Power and Field Strength

Knowing and understanding the math is an important skill for a part 15 operator, but the other difficulty is knowing how to verify compliance with 15.239 without $20,000 test equipment. That's where we get stuck in a rut.

Back when Rich advised using certified FM transmitters for confidence in compliance, I have no doubt he believed in the certification process and trusted it was upheld by manufacturers and the FCC. That's what I wanted to believe, and was proudly running my certified C.Cranes.

Now that Tim in Bovey has found the unexpected over-power of so-far two certified transmitters, we are all standing here with paddles but no boats.

Don't blame us for having wet socks.

The math above was intended to help as we kick around various things regarding possible rule changes and it certainly doesn't address the politics or problems we hobbyists have with trying to be compliant with FM rules.

Carl, you have pointed out, and I agree, that a positive change would be some way that the user can tell if they are in compliance. We really caught a break with 15.219 in that with a $4 (sometimes free) DVM from Harbour Freight and a little electronics knowledge and a tape measure we can determine compliance.

Now, as you mentioned, we find that certification is not as reliable as we had hoped. The calculations I demonstrated do nothing to solve this dilemma.

Even if the FCC agrees to an increase in field strength such as the Canadian rules we would still be faced with the same problem of knowing our field strength. Perhaps we need to find another way to specify compliance and get it written into the rules, something which is within the capability of modestly equipped hobbyists to implement.

Neil

This may sound silly but if you going to continue to use field strength you would have to use an affordable Radio and the makers would always have to manufacture that some model Radio until the end of time. For example the FCC uses a Grundig 450 to state that a compliant part 15 transmitter under the BETS rules would go 1,000 feet. So if your transmitter went 1,000 feet and 7 inches your illegal. Or say it went 1,010 feet your illegal is more like it. However the Radio would have to be an affordable Radio for all. A Sony Boombox for $50 could be the official Radio for measuring part 15 compliance for the average joe. I get why they use field strength instead of power for FM but what we have to figure out is a simple solution and that would be 100 uVm @ 30 meters. At least it will give far more range for the hobbyist to work with which could get up to 1,000 feet.

" For example the FCC uses a Grundig 450 to state that a compliant part 15 transmitter under the BETS rules would go 1,000 feet."

Huh?  Where did that come from?

Is that saying then that if your transmitter has a certain field strength at 1 mile, and you increase power 4x, then you'll get the same field strength at twice the distance?

However, if you keep the transmitter power the same and go from 1 to 2 miles, then the radio gets a fourth of the field strength that it did at 1 mile?

That makes it seem like running low power is more efficient. Maybe transmitters should be spread around a city and linked, instead of having one big centrally located transmitter for AM.

Maybe what we really need for Digital radio is a whole new system that's cellular in nature and operates like SiriusXM when it hands off between satellite and various terrestrial repeaters, just keep out the satellite and stay with the cells on the ground.

"Is that saying then that if your transmitter has a certain field strength at 1 mile, and you increase power 4x, then you'll get the same field strength at twice the distance?"

Sort of. There are simplifying assumptions used, or another way to say this is that other factors are being ignored such as ground absorption, reflections, height above ground, near field, and far field. The math applies to signals in a "free field".

Neil

Nate, we already have multiple things like that.  The internet.  Plus the cellular network.  Right now anyone can go out and receive Artisan Radio's stream on their cell phone.  Or computer.  Or internet radio.  All assuming a connection to the Internet of course.  Plus most people use their cell phones and computers more than radios these days.

And if you really wanted to, one could build a local Intranet to get great coverage and still be meeting Part 15 rules.  The FCC (and Industry Canada) allows 1 watt on wireless frequencies, with up to a 6db gain antenna.  Take some commonly available dual radio nodes, add a mixture of even more common access points, and voila - you have your own little local Internet that you could stream your radio station to.

The biggest difficulty with the latter is not technical - it's relatively easy to build an Intranet.  It's not even cost, assuming that you can afford a Rangemaster or ProCaster.  But it's logistics - finding places to install these things, getting power to them and working out who pays for it - even issues such as insurance (assuming that you're mounting them up high on a rooftop).  Etc.

But I suspect you'll get a lot more range with something like this a lot sooner than you will going after Part 15 rule changes for either AM or FM.

And anyone who says that this isn't radio (a common theme on the 'professional' web sites- 'streaming isn't radio') is fooling themselves - it's just a different, already existing and common type of digital format.  That is relatively low cost to implement.  And that people can receive without having to buy anything over and above what they already have.