Earlier today I posted that a 3-m Part 15 vertical antenna radiating ~9.5 nW could produce a ~27,000 µV/m field at a distance of 35 meters. I slipped a huge number of decimal points in making that statement -- the answer is really ~9.5 mW (milliwatts). My apologies for my mistake in reading my spreadsheet. Now for some elaboration on this.
First, these are RADIATED powers, not the power input or power output of the Part 15 tx.
Earlier today I posted that a 3-m Part 15 vertical antenna radiating ~9.5 nW could produce a ~27,000 µV/m field at a distance of 35 meters. I slipped a huge number of decimal points in making that statement -- the answer is really ~9.5 mW (milliwatts). My apologies for my mistake in reading my spreadsheet. Now for some elaboration on this.
First, these are RADIATED powers, not the power input or power output of the Part 15 tx.
Various methods of calculating the efficiency of a very good 3-m, ground-mounted Part 15 AM antenna and ground system near the high end of the band show a radiation efficiency of about 1% for it. So if a Part 15 tx delivers 80 mW of carrier power to the input terminal of the antenna system, it would radiate 1/100 of it, or 0.8 mW. The rest is lost as heat in the loading coil and ground system (mostly).
The peak radiated field 35 meters horizontally from the Part 15 vertical radiator in this setup will be about 8 mV/m (8,000 µV/m). The method of calculating this will be supplied on request, but it is based on an applicable equation from the engineering text "Reference Data for Radio Engineers."
Now if the station in the FCC violation case was measured to have 27,000 µV/m at 35 meters, that could be the result of increasing the tx output power delivered into the above-described antenna system by the square of 27,000/8,000, which is 11.4 times (field strength changes by the square root of the power change). Or it could be the result of using an antenna system with 11.4 times more gain. Or it could be some combination of those.
It's a lot easier to increase the gain of a Part 15 AM transmit antenna system by 11.4 times than it is to increase the ouput of a Part 15 AM tx that much. All that is necessary to increase the antenna gain that much is to install the Part 15 tx and its ~3-m whip at the top of a 20-25 foot support above the earth, and connect the tx chassis to the same earth ground as before. The added radiation off the conductor(s) from the tx chassis to the earth ground easily can add that much extra gain to the antenna system.
So this violation may have been the result of a rather common "elevated radiator" antenna system popular with some Part 15 operators, and even shown/suggested on the web pages of some commercial manufacturers of Part 15 transmitters.
Hello Rich,
First, is there any substantials improvement if i install 4 radials of the same length as of the vertical radiator? My broadcast goal is not the skywave propagation but the groundwave.
Second, in order to get some groundwave improvements, is it a better idea if i 'open' my radials around 135 degrees from the vertical radiator instead of 90 degrees?
Finally, is there any improvement if i put 8 or 10 radials instead of 4 radials? How many radials would be enough on 20 foot support?
Thanks!
Yves Roy
First, is there any substantials improvement if i install 4 radials of the same length as of the vertical radiator? My broadcast goal is not the skywave propagation but the groundwave.
Using 1/4-wave-long elevated radials is best. Elevated radials only 3-m long are better than no r-f ground at all, but such a system is not very efficient.
Second, in order to get some groundwave improvements, is it a better idea if i 'open' my radials around 135 degrees from the vertical radiator instead of 90 degrees?
If the elevated radials are not installed in the horizontal plane, they are adding to the electrical length of the radiator. If that doesn't matter, then you might experiment.
Finally, is there any improvement if i put 8 or 10 radials instead of 4 radials? How many radials would be enough on 20 foot support?
No. The length of the elevated radials is important, but any number of them greater than two does not improve system performance. Note this is not true for buried radials.
You might also re-read this thread link for a lot of discussion about this. http://part15.us/node/582