FM experimenters have only the field strength guidelines so measuring field strength is a critical task. The maker of this kit says “This Field Strength Meter has been specially designed for very low power transmitters and will assist enormously in adjusting and tuning for optimum output.”
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Learn more about this kit and get ordering info here:
NOTE: This vendor is loacated in Austrailia, and scwis has ordered from vendors there several times. The orders show up in about a week, and are sent USPS priority mail. Inexpensive and painless. I’ll make a note to write up Aussie sources for a future entry.
A Couple of Caveats?
1. The website for this meter says “It requires about 300mV signal to start the process and with a gain of about 12 on the RF transistor, we need about 30 millivolts developed on the antenna circuit to start the detecting process. This makes the Field Strength Meter only sensitive to nearby signals and prevents weaker signals from upsetting the reading.”
Presumably this means that the actual field strength in mV/m (= millivolts/meter) could be greater than 30 — which already is 120 times higher than the maximum Part 15 FM field of 250 µV/m (=microvolts/meter) at 3 meters.
2. There is no means given to calibrate the meter, so it couldn’t be used to determine compliance to Part 15 FM limits.
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Manufacturer Confirms Rich’s Observation
scwis contacted the MK1 Manufacturer and asked:
> Would the MKl, or any other product you offer, be
> useful for measuring 100MHz signals in the hundreds of
> micro-volts?
To which http://www.talkingelectronics.com.au replied:
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The Field Meter MkI is not a particularly sensitive device and has been primarily designed to measure the output of our bugs and compare one bug against another.
Our bugs generally have a range of 200 – 800 metres using a 3m antenna and the consumption is about 7mA @ 3v to 15mA @ 9v. This gives you some idea of the output of our bugs and the size of the antenna.
We have never worked out any other values as we try to keep things simple in our articles and hobbyists just want to get the longest range with the lowest power consumption.
The Field Strength Meter MkI has been specially kept insensitive so that it does not compete with our Bug Detector Mk1 that sells for $150.00
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I think http://www.talkingelectronics.com.au sould be complimented for their honest and prompt answer to my question. I think they’ve provided great guidance in using this product.
Both the tutorials site and the products they offer look interesting and fun, though far beyond Part 15 power levels.
Great observation, Rich – don’t use it for compliance!
Experimental broadcasting for a better tomorrow!