A Wideband Field Strength Meter

One of the more common pieces of equipment that many amateur operators use is the field strength meter. Most typically consist of a diode detector (often germanium for sensitivity), followed by a micro-ammeter, and usually are limited to frequencies in the HF region, although some VHF and even UHF versions do exist. In addition to measuring the output of a transmitter, the field strength meter is also useful for antenna radiation pattern measurements, hidden transmitter locations (such as in a fox hunt), and general RF detection.

This month we would like to introduce you to a more modern version of the field strength meter, one that covers the range from 10 kHz to over 1 GHz (1000 MHz) and is almost as simple to build as its many predecessors.

Our circuit uses the LinearTechnology LTC5507, which is described by the company as an Power Detector.”It consists of a Schottky diode peak detector and gain of 2 buffer amplifier all contained in a tiny SOT type surface mount package. The chip operates from 2.7 to 6 volts DC and therefore can be powered from a couple of AA batteries. The sensitivity of the circuit extends from-34 dBm (about 1/2 microwatt) to +14 dBm (about 20 milliwatts) over the entire frequency range. The resulting DC output has a frequency response of DC up to about 1.5 MHz, so it can even be used as a wide-band AM detector, if you wish. Fig. 1 is a graph showing the input power vs. output DC level (in millivolts) over the entire range of the chip. While not particularly linear, the range is quite impressive.

Fig. 2 is the basic operating schematic of the LTC5507. C1 couples the input to the chip and should be chosen for the frequency range desired. It should be a ceramic type for best results, and you must be careful of self-resonances, particularly at high frequencies. Remember, if you use a 0.1 uF, for example, it does not take much lead length to resonate above a few tens of MHz. As a result, it is not a good idea to use any sort of electrolytic capacitor, since the internal winding of the foil elements in the capacitor package is inductive and will certainly resonate at some frequency over the wide range of the device. C2 is the filter capacitor for the output of the peak detector and will affect the speed of response of the output. As a rule of thumb, both C1 and C2 should be selected according to the following guidelines suggested by Linear Technology:

A Wideband Field Strength MeterC1 = C2 = 1/30 x F

F is the lowest frequency desired in MHz, and both C1 and C2 are in uF.

Of course, you can always experiment with these values.

Fig. 3 is the schematic of a complete field strength meter covering the entire 10-kHz to 1-GHz range using the LTC5507. You will note that we have coupled the input to an adjustable telescoping whip antenna and the output to a 100-microampere panel meter along with a variable ”control. Power is provided by a couple of AA or AAA batteries, and the entire device is mounted in a small aluminum enclosure. Power drain is very low, and the batteries will last for many hours of measurements. Due to the small size of the components and the frequencies involved, try to keep all input leads as short as possible. We routinely use individual strands from common 16 stranded house wire, and they are perfect for the task. A free 1-foot ”from a local home center will provide enough jumpers for numerous experiments. Finally, be sure to download the data sheet from the Linear Technology website for more information.

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