic at the penultimate page of the kit assembly manual). However, as far as I can tell, relative field strength meter circuits are all very similar to one another, each consisting of a detector-log amplifier followed by an additional amplification stage and some form of display. The detector part is basically a boilerplate circuit from the AD8307 datasheet. An op amp or common amplifier transistor circuit boosts the RF chip’s log voltage output to a level suitable for interfacing an analog or digital display.

PCB mounted to inside back of enclosure

     I should learn to make panel labels, or at least to make room for them! The top potentiometer (photo at top of page) is the gain control and the one below it is for setting the zero level of the meter. RF energy is everywhere, so it makes sense to set the meter at some low non-zero reading, to represent ambient RF (broad spectrum noise). The ‘zero’ potentiometer allows setting it to any value on the scale. The amount of amplification needed depends on the sensitivity of the meter (or other display). One LM358 easily drives a 200 μA meter.

    It might be possible to arrive at some physical interpretation of readings by inserting RF attenuators. One could observe and record changes in meter readings that correspond to different attenuation levels. But I doubt this would be of much use, unless the meter were to be calibrated for a specific narrow frequency band. Hint: Calibration is not easy! This article, for example, describes calibrating a field strength meter for the 2200 meter amateur band.

Field Strength from Nearby Antenna     At the time the circuit for this project was first assembled on a breadboard3 I had not completely abandoned the idea of its usability for RF exposure assessment. One exploratory exercise recorded relative RF field strength (microammeter readings) for a sequence of test FT8 transmissions on the 15 meter ham band. Transmit power ranged from 1 watt to nearly 100 watts. The backyard transmit antenna was aimed approximately orthogonal to the field strength meter’s location in the ham shack. For this ‘study’, gain and zero potentiometers were pre-adjusted to produce the range of meter deflections recorded.

    The trend depicted in the graph (left) is systematic but that doesn’t prove anything. Measurements (meter deflections) could not be correlated with meaningful physical units. Moreover, the ‘rubber duck’ antenna attached to the meter was designed for VHF use, a frequency range far above the 15 meter band. In fact, briefly triggering a 2-meter handheld transmitter in the next room causes the microammeter to go off-scale. —It is always possible, of course, to bring the meter within scale, as the gain potentiometer can be turned all the way to 0, which grounds input to the amplifier.

SMA Jack Adaptor    Enclosure: 3D-printed enclosures consume a lot of filament and print time. By design the enclosure bottom included a large rectangular cutout that could be covered by a piece of copper. (See PCB photo above.) To prevent the possibility of accidentally shorting the meter, an index card was taped to the copper on the inside before final assembly. I had thought of shielding the entire box. It would be fairly easy to cover a 3D-printed enclosure or sub-enclosure with adhesive copper tape, but that did not seem necessary. One trick I will mention: The SMA jack is a PCB mount type. To attach it to the panel I made a two part adapter. The back part has holes for the center conductor and four corner ground posts, while the front part has a single hole for the SMA female connector (right).

Holes Test Piece     One more 3D printing trick: I made a small test piece (left) for the potentiometer shaft hole diameter and tab size/distance, as well as the power-switch shaft diameter, square cutout for the SMA jack, and indicator LED hole. A lot less filament is wasted in reprinting a small test piece than in reprinting an entire panel. The meter bezel (translucent part in image at the top of page) is an artifact of this same concept. I test-printed the meter hole and mounting screws placement, then decided to use the test piece as a bezel. The minimum depth of the enclosure is determined by the depth of the meter. Thus, enclosure depth can be reduced by the thickness of the bezel, which is another small advantage.
SMD version of AD8307
   Miscellany: The most expensive components of the present project are the AD8307 and the microammeter (if you don't have one).4 Early in the project I thought that I had fried the AD8307 (DIP version) from Mouser, and therefore I searched for a less expensive replacement. This led to ordering a 3-pack of an SMD IC from eBay ($5 for 3 ICs plus shipping). I replaced the presumed dead AD8307 with one of the eBay chips (using an SMD to through-hole adapter), and after doing so, found my error—it was elsewhere! The replacement was not needed, but it was good to know the AD8307 is available in SMD form at much less cost, and that the SMD is functionally equivalent to the DIP.

    Demo video: RF_Field_Strength_Meter.mp4

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1. White, W (2021). A Sensitive Field Strength Meter for Foxhunting, QST 105(7), 34-35.
2. Ron Davis (K4TCP) private communication.
3. The circuit was not exactly the same, but equivalent from a practical or functional standpoint.
4. A length or loop of wire can be used for the antenna if a small whip is not on-hand.

Project descriptions on this page are intended for entertainment only. The author makes no claim as to the accuracy or completeness of the information presented. In no event will the author be liable for any damages, lost effort, inability to carry out a similar project, or to reproduce a claimed result, or anything else relating to a decision to use the information on this page.