From another thread: Here's something else to speculate on, I wonder if anyone's tried 100 milliwatts into a full length broadcast antenna tower as a test, and how much range did it get?
A typical 1/4-wave vertical tower system used by AM broadcast stations produces an inverse distance groundwave field of about 305 mV/m at 1 km, for 1000 watts of applied power. This has been proven by thousands of real-world measurements using accurately-calibrated FI meters, and as predicted by mathematics.
If the power applied to that system was reduced to 100 mW, the inverse distance field at 1 km would drop by 40 dB -- to about 3 mV/m.
A MW field of about 150 µV/m can produce a fairly useful signal to receivers in locations with low radio noise levels. If earth conductivity is average (5 mS/m) then the 150 µV/m groundwave from this 100 mW system using 1650 kHz would be located about 4 miles away.
This is more than speculation, but hopefully more useful, also.
there was a guy in northern NJ (and as far as i know he is still running it) who had access to a full 1/4 wave tower at an abandon AM site. from my understanding he ground mounted a Hamilton rangemaster near the base of that tower and i think bonded the tower and ground to the hamilton rangemaster. i do not know what kind of range he was getting just that it was done. i would have to assume the results were worth the effort since he had it running for a very long time and very well might still have it running.
This may be a stupid suggestion but my next door nabor has a cell tower in his back yard. What would happen if you stuck a rangemaster at the top of that? Not sure that would be legal though.
1) corporations lawyers will never let you do it without a properly grounded installation if they let you at all.
2) it would not be legal when grounded at the top of even the smallest cell tower.
Ah I see. Well I was mistaken. It is not a cell tower. I asked him about it and apparently he ownes the tower. He is a HAM radio operator! How cool!
see what happens when people try to dissect the regs? now that gray area was shut down and now you can't put your transmitter up on your neighbors tower and play dumb. this site needs to start banning bootlickers and law focused busybodies because they're ruining the hobby
That's a pretty tall tail (no pun) as the Rangemaster will not match a length like that.
Base-fed, 1/4-wave monopole systems such as used by many AM broadcast stations have a feedpoint impedance with a real term of about 35 ohms. About 2 ohms of that is the r-f resistance of the matching network and ground system; the other 33 ohms is the radiation resistance of the tower.
A Part 15 AM system operating under FCC §15.219 also might have the same 35 ohm value. However most of that resistance is from losses in the r-f ground system and loading coil, with maybe 1/10 of an ohm of radiation resistance due to the 3-m limitation in §15.219(b).
So a Part 15 AM transmitter using an external matching setup could drive either of those antenna systems about equally well. Of course a 1/4-wave monopole radiates much more of the feedpoint power than a 3-m antenna system.
If a Rangemaster allows for bypassing the internal loading coil then it could drive a 1/4-wave tower. But the Rangemaster's FCC certification will not be valid then, and obviously a 1/4-wave tower is not permitted by FCC §15.219.
In a preceeding post member Rich said: "But the Rangemaster's FCC certification will not be valid then, and obviously a 1/4-wave tower is not permitted by FCC §15.219."
The statement contains a lingering fallacy that continues being repeated despite frequent reminders of the actual definition of "certified" in FCC parlance...
The purpose of certification of transmitters under Part 15 is to qualify a manufacturer to legally sell prebuilt transmitters. Once that transmitter is sold to a customer the certification has served its purpose and no special rights or privileges are bestowed to the customer.
The owner of a Part 15 transmitter is subject to the Part 15 rules whether he has a certified or uncertified transmitter, therefore the second half of Rich's statement - "obviously a 1/4-wave tower is not permitted by FCC §15.219" - is correct.
Modifications: Any modifications made to this device that are not approved by the manufacturer may void the authority granted to the user by the FCC to operate this equipment.
Mark one up to Rich.
"Once the FCC has certified a device as Part 15-compliant, it may not be modified. Changing anything that affects the electromagnetic transmission of the device will void the unlicensed sta-tus. A change as simple as a different antenna will affect the electromagnetic characteristics and void Part 15 status."
This info rocks, I've wondered what a full size broadcast tower could do with 100 milliwatts, thanks for the breakdown of that and the reactances involved too. It's good to know what the standard maximum is to know where you stand as a part 15.
Inverse distance field would explain how a station local to me that has just an average signal (I think I'm right on the 2.5 mv circle, though they don't even seem to be that strong) can go from 1000 to 4 watts post sunset and if the band is quiet, for example we just had a sun flare, I'm still able to hear them at fringe level.
Nate - In the situation you decribe for your local station, reducing power from 1000W to 4W is a change of 24dB (assuming their radiation pattern doesn't change when they go to 4W).
If that station produces 2.5 mV/m at your receive site in the daytime, then their post-sunset field there would drop to about 158 µV/m.
There was no change in the pattern at night. I was surprised I could hear it when they dropped power that much.
You have posted that with a good radio and quiet location, it's possible to have a listenable signal of 150 uv/m. The problem was, that was a high band station, and every night, by the time they'd drop power, there was at least some skip coming in on that frequency. Sometimes I could still hear the continuation of whatever was playing after the drop. Otherwise it was a jumble of weak carriers beating and audio.
Depending on the tolerance of the listener and the performance of the receiver, a desired AM broadcast signal needs to be 15dB or more greater than the sum of the co- and adjacent-channel interference fields arriving at the receive antenna (including the radio noise level in that spectrum).
Daytime interference from co- and adjacent-channel licensed stations can be very low for some locations and frequencies, so that a 150 µV/m groundwave field from a nearby station could be useful.
At night its a different story due to skywave reception of distant AM stations.