i was told to use a known calibrated fim21/41 as a reference and make a chart of how far off the meter deviates from that known calibrated meter and i already have an engineer willing to do such a verification for me.
Once a field strength meter is calibrated, how long can it be expected to remain in calibration?
"So, if your signal is intelligible at one radianlength, you are probably in violation of 15.221. Something like that was reported by RFB in this thread. If your signal is just barely detectable at one radianlength, you may be OK."
Yes and no. Fact is there are variables that come into play when measuring a CC system vs a typical central point radiator like that of a 3 meter stick or 1/2 wave vertical.
What you want to achieve is the sudden drop off at the 157,000/F point, which would be the specified 15uV, and that is in the noise floor. It's not difficult to achieve that but the big problem is finding that spot when power grid lines are clustered well within even the half radian point of the distance called for in the formula.
No field inspector is going to tag you for your signal being 15uV at 5 feet beyond the maximum point. This is the variable that I am talking about. Heck not even licensed stations precisely hit on the spot of a dime in their measurements. The FCC is aware of and takes into account the variables encountered in the field.
Most CC stations do prefer the lower 3rd of the band due to the longer wavelength and increased distance the signal can travel from the wire. But a CC system's goal is NOT to get maximum signal emission OFF the wire, the goal is to get maximum signal DOWN the wire as far as it can go because a CC system is not intended for typical through the air reception like that of a 3 meter antenna or 1/2 wave vertical. This is why the CC system works in the near field (inductive) range and NOT the far field, two different puppies requiring two different measuring processes.
Thing to always remember is that a CC system is not intended to operate like a 3 meter antenna system or other. It is a "wired wireless" system and we only need enough signal injected onto the line to achieve good coverage along the length of that line, no matter where it goes or how long it runs. It's exactly the same technique as a leaky coax system, except here we use the power grid wires instead of a dedicated leaky coax.
One could spend the $$$$ and run a bunch of leaky coax all over town and get full coverage of that town while still meeting the 221 requirement. And when that leaky coax is placed in the right locations, coverage without gaps can be achieved easily. With the power grid, its a little different because those lines go where the power company planted them so we are at the mercy of where those wires are located and how they are interconnected.
Depending on how that grid system is configured, your CC signal may go all over town, or just a few blocks, or even less. But as long as you meet the 221 specification of signal emitting OFF that wire, your ok, even if that wire carries your signal for 20 miles along the length of that wire. And naturally the longer the wire, the signal decreases in intensity, thus some setups have repeater linear amps and couplers to continue the signal further down the line or to boost it to a useable level.
And the best part of it all, if a proper frequency is found and used, you NEVER have to worry about skywave totally wiping out your station's signal EVER.
RFB
1670 kHz is quite a high frequency for carrier current, making for very narrow strips along the radiating wires where the signal can be heard. But that seems to be fine for RFB's system.
Just for comparative purposes, I want to mention that a the field intensity of a 15.219 station at 1670 kHz is quite a lot higher at about 30 meters than a carrier current station. (I realize that the length of the radiating wire is what determines the maximum range for carrier current.) Even a very inefficient 15.219 station can easily get more than 100 times the 15.221 limit of 15 uV/m. 2000 uV/m at 30 m, and a lot higher, can be obtained with the antenna at the level of the earth.
TIS stations using leaky coax (not carrier current) are allowed a field strength of 2000 uV/m at 60 meters. A 3 km radiator length is allowed, and a maximum transmitter power of 50 W. 15.221 is really very restrictive in comparison.
You may have a slightly higher field intensity at the 30 m distance, but beyond that the intensity decreases dramatically, and even at 30 meters the signal from a 219 setup has a hard time making its way into buildings effectively, not to mention the flooding out of that signal at night due to skywave.
CC does not have that issue, and can maintain the signal intensity farther away (down the line) while the conventional antenna system's signal is decreasing dramatically per distance.
CC will never propagate the signal like a center located radiator does, never meant to, never will. CC works with near field induction, not far field. This is why even at a considerable distance down the line from the TX source, the signal is strong and clear where compared to the central located antenna, that signal is weak and noisy.
Around here, you won't find a power line separated by blocks and blocks. They are so close that from one block to the next, your lucky if there is 120 feet separation.
While the 219 signal gets absorbed by obstructions, CC signal is being delivered right into the homes and to the power outlets, along with a little bit of emission off the line to allow a portable to pick up the signal below the 30 meter range limit. So at a 1/2 mile or even a mile distance, that 219 signal is noisy and weak and most likely won't be picked up on a radio indoors, where the CC signal is right there, strong and clear, day and night.
The two methods can work together, 219 to take care of the outdoors and the CC does the indoors.
Your 219 may be stronger at 30 meters from your antenna, but beyond that, it decreases steadily after that while the CC is still strong within that 30 meters even 5 miles down the line. 219 will never hit that, not without a cluster of em strung together.
Until you separate the two and how they function, ie near field versus far field, you will never see the true difference nor will you fully understand why CC is measured the way it is and why the spec calls for 15uV at 30m based on 157000/F for feet or 47715/F for meters. CC is an inductive form of radio emission/reception...again..near field. 219 is a far field radio emission/reception form, where the signal drops as distance increases from that center point. 219 will fade away long before a CC will down the line. If the lines are just right, you won't notice any signal degrading or fading or nighttime skywave blasting totally wiping out your 219 from being useful.
If you can find one, grab the 1945 copy of ARRL and read up on the CC systems of that era used by HAM's and how it works compared to typical radiator systems. Back then, CC was the only form of radio communication allowed. Those guys chatted about nothing for hundreds of miles on the power wires. If it were not for them, CC probably would not be as advanced or even used during its prime in the 60's thru the 80's.
RFB
Very strong argument for using carrier current. I guess that 15.221 and 15.219 both have their own niches.
Another wartime means of communication for hams that I have read about is conduction of audio power in the ground plane. Audio power is applied to two separated ground rods; and some distance away, an audio preamp, also connected between two ground rods, picks up the transmitted audio signal.
221 and 219 sure do have their own niches. Combined they can work together to overcome each one's problems as well.
The ground audio pump..which is what I have heard it called by a few WWII field radio guys does work pretty well. The same can be done with an RF signal too. A lot less noise, but just like the audio pump, the RF pump would require a bit of power.
If you have one, or can get one, try the RF ground pump method with a couple of ground rods separated by about 25 to 50 feet apart through a TCU-30 or RS CP-15 coupler. Works pretty darn good!
Small article on the audio pump or "Earth Base" system.
RFB
You have presented information on types of induction audio/radio transmission with which I was previously unfamiliar, and it is very interesting. More experimenting ahead.
It is interesting to recall that the audio "base" band is outside of FCC jurisdiction and therefore open to any power level.
What about RF induction into two separated ground rods? Does that come under the CC rule?
The type of audio "base" band induction I know about works within a building or perhaps around a particular open space, such as a sports field. It consists of wire traveling parallel to the ground all the way around an outer boundary, the ends coming together at the location of a power amplifier. I am not sure how many loops add to the inductive effect, but by driving the giant loop of wire a "receiver" consists of a VLF (Very Low Frequency) loop antenna and a small pre-amplifier/amplifier/speaker or head-phones and allows connection-free movement within the served area.
Perhaps this method might be subject to all kinds of VLF interference, one would expect.
I have some friends in a house about
800 feet away.
My Part 15.219 was heard in the house
on a generic boom box with about
70 percent signal and 30 percent noise.
i don't know how sensitive the boom
box was. Not the greatest to listen
to because of the noise, but the
signal was very copyable.
This was before my 15.219 set-up was
really optimized. The reception probably
was better after I got a lot more ground
radials down and made a better loading
coil. But I never thought to go back
there and check.
My main observation was that
my 15.219 signal did penetrate the house,
and it did get inside. And for some
unknown reason, there was not much
electrical noise in their house at all.
I know these people better than anybody else
on the street. There is no problem getting
into their house to check.
So when I get the CC running their house will
be the first place I will check. Actually, I
have some friends about 1100 feet away. I
can try in their house too.
I really really wonder how it will be.
Bruce, DOGGRADIO STUDIO 2
"800 feet away my Part 15.219 was heard in the house
on a generic boom box with about 70 percent signal and 30 percent noise."
See here is where the BIG difference will come in between the 219 and 221. At 800 feet, the 219 signal can be heard, but heard with noise due to the far field signal weakened by the distance.
CC...would be crystal clear because that boom box would be picking up the near field on their power wiring even 800 feet away from the coupling source. 219 emits from a central point and must rely on far field, where the signal weakens as the distance increases, all the while the near field of CC is strong even up to several blocks down the line, and possibly further if the grid is configured right.
Where 219 cant reach, CC can. Where CC cant reach 219 can with clusters. CC can also be clustered to extend coverage even further. Both have their advantages and disadvantages.
RFB
fim pics
Wow, that piece of gear looks really great and gives you a much desired method of monitoring your transmissions.
I understand why you are getting it.
actually have it already along with the iAM tx. both are sitting on my workbench now.
"I understand why you are getting it."
Any serious license free broadcaster/tinkerer is going to have something around to make measurements. And the FCC does recognize other test gear beyond the Potomac FIM's to be acceptable alternatives and accurate enough for measuring.
Make no mistake, such measuring equipment is hard to come by at a decent price and in good working order and getting it calibrated. It is just as important as anything else in the station's inventory, if not more important.
Just going by guesswork and someone else's say so or what's written in an assembly manual is not good enough.
Besides that, it also helps in peaking your antenna system performance, again eliminating guess work and hear say so nonsense.
RFB
Ermi,
I realize that your posting is over a year old. However, if you read this, I would very much appreciate it if you could elaborate.
Assume that I have a single-turn, broadband, shielded loop of 1M circumference (for example) and that I have calibrated Antenna Factors for this antenna. Also assume that I will be using a spectrum analyzer to take measurements in dBuV and later convert to uV/m, accounting for Antenna Factor.
Now, I want to take near-field measurements of a CC station at 1/2 the 15uV/m distance. As I understand things, I should measure 120uV/m maximum (2^3 * 15) at this distance to be in compliance.
But this 120uVm is a far-field value. What level will I really read on my spectrum analyzer, and what calculations do I run to convert this H-field reading into a correct E-field value?
Joe