- Posts
This topic is sure to attact some flak but that’s OK. I can take it.
FIRST: This procedure is not lab accurate but will get you as close as possible without measuring at a lab test site. Also, it is not unlike the on-site measurement procedure FCC agents employ when they make a visit to a suspect pirate.
Acquire or find a cable TV buddy with a Field Strength Meter. Cable TV FSMs usually cover the FM broadcast band and if routinly calibrated, they are very accurate.
This topic is sure to attact some flak but that’s OK. I can take it.
FIRST: This procedure is not lab accurate but will get you as close as possible without measuring at a lab test site. Also, it is not unlike the on-site measurement procedure FCC agents employ when they make a visit to a suspect pirate.
Acquire or find a cable TV buddy with a Field Strength Meter. Cable TV FSMs usually cover the FM broadcast band and if routinly calibrated, they are very accurate.
Set the FSM to 100KHZ bandwidth or something close. A wider BW will result in a higher reading so stay as close as possible to 100KHZ.
If there is a detector setting set it to “peak”. If you’re lucky enough to have a QP detector than use that. Peak will be worst case.
Take a car radio antenna with the appropriate adapter and connect it to the FSM. Set the FSM to the broadcast frequency of your transmitter.
If your radio is a Ramsey Yardcaster with the built in antenna, place it on a table in an open area. Maybe outdoors on a picnic table.
Position yourself 3 meters away from the transmitter.
With you car radio antenna fully extended and holding it at the base, not touching the element, move the antenna from vetical to horizontal and tale note of the highest measurement. If you’re under 250uV or 43dBuV, your complient or close enough that you will not incurr the wrath of the FCC.
If you are 1 mile away and you’re over 250uV or 100uV or 50uV, you’re not legal.
Hope this helps
Total posts : 45366Experimental is just that – trying things 🙂 It’s nice to know there are ways to get an idea if an experimental set up is something to continue working with, or something that needs to be changed before going forward to the operational phase.
Nice contribution wdcx!
Experimental broadcasting for a better tomorrow!
Total posts : 45366Hello all,
wdcx has describe a possible way to get close to p15 FM compliance and it is not an unreasonable approach. Those who know more about this than I can probably add something, but the cable guys I know have instruments which connect directly to the F connector on the wire and measure dBm on the selected frequency. Maybe the guys who chase leakage are better equipped, I just don’t know. For a radiated signal I wonder what antenna will give the required readings when attached to the instrument and do the cable techs have this?
My suggestion for those who do not or cannot commandeer a cable tech is in this post:
http://part15.us/node/993#comment-2269
Neil
Total posts : 45366[quote=wdcx]Acquire or find a cable TV buddy with a Field Strength Meter. Cable TV FSMs usually cover the FM broadcast band and if routinly calibrated, they are very accurate.[/quote]
Note that such cable meters do not, and can not accurately measure radiated field strength.
//
Total posts : 45366[quote=Rich][quote=wdcx]Acquire or find a cable TV buddy with a Field Strength Meter. Cable TV FSMs usually cover the FM broadcast band and if routinly calibrated, they are very accurate.[/quote]
Total posts : 45366[quote=wdcx][quote=Rich][quote=wdcx]Acquire or find a cable TV buddy with a Field Strength Meter. Cable TV FSMs usually cover the FM broadcast band and if routinly calibrated, they are very accurate.[/quote]
That is is a BIG difference, and r-f voltages so measured cannot be used to prove compliance to/with the rules applying to Part 15 FM.
//[/quote]
NOT TRUE. It is evident that you have never done or been involved with FCC compliance testing.
Total posts : 45366The problem with a cable TV meter is that it is not calibrated to measure radiated fields. The meter scale on it will be calibrated either in units of voltage (mV, dBu, etc), or power (mW, dBm, etc). None of those units is a measure of field strength, such as µV/m.
(added later) I just did some calculations for the 100 MHz field strength that a typical, matched 1/2-wave dipole would need to see in order to produce a reading of 250 µV on a cable meter.
If the cable meter reads 250 µV in its 75 ohm environment, that means that a power of 0.0033… mW (-24.7 dBmW) exists in the measuring circuits of the meter (P = E^2/R).
A matched, 1/2-wave dipole would need to be immersed in a radiated field of over 32,700 mV/m in order to produce the amount of power needed by the cable meter to show a reading of 250 µV.
//
Total posts : 45366[quote=Rich][quote=wdcx]NOT TRUE. It is evident that you have never done or been involved with FCC compliance testing.
[/quote]None of those units is a measure of field strength, such as µV/m. These units are not interchangeable.
Certainly if you connect a cable meter input to some kind of an antenna, the meter may show a reading. But it won’t be valid, because the efficiency of that receiving antenna at converting the arriving field strength into whatever the cable meter reads is unknown.
//
[/quote]
Huh? 1000uV = 60dBuV Of coarse this is across 50 ohms.
YOU SAID: Certainly if you connect a cable meter input to some kind of an antenna, the meter may show a reading. But it won’t be valid, because the efficiency of that receiving antenna at converting the arriving field strength into whatever the cable meter reads is unknown.
I SAID: That’s why they have ACF’s.
Total posts : 45366[quote=wdcx]Huh? 1000uV = 60dBuV Of coarse this is across 50 ohms.[/quote]
But neither of those is a measure of field strength, of “coarse.”
Total posts : 45366So if I use an old Eaton/Stoddard FSM with a added or subtracted ACF using a dipole or biconical dipole, plus cable loss, I should just throw that stuff in the crapper because I’m too dumb to have it register…”
BTW, the FCC for spot compliance uses a FSM and a 1 meter vertcal. Like it or not. I was simply attempting to bring this to a level where all can understand and some may want to experiment to draw thier own conclusions.
It was not my attempt to make this an engineering pissing contest.
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
Total posts : 45366Hello all,
The FCC rules for part 15 FM are based on well defined physical and measurable parameters. Any reponsible and competent engineer can understand what the rules say and does understand the principles and limitation of the measurement techniques. For those who are not engineers with experience and expertise in the use and measurement of RF technology I ask why do you not trust the advice and judgement of those who have made a career in this field?
The personal attack on the poster concerning his background and experience was not only ignorant of his background but also uncalled for. Why make such a statement when you know nothing of the poster’s credentials? I think that was corrected.
A moderator of this board has posted and has communicated to me privately about being positive and encouraging experimentation. I really like to be positive, but when things are said that are not founded in engineering or physical fact and could lead readers to legal problems I see no value in not calling the question.
I, and some others, are just trying to share years of personal and professional experience and guide people in their journey into part 15 activities in such a manner that the laws are explained and the liabilities are known. What readers do with the information is their business but at least they have been advised. If you don’t want to listen to my advice or the advice of others who have made this their profession then don’t. But stop misleading readers with hearsay about technology or what you think the FCC inspectors will do which is not based on citable reliable information. I am not interested in such speculation.
Neil
Total posts : 45366[quote=wdcx]It was not my attempt to make this an engineering pissing contest.[/quote]
Let us allow readers of our posts in this thread to verify whether your posts or mine stand up to reality using the laws of Physics.
Total posts : 45366What did you view as a personal attack?
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
Total posts : 45366This:
You wrote: “It is evident that you have never done or been involved with FCC compliance testing.” Reading this literally would indicate that the statements should be summarily disregarded due to incompentence on the part of the poster.
You may not see this as a personal attack and that may not have been your intent, but I see it as personal when one engages in a discussion by questioning the credibility of those participating rather than addressing the issues and technical facts. I trust you understand why I responded as I did.
Several of your posts address the technical topics and I appreciate that approach and your contributions to these discussions.
Thanks for asking.
Neil
Total posts : 45366John,
You have met with the same success, as many of us on this forum have, with trying to be reasonable with Rich. He is a brilliant mind when it comes to the science of radio theory. But, he lacks the sensitivity to reach down to where most of the readers of this forum are. Where as my comments may ring of negativity, I have a true appreciation for the intellect Rich possesses on the subject. That intellect, however, is beyond most peoples understanding of the subject, their personal resources or personal motivation to learn what he is talking about. I. too, have been a broadcast engineer (in the field) for decades. But, I have not been able to convince Rich we all appreciate his contirbution, but his, sometimes, combative demeanor is not appreciated.
And to Rich: Keep up the good work. And at the same time, I encourage you to “dumb it down” for the people who really would be helped by your knowledge and passion for radio science. Practical solutions to complicated subjects could make you everyone’s hero here. Try it and see what I mean.
Marshall Johnson, Sr.
Senior Pastor, President
Rhema Christian Fellowship, Inc.Rhema Radio – The Word In Worship
AM 1660 – FM 93.5
http://www.rhemaradio.org
- You must be logged in to reply to this topic.
