I just don't buy it.
With that Talking Sign installation I talked about earlier in this thread, in the direction of open fields I got well over a mile range (almost 1 1/2 miles).
In the direction of a significantly built up area, probably 1/2 mile at most.
Something is reducing the field strength in that latter direction. If not obstructions, exactly what?
I've written before about trouble I've had getting my AM signals to go uphill.
A solid grounding to the electrical ground didn't improve it.
Burying ground radials pointing toward the uphill on one side of the antenna and away from the downhill on the other side of the antenna shot me up and over the hill.
In my final opinion no installation without ground radials will do as well as an installation with ground radials.
... Something is reducing the field strength in that latter direction. If not obstructions, exactly what?
It might be related to the radio noise level in your built-up area (SCR dimmers, switching power supplies, etc) compared to those open fields.
"It might be related to the radio noise level..."
Or it might not be.
Speculation is a starting point for tracking down a technical explanation but one needs a complete set of conjectures of which "noise level" might be one.
Perhaps there are ten or twenty other possible explanations.
For example, maybe the ground conductivity is poor in the weak direction.
Curious about the claims or intimations made in this thread about elevated installs I took a tour of the websites of three certified part 15 transmitters to learn what they say.
Grain Industries Model GI-100/1000 advises: "For the best radiation of the transmitter it should be located at a high point that is away from trees, buildings or other obstructions."
Chez Procaster writes: "The advantage of the elevated ground system is that the transmitter is higher and in the clear and more likely to have better range."
Keith Hamilton's Rangemaster1000 makes available 50' or 100' lengths of 2-pair shielded twisted pair cable for audio/power connection to the transmitter, and both the Hamilton and Chez instructs the shield be grounded ONLY at the send end and not at the transmitter so as to avoid noise due to voltage trickling on the shield.
Note to Artisan: the major transmitter manufacturers agree with you.
both the Hamilton and Chez instructs the shield be grounded ONLY at the send end and not at the transmitter so as to avoid noise due to voltage trickling on the shield.
That is correct. Even in commercial AM radio installations where long balanced pairs are run, one terminates the shield on one end.
Reading again the original post it is apparent that the data presented was the result of measurements and not simulations or speculation. It is possible that there are confounding factors which can affect such a measurement but a change in the transmitting antenna configuration or location would not be such. The observation would be strengthened if more data points, obstructed and not, could be taken.
ArtisanRadio's report of his experience describes an observation where both the transmitting site and the transmitter configuration changed which raises the possibility that factors other than obstruction contributed to the observed result.
The three manufacturers' literature cited make statements which may or may not be supported by experimental data but this is not known since no references were provided.
The comment regarding twisted pair shielded feedline raises the question of what does "to avoid noise due to voltage trickling on the shield" mean? It is common practice to ground only one end of an audio feed shield in order to prevent ground loop current from happening but I have never heard this explained in terms of "voltage trickling".
It is not a trivial matter to draw conclusions based on experiment. There are personal biases which must be overcome, assumptions which may not be correct, errors in measurement, and errors in interpretation to name a few. Good science results from critique of and replication of experiments. Without this we are left with "the data suggest..." rather than a proof.
Neil
My paraphrasology came up with "voltage trickling on the shield." Here is what Keith Hamilton literally said:
"If you connect the shield at both ends current can flow in the shield inducing noise into your audio."
In my half-schooled way of thinking flowing and trickling are interchangable. By "voltage" I meant "current."
The data suggest we need more proof.
"ArtisanRadio's report of his experience describes an observation where both the transmitting site and the transmitter configuration changed"...
Actually, no. I reported a number of observations with different sites and configurations, but the critical one is that with a Talking Sign, in exactly the same installation and location, with the wire antenna NOT elevated, I got significantly less range than if I raised it over the roofline to get it in the clear and line of sight (easy to do since it was threaded through a PVC pipe).
And even with it raised over the roofline, I got much more range (about 3 times) in a direction that had very few obstructions, essentially open field.
The Talking Sign is an ideal candidate for reporting these results, as it is grounded through the AC adapter, and thus the length of the ground doesn't change. In fact, nothing changed other than the height of the antenna, and the obstructions that resulted from the height.
I do agree that there are many factors that go into any scientific experiment. But if everything is kept constant other than the height of the antenna, and whether it is in the clear or not, it's difficult to imagine that obstructions do not play a part in observed range, theory notwithstanding. It's also difficult to believe that noise plays such a significant part in just one direction to reduce that range by a factor of about 3.
This discussion is useful, as is theory, but I tend to go by what i actually observe more than anything else.
Postscript:
It would be useful to gather observations in completely rural setting, where noise is not nearly a significant factor. But that's not going to happen, at least for me in the short term. I can't even get my signal out of my townhouse complex right now.
... both the Hamilton and Chez instructs the shield be grounded ONLY at the send end and not at the transmitter ...
For calm discussion, please:
The Procaster User Manual, underlined on page 7 ( http://chezradio.com/index_htm_files/AMTX200%20-%20User%20Manual%205Jan14.pdf) states that the shield should be connected at the transmitter end, and UNconnected at the studio end. This is the reverse of the information in the post quoted above.
Therefore, if the shield of that cable connects to the "circuit ground" bus of the transmitter, and that path is unfiltered at the transmitter for the r-f signal produced by the transmitter -- then that shield will add radiating length to the ~3-m whip attached to the transmitter.
And if the "ground" wire of the pair of conductors supplying d-c power to the transmitter connects to the circuit ground bus of the transmitter and it is unfiltered for r-f, it too will add radiating length to the ~3-m whip attached to the transmitter.
Quote from Reply 45:
... The Talking Sign is an ideal candidate for reporting these results, as it is grounded through the AC adapter, and thus the length of the ground doesn't change ...
The length of the conductor connecting the circuit ground bus of the Talking Sign transmitter to the a-c adapter and its connection to a grounded a-c outlet might vary with the installation, which can effectively change the length of that total path to "ground" (typically a ground rod at the a-c service entrance of the building). That conducting path if unfiltered at the transmitter end can add to the radiating length of the ~3-m antenna conductor attached to the transmitter.
I think a fairly easy test could be performed to see if height matters for a letter of the law/spirit of the law compliant part 15 am transmitter.
Get a lifting kite.
Get a part 15 am transmitter that can run from a battery.
Put a 3m antenna on the transmitter.
Feed the transmitter with an MP3 player using as short as practical of a lead.
Attach the transmitter to the kite.
Fly the kite with dry nonconductive string.
The MP3 player and the battery being lifted by the kite ensure there is no path to ground. (an alternative would be to feed signal to the transmitter with a non-shielded optical cable that was free of any sort of conductive path, but the transmitter would still need to be battery powered).
Fly the kite at various heights and see what the range is. Don’t go above 150’ if you are more than 5 miles from an airport; if you are within 5 miles of an airport there are other rules to follow. Make sure you obey all local, state, and federal laws for kite flying.
As a bonus, compare the coverage to the same transmitter ground mounted with radials.
If there is improvement from a ground mounted antenna to a 10’ in the air mount free of anything radiating except the antenna, then flying the transmitter at 150 feet should cause significant improvement. On the other hand, if the coverage sucks at 150 feet because there is no ground then something was grounding the transmitter and at least academically the 10’ install was non-compliant.
(I think practically the best way to do a letter of the law/spirit of the law compliant elevated part 15 AM install would be to have the transmitter feed two 1.5m ends of a 3m dipole with the audio fed optically and the power fed differentially with twisted pair that has the hell choked out of it so MW frequencies really don’t want to use it; all mounted on a non-conductive mast.)
Until every effort has been made to eliminate any sneaky grounding/coupling of RF to earth, there is always a chance for a radiating ground of some sort to cloud the discussion of range from mounting height.
How fortunate that Rich detected a difference between the shield connection instructions given by Hamilton as compared to Chez.
I started with the Hamilton when studying the published information, and for the Rangemaster the detail calls for connecting the shield only at the audio/power feed-end and not at the transmitter.
That the Chez Procaster says exactly the opposite slipped past my notice.
Therefore, in light of this meaningful difference in shield connection strategy, having been informed by Rich that connecting the shield at the trasnmitter is apt to apply radiating RF to that shield, is it reasonable to suppose that doing it Hamilton's way... connecting shield to ground only at the audio/power feed-source... will AVOID a radiating shield?
My comment which you refuted in post #45 was based on what you wrote in post #2 which is:
"I have a Talking Sign, certified. Currently it gets poor range, mounted inside at ground level, with 2 stories above it and many townhouses surrounding it. That same transmitter, installed in a rancher, with the wire antenna just reaching over the rooftop (about the same height, maybe a little higher), got over a mile range in a direction with open fields."
Did I misinterpret that this explanation was for different sites (townhouse and rancher)?
You could help me understand better if you will provide more details about the experiments, especially how you raised the antenna as described in post #45 without changing the site configuration. In other words, how do you know that the range change was due to changing the antenna height only and not something else?
Neil
... is it reasonable to suppose that doing it Hamilton's way... connecting shield to ground only at the audio/power feed-source... will AVOID a radiating shield? ...
I just spent quite a few minutes clicking through the appropriate website, and couldn't locate any page there that would restrain anyone from connecting the shield for the program and d-c power cable at the tx end of that cable.
Connecting that shield at one end only, yes -- but having a restriction about which end of that shield should be connected to "ground?" No.
Can anyone supply such a link from that OEM?
TIA.
The link requested will be given after my remark...
Unless Mr. Hamilton changed the text since earlier today when I viewed it, he actualily advises connecting the shield ground at EITHER end of the audio/power cable, but not both.
Therefore his instruction is not the opposite of that given by Chez as I'd errantly stated.
But the core of my request remains valid... so again my question is...
"If the shield of the audio/power cable is grounded only at the supply/feed end and NOT at the transmitter end, will that shield tend to radiate?"
http://www.am1000rangemaster.com/cablexmit.html