CC solutions and power transformers

how do I tell where a transformer which is going to kill the signal is?

Unfortunately all of the transformers are built with electrostatic shielding to prevent noise and induced EM on the high tension lines from traveling down into homes and businesses. So every transformer you see hanging on a utility pole or sitting on a slab of cement on the ground is a road block to a CC signal.

However there are 2 things that effectively bypass this problem

The first is to couple to the neutral line. The neutral line, though some do actually connect through these transformers, are still connected to the power grid neutral even on the high voltage side. They connect to a common neutral point and then to a ground lead that runs down each utility pole. One would think that this common point connection might just take your signal right into ground...well it does...somewhat. However a majority of the signal continues to travel down the neutral line, effectively bypassing those stubborn overhead and ground mounted transformers.

The second way that a CC signal could bypass the transformers is if there is a service provided via the power grid called BPL, or Broadband over Power Lines. It is a new type of broadband net delivery service, mostly found in rual areas and small towns where there are no major ISP services with broadband to offer. The transformers are modified with bypass capacitors by the power company and allow RF signals to "jump" across the transformers. BPL frequencies begin at 1.720 Mhz and use what is called "spread spectrum" transmission..meaning they span a very wide range of frequencies up to about 50Mhz.

In the forum library you can find useful information about neutral coupling by LPB as well as a couple other publications covering the same subject.

Carrier Current broadcasting is regulated in Part 15.221 of the rules.

Quick review of neutral line coupling:
Neutral coupling is done this way. (Based on the LPB TCU-30 coupler and TC-8 coupler)

There are 4 connections in the coupler, labeled N/L/L/L. N is the neutral connection, the L's are the "hot" lines. Normally you would connect the power grid neutral to the "N" connection in the coupler, and the "hots" of the power to the "L" connections.

But in neutral coupling, you connect the "N" connection in the coupler to an Earth ground. This can be the typical 8 foot ground rod or series of ground rods. Keep this connection path as short as possible, typically less than 5 feet if you can.

Now connect 1 of the "L" connections in the coupler to the power grid neutral wire in the breaker box. This wire will have white tape wrapped around it and is where all of the white wires to each outlet in a building connect to on a screw terminal buss. After making this connection, you are done..do not make any further connections to the power grid.

For further coupling effectiveness, you can take one of the unused "L" connections in the coupler and connect that to the green wire in the breaker box, which is the power grid Earth ground. This works really well for the upper portion of the MW band. It merely adds a little more inductance loading for the coupler and provides a better VSWR in most circumstances, but is not necessary.

For the lower frequencies of the MW band, utilize the same connecting sequence as described above.

It takes roughly about 10 watts input to the coupling unit to effectively induce the signal onto the power grid wires, no matter if you are coupling to the neutral or the hots. In some locations, it may take a bit more power or it may take less.

To meet FCC specification and requirements, the signal must not exceed 15mV at a certain distance which is dependent on which frequency you use. Naturally the lower frequencies will allow for a greater distance of signal off the power line neutral wire. For example, my station operates on 1.670 Mhz. The formula used to calculate the 15 microvolt maximum signal yields a distance of 93 feet. That means that anywhere my station's signal travels on the power grid neutral wire, the signal cannot exceed 15 microvolts at a distance of 93 feet from that wire, even if that wire carries my station signal all over the city, the signal must not be more than 15 microvolts at 93 feet distance from that neutral wire.

The formula for this calculation can be found in the publications mentioned in the forum library.

Hope this helps! 🙂

Just a side note to add...I do engineering consulting for commercial radio and television engineering as well as for community radio. Be glad to help.

RFB

RFBurns thank you for sharing your real world experience about sending CC on the AC neutral. Previously I have only seen theoretical discussions and the application sheets.

Since the transmitter power need might vary, it would be best to have variable power, which LPB once had available, but now, does anyone manufacture a variable power AM transmitter?

thank you for sharing your real world experience about sending CC on the AC neutral

Your welcome.

Traditionally these systems would use the "hot" wires. Well as the old saying goes...tradition does not mean it is the only way. Neutral loading is actually no different from coupling to the hot wires. There is only one basic difference..and that is the hots go to a transformer where the neutral does not. In the end, your still coupling to the power grid and regulated by the same specifications. Nothing changes, except that your signal no longer gets road blocked by those transformers.

Many would say that carrier current is only for educational institutions. WRONG! Again there is that stubborn old cow way of thinking because of tradition.

There are NO rules that state a carrier current station is only allowed to be set up and operated by colleges. Although carrier current was started by students of a college, that does not mean that it is strictly limited to colleges. There are examples of carrier current stations outside of the college campus. *Military bases, sports stadiums, convention halls, mental and penal institutions, trailer parks, summer camps, office buildings, and drive-in movie theaters. (*Taken from Wikipedia article: http://en.wikipedia.org/wiki/Carrier_current)

Since the transmitter power need might vary, it would be best to have variable power, which LPB once had available, but now, does anyone manufacture a variable power AM transmitter?

Yes. Radio Systems Inc makes a 30 watt variable power transmitter for carrier current uses.

http://www.radiosystems.com/carriercurrent.html

They also make a coupler unit similar to the old LPB units.

You can also find LPB gear often at auctions sites such as ebay or ibid. Not often because those are the most sought after units, but sometimes I have seen as many as 5 at a time being auctioned off. Most of them come from college setups and movie theaters where these locations have moved to newer systems or as in the case of the college campus, have upgraded to a licensed station.

As long as you adhere to the Part 15 221 specifications, you are perfectly legal. I have owned a carrier current station since I first got into radio engineering way back in 1973. I remember my first setup, all home made gear, even the coupler. The transmitter was a tube based unit and the coupler consisted of tuned circuits. It sure was fun to experiment back in the day.

Let me make a very important note about setting up a carrier current station, especially those who are sitting there with these 100mW transmitters. IT WONT WORK! There is simply not enough power from these transmitters and your signal will be absorbed by the wiring within a few feet as it travels down the line. You need WATTS and PROPER COUPLING.

Also, it is VERY important to have a proper coupling unit, be it home built or professional unit. One publication found in the forum library makes reference to just use a length of coax and connect that center conductor to the neutral wire or strain-relief wire at the service drop. It shows the use of an alligator clip. WRONG WRONG WRONG!!!! Here is why.

That neutral line, which is found ALWAYS at the upper most and center part of the power grid system, even on the very high voltage, high tension power lines, is positioned there for a reason. That neutral not only serves as the current return of the AC power system, but is also where lightning strikes will most likely hit. Without the proper isolation and fusing, connecting to that neutral line or service drop strain relief line, you run the risk of having lightning potentials coming right into your transmitter and POOF!!! Not to mention the risk of a nasty fire and the risk of life!! PLEASE PLEASE PLEASE use adequate isolation and fusing if you decide to set up a carrier current system with that technique!!! I am surprised that the author of that publication does not include isolations and fusings for that type of connection! BE SAFE, NOT SORRY!!

Also, even though your working with a line that does not contain any voltage potentials, your still working around lethal voltages elsewhere in the breaker boxes or power outlets. Take the appropriate safety measures when working around power lines and breaker boxes!!

RFB

I've failed so far to find a dealer. I do business with BSW, who is supposed to be a dealer, but I can't find it in their catalog.

What about a vintage unit like the LPB AM-25B? There is one on eBay.

What about a vintage unit like the LPB AM-25B? There is one on eBay.

Yes. That unit is a true red, white and blue carrier current transmitter. The AM-25 is a fantastic unit. I have one as a spare with my two LPB TX 2-20 units. I keep it protected in plastic wrap and inside a foam packed box. I have three sets of low pass filter components so any one of my transmitters can be configured for any part of the MW band.

Its not a bad price at the moment. Last I checked the bidding is up to 99.99 with just over 1 day left in the bidding.

I say go for it if you can. It is an FCC certified transmitter for Part 15.221 use. All you need is a coupler. The Radio System's coupler runs around 450 bucks. But keep tabs on ebay because sooner or later there will be a coupler thrown up for auction.

RFB

I sent a message to the seller ... dunno why, I can't really afford it right now unless I get a check in the mail quickly. He has a coupler and an additional unit, too.

Keeping watch ...

He has a coupler and an additional unit, too.

Indeed. I too checked on that with the seller a few days ago just out of curiosity as the auction has in the description about a complete package. I believe the seller will be posting those items soon from what I was told.

Yep keep that on the watch list for sure! 🙂

RFB
(196) 😉

A couple of years ago I connected an LPB transmitter
through a coupler into the power line. I was hoping
to get a powerful signal into the AM radios of maybe
a few neighbors. I was mainly interested in seeing
if it could be done AT ALL. (Oh, yeah, I tried neutral
loading also.)

the LPB transmitter was a small one, and only put
out about five watts. It had worked wonderfully for
years. Then (of course) when I really wanted to use
it for something - it developed a problem with the
modulator. Audio was weak and very distorted.
I still have plans for this transmitter, though.

The coupler I had was on loan from a local broadcast
engineer. At some point after my experiments, he
needed it back.

I only had one crystal, 860 kHz, which is right between
2 strong stations here. So that didn't help either.

However, this project is not closed.

Also, RFB, is there any info you can tell us regarding the
range of your carrier current operation? It must not
be easy to figure out, though. You can maybe get into
the houses of a few nearby neighbors
to check the signal? If you know them well enough?

That must not be easy either.

Best Wishes,
Bruce, MICRO1690/1700

There is an old (73) LPB Signature II series mixer for sale by the same seller of the LPB AM-25D transmitter.

http://cgi.ebay.com/Vintage-LPB-Studio-Mixer-and-Monitor-Signature-II-/330533482769?pt=LH_DefaultDomain_0&hash=item4cf554b111

I remember those things. Good mixer...when the contacts are constantly cleaned on the switches and the wipers in the pots cleaned and lubed. Not exactly a smooth linear scaling in level control..but wonderful big ol knobs!!

They sure do bear a striking appearance as the old RCA's don't they!!

Also, RFB, is there any info you can tell us regarding the
range of your carrier current operation? It must not
be easy to figure out, though.

Not easy, but not too difficult considering I have a Motorola R2012D coms analyzer and a HP 8520A and IFR 3000 to do measurements. All recently calibrated about 6 months ago.

Big equipment to haul around as well as the generator, but when I did, I was able to measure the signal off the wire 8 blocks from the studio at about 11mV at 93 feet from that neutral on the utility pole. I have a friend who lives about 3 miles distant who picks it up clearly in his house on a small clock radio. At night its a little noisy but not enough to really affect listening.

Using the vehicle radio only, at about 6 blocks out you can hear the signal come and go as you drive along passing the power lines hanging overhead. If you drive down a block where the power line runs the length of the block, its relatively steady and gets very clear if you park next to a utility pole.

This is to be expected with a CC setup feeding the neutral line. However the good thing is that the signal is mostly where I want it to be and going right into the homes, apartment complexes and businesses in the area I was hoping it would.

Its nice to be walking down the isles of the local wally world and overhear conversations about programs my station carries.

Was your LPB unit the LPB 6A?

RFB

Reception on a clock radio 3 miles away!?

Wow!!

Well, that goes to show what an experienced
engineer can do. That's very impressive!

I don't remember the model number of my
LPB, but it's in the cellar and I know where it
is. I'll have to check. I do believe that the
modulation transformer is rated at 10 watts
(which seems like a small number considering
how big it is.)

I think also that the transmitter uses 3 or 4
6AL11 compactron tubes. It is a beautiful
piece of gear, even though it doesn't work right.

It's sounds like it would be the LPB 6A.

I'll take a look. Years ago, a good friend of
mine did have a 25 watt LPB tube CC transmitter.
Somewhere along the way it was sold. I don't
have the money right now to be buying equipment -
but I wish that 25 watt transmitter was still around.

Thanks for the info RFB!

Best Wishes,
Bruce, MICRO1690/1700

P.S. Are you on the low end of the dial? From
what I understand, that's where the CC rigs
work best.

Are you on the low end of the dial? From
what I understand, that's where the CC rigs
work best.

Usually in most cases the low end of the band is best because of mainly one thing...the distance from the line that the signal can reach is farther than what is allowed at the high end of the band, and the signal should travel farther down the grid.

But this is a 50/50 shot because of the various configurations on the power grid layout. I found for my location and power grid configuration, the upper part of the band works best than the low end of the band. My CC station operates on 1670 with 20 watts drive into the coupler from the LPB TX 2-20 (standby TX is also a TX 2-20, both use the C-CUFF C-QUAM exciter). Using the very same coupler (I have 2 TCU-30 couplers and 2 TC-8 couplers) and firing up the LPB AM 25 unit which is configured for the low end of the band, on 670Khz with the full 25 watts into the coupler, I get less performance, but not severely. Its odd..but that is how it turned out to be doing my testing to pick a spot where to park the station.

Biggest thing about setting up a CC station is finding that right combination for effective coverage. The frequency, the way the power grid is configured, ground conductivity, coupling efficiency. Mostly its the way the power grid is laid out that is the unknown in how it will deliver the signal for a given frequency.

Yes it sounds like you got an LPB 6A unit with the tube description..but I believe there were a couple other models that uses the 6AL11's. Could just be a weak tube causing the audio issue or some circuitry below.

RFB

The recent auction for the LPB AM 25-D unit just sold for 430 bucks! OUCH! That is a tad high. But it is an example of just how popular and sought after these things are. They are built like tanks and top quality components were used in their design, which is why they will run and run. Short of a direct lightning strike, they can maintain a solid carrier and frequency even with the stock exciter while exposed to a wide operating temperature range.

430 bucks! Last time I won on an LPB auction was for an AM 25 25th anniversary edition with its matching 25th anniversary coupler and got the package for 350! This recent auction is the most high dollar auction on an LPB transmitter by itself I have seen ever.

RFB

Thanks for the info about your
CC set-up. Again, very impressive!

The rig in my cellar is an LPB RC-6A.

Here's a brief report on the unit. (Only
because it's so late. I'll add more later.)

It's running right now, putting about 5.5 watts
into a dummy load. The modulation was all
distorted at first - then I switched some of
the 6AL11s around and now it sounds fairly good.
Modulation seems to be about 70 percent max.
Audio quality is good within that modulation range.

Who knows what's going on inside of this transmitter -
from an electrical standpoint. I'm sure a lot of the
component values have changed somewhat.
I've had it for about 25 years and have hardly used it.
Years ago, I tried to operate it under Part 15.221.
(I believe I'm mentioning the correct reg - if I have the
wrong number -- I mean the reg that allows you
to operate with a certain field strength.)

Anyway - I have a few more questions and comments
for you. I just
have to figure out what they are!

Best Wishes,
Bruce, MICRO1690/1700

I switched some of
the 6AL11s around and now it sounds fairly good.

Yep, there is a weak tube or two in there. As to the other components and their state, chances are good also that some of them will need replacing due to value changes because of their age. That is the neat thing though of those and other tube based gear. It is the fact that tube circuits have such a wide swing tolerance that the gear will still operate, though not at its peak, but enough to be able to easily track down the weak link by tube swapping, and by using fresh tubes, track down support components below that need replacing.

Hopefully you have the schematic for part values to caps and resistors, or hopefully the value numbers are still readable on the parts themselves.

Since its putting out 5.5 into a dummy load that is also good! Looks like the unit has managed to keep itself in the ball park range considering the age. With that in mind..there cant be too much wrong with it.

I would begin by simply replacing the tubes with new ones. Something tells me that by doing that..the ol gal will fire up and produce rich smooth audio on a strong carrier and be good to go for the CC project.

Here is a photo of one of my LPB TX 2-20's with the C-CUFF C-QUAM exciter currently on air as the primary TX...

And here is a sound sample using a Sony SRF-42 receiver...

http://krocksradioone.com/files/AMSTEREO1670soundsample.mp3

Be glad to help out where I can. 🙂

RFB