Milliwatt meter?

[TechTot]

What strategies are folks using for measuring and adjusting for 25mW on the FM25c? I don't want any trouble. [img]smile.gif[/img]

~TT~

[Joe Chezk]

I made a small 1/4 watt dummy load, and then on the hot side of the resistor which is connected across the RF out of a micro transmitter, I have a 1N34 diode, and connected to the top side of diode there is a small ceramic cap to ground, value not critical. From the junction of the cap and diode, connect a dc volt meter set to a range of 3 volts or less. Use the volt meter reading and the formula watts = E squared over R. R being the 50 ohm load, example is 1 volt will =20 MW

[rfry]

Two issues with the resistor/diode/cap/meter idea:

1) Part 15 FM compliance is shown by measuring the field strength near the antenna for a maximum field of 250 uV/m, 3 meters in any direction from it. The input or output power of the tx is not covered by FCC Rules for Part 15 FM.

As has been posted here already, the r-f power necessary to generate that legal field that can be radiated from almost any antenna is something like 1/1,000,000 of the power produced by a tx rated at 25 mW.

2) The tx may not provide the same power to the real load impedance provided by whatever antenna is connected to it as it will into the resistor & network used in the test.
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[ June 08, 2006, 03:53 PM: Message edited by: Richard Fry ]

[TechTot]

Ahhh. Good point on field strength vs. rf output. Does anyone know of a good tutorial on the subject of measuring field strength?

~TT~

[Joe Chezk]

If you want to meet the Part 15 rules for FM, then just remember if your transmtter cant transmit more than five feet, then your legal.

[rfry]

Originally posted by Diode:
If you want to meet the Part 15 rules for FM, then just remember if your transmtter cant transmit more than five feet, then your legal.

It's not as grim as you state. Here is a link to plots for various peak radiated powers from a 1/2-wave dipole. The legal field defined for Part 15 FM is shown by the red line there.

This data shows that a good FM receiver with a good antenna and a clear transmission path should be able to receive a fully legal Part 15 FM setup for a radius of around 175 meters (~ 575 feet).

http://i62.photobucket.com/albums/h8...0/6417f684.gif

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[JOHN ORTIZ]

Hi Mr. Fry, are there any "table" for the long of the FM whip antenna, for especific frecuency?
Thank, JOHN

[DigitalJunkie]

This,in my opinion,is the problem with the Part-15 rules.
We all know the 250uV/meter rule.(or should!)
But how does a hobbiest go about measuring this,to be sure he's compliant? Sure,you can get a Field-strength meter,but it would need to be calibrated,and perhaps even FCC certified.
Certified/Calibrated Field-strength meters are expensive,out of grasp for anyone considered a "hobbiest".. Whats the point of buying a $2K meter for a $100 transmitter anyways?!

So how can "Average Joe-Radio" comply?
It seems it's a Catch-22.

Thus,I say- Run with the 25mw,put up your antenna,use a clear frequency,and clean content. Nobody will be the wiser. Everybody here does it on a daily basis,and so far I think there's only been one or two people that got a letter from the FCC,probably both tipped off for offensive content,etc.

If I were to "get the visit" I'd simply state the above,and ask them if they would help me get everything setup correctly since he's already there,and has a Field-strength meter in hand. Take advantage of the opportunity,and let him know that you're more than willing to comply,if they'll lend a FSM for a few minutes,so that your *able to*.

Obviously if you're *WAY* over the limit (running an amp,etc.) they'll know,and the above plan might not work out as you'd wish.
Your best bet is to comply,and fast -they'll be back in a few more days to check again,if you're still way over the limit,you're screwed! They only give you 3 chances,at the most.
That is,if you even get a warning..If they see power levels that high they'll probably just send the "Cease and Desist" letter. (pull the plug or you're in for it!)

[ June 09, 2006, 02:17 AM: Message edited by: DigitalJunkie ]

[radio8z]

A Part 15 FM broadcaster probably cannot measure the field strength of their transmitter to verify compliance, but a reasonable guideline is that the signal should fade into noise about 200 feet from the transmitter.

Neil

[rfry]

John Ortiz: are there any "table" for the long of the FM whip antenna, for especific frecuency?

DigitalJunkie: We all know the 250uV/meter rule.(or should!) But how does a hobbiest go about measuring this, to be sure he's compliant?

For Part 15 FM, usually the legal field at 3 meters will be produced by a random, short wire, probably a foot or less in length. The field it can produce will depend on how much power the tx can supply into that wire, much more so than the length of the wire itself. So this is a system issue. There is no single answer or table possible, because Part 15 FM txs differ in the amount of power they will deliver into mismatched antennas. The length of the antenna has to be determined experimentally for that tx, by measuring field strength.

This situation was recognized by the FCC when they set up a certification process for Part 15 FM. The process includes professional field strength measurements when the tx is using an antenna defined and supplied by the system manufacturer.

So if a Part 15 certified FM tx+antenna system is used in the same configuration as in the certification process, its compliance has been pre-determined.

If a user builds his own Part 15 system, or changes a certified one, then Part 15 compliance becomes that user's responsibility, and proving that compliance is technically difficult.

But as shown in the graph I linked to earlier in this thread, even a simple dipole, j-pole, or ground plane antenna radiating all of the power that can be supplied by a typical Part 15 FM tx (10 mW and more) produces fields that are far, far above the Part 15 limit.

It's everyone's own choice how they operate. This information just provides some background to use when making that decision.
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