Here is a simple test to investigate the r-f characteristics of a conductor connected at one end to a ground rod buried in the earth.
First, locate the utility pole used to distribute a-c power to your (or anybody's) home. It should have a vertical wire attached to the side of it, connected to a rod buried in the earth.
While standing at least 10 feet from that pole, tune a portable AM receiver such as a Tecsun PL-310 to a weak AM broadcast station. Rotate the receiver on its vertical axis to find the strongest signal that weak station produces when that receiver has its internal loopstick antenna parallel to the earth.
With due regard for your personal safety, walk over to the utility pole and hold the receiver an inch or so from that ground wire, at about 5 feet above the earth. Its loopstick should be oriented in the horizontal plane (as before). The signal at the receiver will be noticeably stronger.
Now hold the receiver near the ground wire but within a few inches of the earth. The signal will be weaker than before.
In fact, the higher along that wire the receiver is located, the greater is the net r-f field arriving at the loopstick.
This illustrates that an r-f ground does not exist along that "grounded" wire except at the exact point where it connects to the buried ground rod.
A valid analogy can be made between the results of this excercise and the belief that the ground terminal of an a-c outlet provides a true r-f ground to Part 15 AM systems.
The analogy demonstrated in this instructive experiment tells us that a ground lead located above earth soil produces a radiation field starting at zero near the earth and expanding cone-like by a few feet as we rise higher in altitude.
Making a wild guess the increase of radiation thus produced wouldn't be significant until getting pretty high in the air.
Below is a chart showing the effect of the length of the ground lead on radiation, based on the Part 15 AM system described there.
The difference in radiated fields results only from the changing length of the ground lead. Everything else in this system is constant, otherwise.
Of interest is that the change in field for a 10-meter ground conductor compared to a zero length ground conductor is 171/30, or an increase of about 5.7 X.
Field intensity varies as the square root of radiated power. So a field increase of 5.7 means that the radiated power using a 10-meter ground lead is 5.7² or about 32.5 X more than with a zero length ground lead.
Does the percentage of radiation produced by an elevated ground lead mix with the radiant field from the antenna in positive phase, which of course would increase the overall radiation strength, or are there varying circumstances that can put some of the ground-lead-radiation in a phase-canceling relationship as it mixes with the antenna field, which would reduce the overall radiant strength?
The r-f current flowing to/from the "antenna" connection of the transmitter into a monopole conductor such as a vertical whip is identical to that flowing to/from the r-f ground conductor of that antenna system, but the two waveforms are always 180° out of phase with each other.
However the radiated far fields from the two sections of this modeled antenna system always reinforce each other, as the r-f currents producing them always are traveling in opposite physical directions along the two conductors. (A more complete treatment of this may be found in antenna engineering textbooks.)
NEC software correctly shows the total radiated field for these conditions.
Talking about vertical above-earth ground leads in AM transmission systems is one and the same as talking about elevated antennas.
There are two schools of thought on elevated AM antennas and I'm on the low-side of the argument which says that ground mounted transmitting systems are most effeicient.
Others will insist that raising the AM antenna improves outreach, and with these little transmitters we're talking about the probable reason for an increase in range being the added length of ground wire functioning as a longer antenna, which makes the comparison unequal.
If two transmitter/antenna systems were designed exactly alike but with one on the ground and the other up high (no long ground lead) I would expect the ground mounted transmitter to reach farther.
This discussion has not included ground-radials, which are a separate matter.
"If two transmitter/antenna systems were designed exactly alike but with one on the ground and the other up high (no long ground lead) I would expect the ground mounted transmitter to reach farther."
To investigate this concept I used a NEC4.2 model of a Part 15 AM transmitter where the sum of the vertical length of its circuit board and the vertical length of an antenna conductor totaled 3 meters. The system has no physical conductor/s connecting it to the earth (or anything else).
The groundwave field intensity it produces at 1 km when the base of the transmitter system is elevated 10 meters above the earth is virtually identical to the field it produces from 0.5 meters above the earth, all other things equal.
Usually.
A very close friend of mine has used this similar trick to set up CB and HF antennas for optimum skip or DX conditions. By tuning into a weak signal on the AM BCB and walking around his property listening to the signal rise and fall he is able to find the best place in his yard to mount a base antenna for ham or cb radio.
Does this work? Seems to work like a charm. Now it's not 100% guranteed that he or you would harness the power of DX every single time he keys the mike or receives a signal on a shortwave radio. Other conditions are involved here but it seems to help out on both transmit and receive.
I think it has something to do terrain and the orientation of the antenna but of course that one station he tuned into at it's weakest point is in one direction and could have been north or south of him or even east or west. Hard to say.
What Barry describes is very much like a discovery I made from inside the house where the portable FM receiver would get slightly better results trying to listen to the weak signal from 90.3 as I moved around.
In some locations it came in fairly well but one spot... with the radio flat against the wall and the antenna vertical right at the front porch light switch.... in that one and only spot I got a fabukous signal.
Because of the law of reciprocity I assume that placing an FM transmitter in that spot would reach out with enormous coverage. I do believe I'll try it very soon.