Another Part 15 AM'er Nailed

The field strength of the signal on frequency 1620 kHz was measured at 23,600 microvolts per meter (uV/m) at 75 meters, which exceeded the maximum permitted level of 250 uV/m at 3 meters

Good grief!

Not likely 100 mA DC input to the final RF stage...

Experimental broadcasting for a better tomorrow!

This case appears different from the two previous cases recently discussed in this Forum. Antenna height was not mentioned in the NOUO. When working backwards from the measured field strength of 23.6 mV/m at 75 meters, we get a radiated power for an electrically short antenna of about 35 mW. Under ideal conditions, a legal Part 15 AM system might produce a maximum of about 1 mW of radiated power (0.1 mW is a lot more likely). A really efficient system using a "whip and mast" might produce about 20 mW. It was pointed out in a previous thread that the output of a Part 15 AM transmitter may not even be as high as 35 mW when tested with a resistive dummy load. So, a radiated power of 35 mW is unlikely from a transmitter with 100 mW of input power. The operator may actually be a genine low-power AM pirate rather than someone trying to stretch the Section 15.219 rules to the breaking point. However, since the apparent radiated power is less than 100 mW, the use of a Part 15 AM transmitter cannot be competely ruled out.

The FCC made a mistake citing 250 uV/m at 3 m as the field strength limit at 1620 kHz. Section 15.209 specifies about 14.8 uV/m at 30 m. The FCC is so much more accustomed to busting illegal FM stations than illegal AM stations, that they cited the Part 15 FM limit rather than the AM broadcast band limit. Section 15.219 does not specify a field strength limit, but assuming 1mW to be the maximum possible legal limit for radiated power, Part 15 AM should not have a field strength of more than about 4 mV/m at 75 meters.

I agree with Ermi.

Using Rich's FS equation: E = SQRT(49.2*P)/ R and assuming a 3 meter base-loaded antenna (.1 ohm radiation resistance) and a pretty darn good ground resistance of 20 ohms, the OUTPUT power of the transmitter in question would need to be 13 watts. That's output power. Even with extremely good overall efficiency of the RF output stage and tuning network of 75%, the input power would be about 17 watts.

This guy is using a grossly illegal transmitter, antenna, or both. Whatever, he is a pirate and NOT a part 15 wannabe.

One would hope the FCC was able to view the antenna when they took the FS measurements, so I assume they saw something that looked legal or they would have cited it in the NOUO. But, then again, you know what they say about people that assume.

PhilB

Thank you for your confirmation, PhilB.

Since we used vastly different numbers in our posts, the casual reader who has not had time to analyze our presentations might think we disagree. Of course, we actually don't.

Rich has contributed many Part 15 AM antenna calculations to this forum, with the biggest variable being the ground loss resistance, which is very difficult to measure or calculate. It is usually guessed at and assumed. I think that, in the particular post you referred to, Rich said that the antenna loss is 25 dB. 25 dB corresponds to 0.316%. For 75 % efficiency for a 100 mW input, the radiated power is .237 mW. Rich described this as "good" performance for a legal Part 15 AM antenna. To get the 35 mW of radiated power that I mentioned in my post from Rich's hypothetical antenna requires about 11 watts. You mentioned 13 watts in your post. I consider this to be substantial agreement.

Whether the transmitter output power is 11 or 13 watts, this is, of course way beyond the Part 15 range. A longer, more efficient, antenna, such as of the "whip and mast" type, would requre less power from the transmitter to get 35 mW of radiated power. The point I was trying to make in my post is that even a whip and mast is unlikely to have enough efficiency to get 35 mW of radiated power from a 100 mW input power transmitter stage.

In one of my posts in the EH antenna thread, I stated that the EH antenna, and some related antennas, such as the CFA, are all whip and mast antennas. The CFA is the most well-studied, because it has been around for nearly two decades. The CFA has a loss of between 6 dB to 10 dB. I will take 6dB to be the lowest loss that can be obtained with the whip and mast conguration. To get better efficency than that, you would have to drop the pretense that the antenna is shorter than it really is. With 6dB antenna loss, 140 mW is required from the transmitter to get 35 mW of radiated power. With 75 % transmitter output stage efficiency, the input power would be 187 mW.

PhilB wrote: Using Rich's FS equation: E = SQRT(49.2*P)/ R and assuming a 3 meter base-loaded antenna (.1 ohm radiation resistance) and a pretty darn good ground resistance of 20 ohms, the OUTPUT power of the transmitter in question would need to be 13 watts.
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Phil,

That equation applies to a 1/2-wave dipole in free space, and gives the wrong answer for the short, loaded monopoles used on Part 15 AM (sorry).

In my post in another thread here I gave an estimated field strength value of 2.4 mV/m at 62.5 m (1/16 km) from a very good, ground-mounted Part 15 AM setup. That field would drop to 2.16 mV/m at 75 m.

The tx output power for my example was 80 mW. As field strength varies by the square root of a power change, the tx in my example system would need to produce 119.4 times as much power, or 95.5 watts, in order to generate the 23.6 mV/m field that the FCC measured at 75 m.

The gain of an elevated (whip/mast) Part 15 AM antenna system with a long, radiating ground conductor could be about 10 dB more than when the tx+whip base are at earth level, other things equal. A power gain of 10 dB is a multiplier of 10, so in that case the tx output power needed would be "only" 9.55 watts.

Rich

Actually, it's a pretty interesting case. If you do a search for the name of the person and the town, you can find this article:

Station Has Down-Home Appeal

The article is a bit confusing as to what was actually going on. It says that it's a 1000 watt station, but with a range of 2 miles and with a 10 ft antenna?? Maybe the reporter didn't get that part right.

Daniel

Please ignore the last two paragraphs of my last post, and read these below. I shouldn't post so early in the morning, I guess. I was off by a factor of ten in those powers because I referenced the changes to 0.8 watts and not 0.08 watts (80 mw). Sorry about that.
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The tx output power for my example was 80 mW. As field strength varies by the square root of a power change, the tx in my example system would need to produce 119.4 times as much power, or 9.6 watts, in order to generate the 23.6 mV/m field that the FCC measured at 75 m.

The gain of an elevated (whip/mast) Part 15 AM antenna system with a long, radiating ground conductor could be about 10 dB more than when the tx+whip base are at earth level, other things equal. A power gain of 10 dB is a multiplier of 10, so in that case the tx output power needed would be "only" 0.96 watts.

Trying to make sense of the newpaper article from the earlier post, and compared to Rich's calculation, 1000 milliwats, or 1 watt 🙂 might be what was being referred to.

So, in short, this broadcaster was using a W/K four letter call, was selling advertising (not prohibited, but a GREAT way to make any licensee FURIOUS), getting write-ups in the paper (also not prohibited, but see selling advertising, above) and was using equipment that, no matter how you calculate it :-), is far outside of Part 15 compliance requirements.

Gee, I wonder why they got a visit...

Experimental broadcasting for a better tomorrow!

I just did an AM query Call Sign search on the FCC site for WKMX. The call letters mentioned in the article. The search had 0 results. So WKMX is most likely a pirate station that did a good job of fooling the people in his town into thinking he was licensed.

But it does show up in FM Query...
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WKMX ENTERPRISE, AL USA

Licensee: MAGIC BROADCASTING ALABAMA LICENSING LLC
Service Designation: FM 'Full Service' FM station or application

Channel/Class: 294C Frequency: 106.7 MHz Licensed
File No.: BLH-19870105KB Facility ID number: 73179
CDBS Application ID No.: 96613

31° 24' 41.00" N Latitude
85° 57' 32.00" W Longitude (NAD 27)

Polarization: Horizontal Vertical
Effective Radiated Power (ERP): 100. 100. kW ERP
Antenna Height Above Average Terrain: 326. 326. meters HAAT
Antenna Height Above Mean Sea Level: 422. 422. meters AMSL
Antenna Height Above Ground Level: 301. 301. meters AGL

by MRAM 1500 kHz
The WKMX webpage link found in the new article regarding this Wellsville station is dead.

Perhaps he folded up and left town.

An archived copy of the page from Feb 2006 can be found here:

http://web.archive.org/web/20060206222545/http://www.wkmxlive.com/

Be patient - it make take few minutes for the server to retrieve your request. Not much of it was saved, but some of the site is visable.

I wasnt going to post but I didnt want this to get out of hand. Yes we run a small local part15... well for the most part... we have several rangemaster transmitters.. and someone else connected one( the one in question) and I wont get into that part. but anyway. this is a community thing that is going on.. Hell we even have the mayor of our town helping.. No one make ANY money. we put this together because Keymarketing, who owns most of the radio stations around us pulled out our local talk radio and went to 24/7 sports ESPN.. so we all got together and started this. We have covered jsut about every local sporting event for the high schools.. and that includes live audio and video streaming over the internet.. It has been great.. ALthough I guess one of the transmitters that one of the guys put up was more power than supposed to be. yes we got a visit... but Ill tell you the FCC guy was really nice AND helpful. and has given us alot of information on getting a license too.. We are not off the air as someone said. we are currently getting ready to move the servers and are waiting on a few other groups to join us only to get bigger..Well with less power....gotta do it right this time.. but we have so much support its not funny.. jsut about every busisness has donated money, and yes we ran their commercials in return.. but we had to pay a fee for every game that we covered so their money took care of that., no other money was made.. there are alot of town people involved in this including the area churches and groups.. There are alot of things that we are doing here and I jsut didnt want this post to get too ugly and someone saying something that really wasnt true.. Yes we were stupid for going against the part15... but did we due it to BE a pirate. no. we are only trying to bring programming to our small community. We do still do the audio and video and the FCC agent whem ive talked to quite often and still do.. gave us the ok to go back on the air with our other transmitters..

Thanks

We just do not know. Here we go again with a report of an AM station getting a "visit". Here we go again calculating back to estimate what power he was running. Here we go again guessing whether he was a part 15 operator. Here we go again getting a report that a station had a 20 foot elevated transmitter only to have the report changed to modify the height (on another thread). Here we go again when a church gets busted while perhaps following installation advice from a manufacturer. Here we go again, the FCC guys were really nice and will help us get a license. Here we go again with being scolded by the FCC but we are going to do it right this time. Be nice and polite to the inspectors and you won't have a problem.

Those cited or those who know of citations have not been willing to discuss the circumstances in detail. It is all hedging. We read second hand reports where identies are hidden, circumstances are clouded, and a general reluctance is shown to make public the actual facts. How can anyone make any decisions based on this total lack of good information, or on bad information?

The simple truth is that the rules (part15.219) are easy to understand if one has any technical knowledge. Bend it or interpret it as you may, but I have no trouble understanding the rules.

The other truth is that an enforcement agency will not share specific details regarding their activities. I know this first hand from my contact this week with a detective in my city. He flat out told me this was the case.

In light of the lack of good enforcement information, which appears to be unavailable, the only safe way to engage in part 15 AM transmitting is to follow the part 15.219 rules, which are clear to me. Three meters total length of antenna, transmission line, and ground lead is pretty simple to understand and it means just what it says. To do otherwise is to roll the dice and fantatize that the FCC inspector is "a nice guy who will be very helpful, and will help me get a license.".

Neil