Transmission Line

Academically, even cheap coax is very low loss at MW frequencies. RG6 which is readily available loses 0.2dB/100ft at 1MHz. That means 95% of the power delivered into one end of a 100’ piece of RG6 at 1000 kHz comes out the other end. For that to be seen in the real world, the final transistors of the transmitter would need to be matched to the feed line impedance, and a matching network would be needed between the transmission line and the antenna at the other end. Or if the transmitter and antenna were well matched, a ½ electrical wavelength piece of coax would show the transmitter the antenna’s impedance and the antenna the transmitter’s impedance.

stvcmty, you have given a very specific answer to my 2nd question, which also serves to emphasize my 1st question. Let me base a re-statement of my opening question based on your answer to my 2nd question:

Since using a transmission line for an AM part 15 sytem causes a 5% LOSS in efficiency, why does the FCC not allow lengthy transmission lines?

The thread linked below (3 pages long) has information about this.

http://www.part15.us/forum/part15-forums/antennas/talking-house-atu-roof-house

The graphic below shows how the r-f-currents are distributed along the 3-m whip and coax shield of a system using an elevated ATU fed by a coax cable from the transmitter.

The greater the length of the radiating conductors in such systems, the greater the r-f field that is radiated (other things equal).

The pages linked by Rich, and thank you, are marginally related to my original question about transmission line between transmitter and antenna, but...

That post focuses on the now-famous Talking House with 300'-coax to ATU-antenna... now a "moot point" as PhilB points out because the ATU has been withdrawn from the market. No need to give the dead horse a 2nd beating (John WDCX).

My issue in that older post is the definition of "Final RF stage" and my current post is not trying to resurrect that question.

Neither am I asking about "elevated" antenna installations, just simple "external" installations, with transmitter safely indoors and the antenna out on the ground.

Hit the reset button and let's start aagain.

why does the FCC not allow lengthy transmission lines?

Because in "Part 15 AM" systems, they radiate -- even if (unfiltered/unchoked) coaxial cable is used as the transmission line.

However using coaxial cable between a Part 15 AM transmitter and an external ATU could still meet FCC § 15.219(b), as long as the sum of all the radiating conductors was 3 meters or less.

This analysis is based on physical laws, not on any configuration or approval process resulting in an FCC certification for a given product.

It radiates.

That settles that question.

That law of physics can be unfair at times. It's as if it deliberately targets part 15 radio stations with the FCC complicit, making a conspiracy.

Ferguson has its problems and we have ours.

When I was experimenting with my ground mounted coil loaded antenna I used a temporary transmission line of about 20 foot length as a feedline placed on top of the soil. I used a large coil with a meter as a basic field strength indicator and could detect a relatively strong field along the line indicating that it was radiating.

I also detected a field along the buried ground radials but presumably such fields will cancel at a distance due to equal and opposite lengths of radials.

Neil

How To Make a Transmission Line Work for 15.219

Alright, per Neil's example we have a given length of transmission line along the ground to the antenna.

Meanwhile, in equal-but opposite phase we have a ground-radial leading away from the antenna along the ground, same length as the transmission line.

Radiation is nulled.

Carl Blare earns lifetime ALPB membership and veiled mention at Hobby Broadcaster.

Rich stays up all night double checking the law of physics.

This may be the big loophole we all feel by intuition.

Carl Blare earns ... veiled mention at Hobby Broadcaster.

Odd that Mr Blare might contemplate/seek this, given the disdain posted by "some" about that venue.

Rich stays up all night double checking the law of physics.

I don't.  But I have spent many years studying such physical laws, and recently using them to support my posts and graphics relating to the performance of Part 15 AM/FM systems.

Should at least the same amount of study/experience be expected of those who critique my posts?

For training and dedication to RF technology, law of physics included, you have respect from us who benefit from corrective reality where it may apply, but in this case I'm right, though, correct?

I didn't hear a refutation.

I didn't hear a refutation.

No "refutation" is necessary to those having the required technical background and comprehension of this subject.

My background is comprehensive!

When thinking of a 50 kw AM station, how much of that 50 kw power flows along the outside of the transmission line from the tuner at the base of the antenna to the transmitter and is it part of the radiated signal?  

Per the above responses it would seem a fairly substantial amount of RF would light up the cabinet of that transmitter.

From practical experience I would agree that radiation from a transmission line occurs when the load impedance does not match the transmission line impedance.  But when properly terminated at both ends the only signal radiated from the transmission line would be due to imperfections of the transmission line itself which is usually minimal.

In the case of our coil loaded whip antennas, the coil reactance nulls out the capacitive reactance and leaves the radiation resistance of the whip which is on the order of tenths of an ohm plus ground losses.  This does not present a 50 or 75 ohm load impedance and  I would agree that the mismatch would cause common mode currents along the outside of the transmission line and result in radiated signal.  

This is not the fault of the coax but rather that of a poorly designed antenna system.

While seeking the next breakthrough idea hoping to advance the art and science of part 15, allow us to replay this from earlier in this thread:

Carl Blare earns ... veiled mention at Hobby Broadcaster.

"Odd that Mr Blare might contemplate/seek this, given the disdain posted by "some" about that venue." (words of Rich)

But don't you see? This website has all the intellectuals. It is we who go disdained by the anti-intellectual set.

The higher academicians find that once an individual becomes intelligent, there is no way of reversing the process. Evolution is a one way street.