I know that this has been discussed before, but I was hoping that someone with the appropriate experience would give me a short list of potential Field Strength Meters for both AM and FM Part 15 broadcasting. I'm probably more interested in FM, since that's the band I primarily use. Although it would be nice to also get something that could measure the field strength of a 13.560 signal.
forget it. a calibrated f/s meter whos readings will be accepted by the FCC (or in your case IC) costs in the $1000's for a used solid state one. you have a calibrated motorola, IFR or HP SA with the appropriate calibrated EMC antenna's for your frequencies of interest or on AM PI FIM 21 or 41 or NC 120E, on FM PI FIM-71. the 60 y/o NC 120E costs $400.00 used and only goes to 1600 khz the FIM21 about $2-3k the FIM 41 and 71 about $5k+ USED!!!!
a decent calibrated SA and EMC antenna's will go about $5k for Calaibrated SA (USED) and New antenna's for your 3 bands of interest.
the new cost on many of these items can easily reach $20k+
A probable concern with those meters for usefulness to Part 15 operators is...
There is no provision to tune the meter to a specific frequency. It receives/measures some net r-f waveform consisting of all the r-f fields arriving at its probe up to a frequency of 3,000 MHz.
So for example, if one wanted to measure a 250 uV/m field from a Part 15 FM transmitter, the reading would include all the radiated fields from all of the AM and FM broadcast stations arriving at the probe -- the sum of which would be many times greater than the 250 uV/m field expected from the Part 15 FM transmitter.
From http://www.emctd.com/documents/Smart%20Fieldmeter%20Brochure.pdf
add one of these https://en.wikipedia.org/wiki/Anechoic_chamber to the expensive list of required equipment. which now brings us to the question of how deoes the FCC truly know they are measuring your signal and not another stations?
That's useful information, Rich, about the meters I linked; which I did not previously grasp.
I spent a moment wondering what could be the usefulness of such an amalgamous meter, and the only thing I can think of is for RF safety level measurements around towers and buildings.
Is this correct, and do they have any other earthly use?
how deoes the FCC truly know they are measuring your signal and not another stations?
They use FI meters tuned to the frequency they are measuring. There are techniques that will determine whether the field they measure is significantly contaminated with signals from other than the source they are measuring.
Carl - yes it would be most useful for measuring high fields fairly close to the radiator.
I kind of figured that this thing would be expensive. And these instruments would certainly not be in the studio of a typical Part 15 broadcaster. Thanks for all the info.
But it also makes me question ever further recent events with the FCC. Part 15 certification allows manufacturers to sell transmitters, yes, but that's primarily a bureaucratic thing. Certification also should be a validation to the user and the FCC, if installed according to the manufacturer instructions (which are supposed to form part of the certification), that you are operating legally. Otherwise, what's the point?
If the FCC is going to ignore certification, and few can afford the instrumentation necessary to validate field strength, then everyone might as well use non certified gear, use best judgement for legality and buy a lottery ticket (i.e., hope that no one complains).
Thankfully, at least for Canadians, Industry Canada isn't following the same path that the FCC appears to be taking. Although I think I'll be keeping my eye open for a FIM at a reasonable (!?) price, just in case that changes as well.
I am a chief engineer for commercial radio and work for both a 100,000 watt FM and a 5,000 watt AM that is directional at night. I own and use regularly a Nems Clarke FIM 120E for FI readings for the directional station. I've done this for about 35 years now.
You are correct that even a used, beat up, uncalibrated NC FIM is expensive. Not to metnion they use TUBES and 67 1/2 volt batteries! But they ARE accurate, and mine is regularly sent out for certified calibration and has been used on several occasions with the FCC on site verifying FI readings for the directional array for the AM station.
I just finished doing a rather extensive although somewhat random set of FI readings of my Part 15 station. Soon as I track down a decent map of the area I'll have to post it somewhere. Very interesting. Tracking a tiny signal is a lot different than tracking 5000 watts! But I was quite pleased with my flea power results. I run my Part 15 at 1620 and my FIM just tunes that high with a bit of room to spare.
Sadly it's not really possible to do any sort of field intensity readings that actually mean anything without a real device. AGC in a typical radio makes reception testing meaningless. A car is a long way from an in home radio. But I'll do a report on my FI testing when I have a bit of time. A real FIM actually tells you how much current is in the air at a given spot. Amazing to see the effects of signals being blocked and signals being reflected. Just moving a few feet one direction of the other can make a noticeable difference.
You might be able to find a local engineer, who out of curiosity may do some readings for you. Although we're getting harder to find as stations go corporate and have corporate engineers taking care of dozens of group stations. But maybe! I'd be glad to do it for anyone in the area if you cover my gas and maybe a box of donuts -- but I'm in northern Minnesota, probably not to many part 15 folks up here!
Tim in Bovey
Iron Range Country