Site Attenuation: A How To Explanation

So, when the FCC gives someone a NOUO they best have done this work before trying to say you're out of limits, eh?

And there is no point at all to taking my FIM out to check these two transmitters, as my site has not had it's attenuation characteristics analyzed. I'm not trying to certify anything, but it's nice to know that a possible defense would be that the FCC inspector did not complete this study in the area where they were monitoring your signal.

I'm getting a whole lot of "your data will be wrong" posts.  Maybe it's just not worth the trouble.

Tim in Bovey

"I'm getting a whole lot of "your data will be wrong" posts.  Maybe it's just not worth the trouble."

I simply posted this to show those who might not be aware of what is really involved in compliance testing. When the FCC issues a NOUO the levels are so far over the limit that any near field effects are moot.

Directional AM broadcast stations use calibrated FI meters to measure field intensities at the monitoring points of their DA patterns.

Those propagation paths are not "qualified," yet the FCC accepts the accuracy of those measured fields.

Indeed they do!  I do directional monitor points for the AM I work for at least quarterly.  It used to be required monthly -- and some stations even more often than that -- but in deregulation over the past few years,  like so many other technical details, it's been left up to the stations to do so as often as they deem necessary to insure compliance. 

I do this with my trusty Nems Clarke 120E FIM that I've used for this purpose since the late 1970's.  Every three years our station participates in a mock FCC inspection in conjunction with the Minnesota Broadcasters Association and the FCC (stations that pass are exempt from any inspections for three years unless there is a complaint that needs followup).  We've passed every time. I then do the monitor points with a real (but retired) inspector and his equipment, and my FIM readings have always been the same with his (he usually has a Potomac FIM-41, but last time had one of the new fangled Potomac $14,000 units).  Of course his new machine takes the reading out to a few decimal points -- can't do that on the analog meter!

Tim in Bovey

I think that what you're proposing to do is a very useful exercise, Tim.  I thank you again for offering to do it.

But I think that you should be prepared for a number of people to challenge the results, no matter what they may be.  There are an awful lot of biases out there.

Personally, I'm just interested in the actual numbers.

Please do the tests....me and some others I'm sure are waiting to find out what you get!

Mark

I think that what you're proposing to do is a very useful exercise, Tim.  I thank you again for offering to do it.

So do I, and I hope you will proceed with your plan, Tim.

My posts about this were not made to discourage such measurements, but to provide insight into the fields that may exist from such systems at various elevations and distances over a real-earth path -- which may affect the conclusions attributed to those measurements.

I should have the Wholehouse 3.0 next week. Once it arrives I'll test both transmitters one after the other in the same setting and see what we have. 

I realize I'm not a high tech lab, I don't have a certified antenna testing site, but I do have the R-506 with it's calibrated antenna with matching cable and all the antenna factors, cable losses, etc directly programmed into the unit. So taking an actual field intensity reading is pretty straightforward.

Rich - couple questions. Is there a preferred height I should work at?  I was thinking of heading out to a nearby open field however if I do that, there is no power available and I'll have to run transmitters and meter on batteries. I'd prefer to be on AC power so I don't have to be concerned with any variables caused by power source.  Not to worried about the meter as it's designed to be used portable and has it's own built in battery pack and extensive regulation, etc. But I don't know about the transmitters.  If I simply test on the sidewalk in front of my house, or out in the street I can run AC out easy, but there would be pavement and old wood homes about 75 feet away or so on either side. Thoughts? Plus, I already have distance marked out in front of the house at 3 meters, and at 20 foot intervals out to 200 feet.  So for convenience out on the sidewalk or in the street would be handy 🙂

Tim in Bovey

Here's my assumtion....If the batteries are fresh and giving the Decade the full 12 volts and the wholehouse a full 4.5 volts the transmitters should be fine but I don't think going to a field or doing it out in front of your house makes a difference. you have open space outside the house and you are just 3 meters in front of the transmitters antennas....the houses next door are not large buildings with steel structure and wouldn't reflect anything that would affect you in front of your house. I would think the only thing that would affect it would be other signals on the frequency your transmitter is at, if at all.

So maybe not nessessary to take all this to a field.

Just my opinion

Mark

Given this (to me) new concept of "site attenuation" I decided to probe a little bit to make sure site attenuation is what I thionk it is.

It is.

Using John WDCX's Link, Post # 1, refer to "Introduction to Site Attenuation."

It suddenly becomes clear that ANYTHING standing around on "good earth" is a potential attenuator, including fences, people and soda cans.

We tend to reside in a highly attenuated world.

Are you paying attenuation to what I'm saying?

Hmm I never thought about a pop can on the ground.  OK I know people walking around you can cause multipath on FM we know this from the old analog days of TV.  God I remember issues with the rabit ears and people walking around.  Digital TV is even worse through an antenna.  When we had to go without cable for a few months it was aweful dealing with that for our entertainment.  Well FM Radio can have the same issues that is for sure.  I don't think AM is quite so bad with people walkiing around but AM doesn't like some structures either.  I still very anxious about the FM tests.  Oh Johnny Punk has not tried to jam my station anymore.  I think it may have been someone in their car riding by and seeing me with my boombox over my head on 96.3 I don't know.  But so far so good.

When TheLegacy mentioned hearing mysterious dead carriers on his FM dial, I was reminded of something from the wayback part of the brain.

So I tuned my Sangean Radio to a blank spot on the FM dial, turned off ALL my transmitters, and used a second radio, a Grundig, and started tuning it across the FM dial until a silent carrier appeared on the Sangean.

Anyone can do the same thing using two radio. What is it? It is the LOCAL OSCILLATOR part of the radio's reception circuit! It's one of the reasons radios also have a Part 15 Sticker. Radios Radiate!

It is even possible to gut an old FM tuner and convert it into a transmitter.

I remember when you received around 95 Mhz your Radio would throw a carrier around 105.7 Mhz and this was for 50 feet or more.  It was why I was adamat about transmitting far beyond 50-75 feet.  To throw a carrier on 87.9 you would be receiving around 77 Mhz and for 96.3 it would be 86 Mhz.  This carrier was very clean.  Made me wonder if mike was playing a trick on me.  It was why I thought it was a johnney punk but it could have possibly caused by a cable box.  I have not heard it since a few days ago.