The italicized text below was posted on a website used mostly by broadcast engineers by someone well known here and on many other "Part 15" boards.
Hopefully the response I posted to it will be useful to those wanting more information on this topic.
I'm going to share a little known secret or maybe a forgotten one. this is how you make poor mans radiating cable. take any 50 ohm cable put a suitable 50 ohm dummy load rated for your power level on one end and reverse the center and shield so that tx goes to shield and ground goes to center. there you go. it will be a leaky cable just adjust your power levels for the coverage you need. I learned that from the late pastor James R Cunningham's carrier current manual.
Response -
Note to the poster of the above clip:
For a 1:1 SWR dummy load at the far end of a coaxial cable, reversing the conductors of that cable where they connect to the output terminals of a transmitter produces no more r-f radiation from that coaxial cable than if its conductors were _not_ reversed.
In such cases/configurations, equal r-f currents are conveyed in the cable along the outer diameter of the inner conductor, and the inner diameter of the outer conductor, due to skin effect. Those currents are 180 degrees out of phase with each other in both of those configurations.
Therefore radiation from that cable after such a conductor reversal is unchanged (other conditions equal).
The Radio Systems TR20/CP15 manual also suggests connecting a length of coax "backwards" to make a radiating cable.
As for the explanation as to why it wouldn't radiate, that would be true if fed by a balanced and "floating" output. Most transmitters, however, have an UNbalanced 50 ohm output, and the outer (normally shield) conductor is also connected to ground inside the transmitter - which is (usually) further grounded at least via the grounding prong of the mains supply, and possibly via additional RF and/or safety earth grounds.
So - a properly terminated balanced line will not (in theory) radiate - and balanced feed lines are frequently used in amateur (ham) radio (450 ohm "ladder" line), and used to be common for receive feedlines (300-ohm flat TV and FM antennas), as well as audio (600 ohms twisted pair) and even ethernet - but an unbalanced line must be shielded, and connected in the proper "polarity" to prevent leakage (out of a transmitting line, or into an audio or receiving line).
If the leaky cable did leak, and enough signal bled out of it to meet the field limitations, where would the cable be located?
Would it be buried along the fence-line of the yard?
It couldn't do as well as carrier current because it wouldn't go out into the neighborhood and into the houses.
After midnight you could drape it through the trees in all the backyards, just watch out for dogs.
Pay a cable line-man a 50 and he might string it for a block up on the poles for you.
...an unbalanced line must be shielded, and connected in the proper "polarity" to prevent leakage (out of a transmitting line, or into an audio or receiving line).
But an a-c waveform has no particular "polarity," as the amount of energy in its positive-going alternations is the same as in its negative-going alternations.
When an r-f (a-c) waveform is applied to a coax cable, it doesn't matter to radiation from that cable, the power available at the far end of that cable, and personnel safety whether the center conductor is driven by the source and the outer conductor is connected to ground, or vice-versa.
There are coaxial cables designed to be "leaky," such as RADIAX®. They have a series of gaps in the construction of their outer conductor, which enable the inner conductor to radiate a small amount of the energy flowing along it into space, at each gap. However their application is designed for higher frequencies, generally above 50 MHz.
... Most transmitters, however, have an UNbalanced 50 ohm output,...
Many transmitters are able to drive a 50-ohm, unbalanced load, but that doesn't imply that their output impedance is 50 ohms. If it was, then when that transmitter was connected to a 50-ohm load, 1/2 of the output power it produces would be dissipated within the transmitter.
For highest power delivered to the load, the output impedance of the transmitter itself must be as low as possible -- less than a few ohms, typically.
My comment about inverting the connections of a coax cable to a transmitter were considering the performance of the cable, itself. However that system would be awkward to implement.
This thread opens by presenting an idea from a person "well known here at this and other part 15 websites," then proceeds to discount the man's idea, rendering it null.
???
Is this "well known person" to be celebrated for presenting useless ideas?
Is this "well known person" to be celebrated for presenting useless ideas?
The point of my post was not to "celebrate" anyone, but to comment on the technical accuracy of the belief that was posted by that person.
Good morning, Rich. You're making me work over here, looking through file folders and stacks of papers.
You said: "There are coaxial cables designed to be "leaky"... However their application is designed for higher frequencies, generally above 50MHz."
BUT the FCC Rule 15.221 states "Carrier current systems and transmitters employing a leaky coaxial cable as the radiating antenna may operate in the band 526-1705kHz..."
THEN we find in Radio Systems documentation for their TR-20 Transmitter, and AM transmitter used in carrier current, T.I.S., and, yes, leaky cable systems, with a paragraph on Page 5 describing options available when electrical wiring does not exist...
Option #1 - "Leaky" Cable Systems - Several coaxial type cables are now being commercially manufactured to specifically "leak" or radiate energy relatively uniformly along their length. This is accomplished by either utilizing slots in the cable shield or a thinly braided shield to radiate signal."
As the resident RF engineer you have put us at an impasse regarding leaky cable.
Radio Shack coax IS leaky cable.