So what was the purpose of the original graph in the first place?
To show the operating parameters of that system for the conditions stated there.
The frequency, power, coil loss, 102" whip, use of a ground rod, and earth conductivity represent common choices/values present in Part 15 AM installations.
The graph does not apply accurately to every Part 15 AM system, just the one described there.
It might give some insight into the performance of systems whose component values and operating conditions are close to those in the graph.
But as with all mathematical evaluations, if the values in the analysis change, then so does the answer.
No doubt, theoretical projections are a great tool to provide a starting point as one begins their experiment to see how changing parameters affect performance.
Of course as they say, the proof is in the pudding. Theoretical evaluations, being a great starting point, aren't going to take into consideration the unknown variables as the plotter has already pointed out.
Now as a tool, these theoretical projections may circumvent some real-world test configurations but where's the fun in that? It has been said you learn from your mistakes. And even if you are re-inventing the wheel sometimes, if you didn't know we already had a wheel, you can learn a lot by doing so. Just ask the mathematician that developed calculus because in his isolation didn't know it had already been done. He was held a genius!
Theoretical projections are fine but theories have to be proven. Go outside and prove it!
Yes, but why that particular chart, with those particular conditions (out of many that could have been chosen) in that particular context?
...why that particular chart, with those particular conditions...
Repeating from my post earlier this morning:
The frequency, power, coil loss, 102" whip, use of a ground rod, and earth conductivity represent common choices/values present in Part 15 AM installations.
But not all of them, obviously.
I just think that when these things are posted, care should be taken to ensure that they are not misinterpreted. It wouldn't have taken but another sentence to do that - something like, while these are common values, it is possible to get higher field strengths with better conditions (as Neal has proven, and as I have seen in a few of my installations).
... it is possible to get higher field strengths with better conditions ...
It is equally possible to get lower field strengths with worse conditions.
(as Neal [sic] has proven, and as I have seen in a few of my installations).
As I posted earlier in this thread, my NEC model of Neil's system calculated higher fields than those shown in my graph for a more typical system.
No surprise there, as that is to be expected for the physical differences between Neil's setup and those applying to my original graph.
That reality _supports_ the use of NEC, rather than questioning or dismissing it.
What is the difference between mathematician and an engineer when their checkbooks do not reconcile by a few cents? The mathematician will spend a lifetime, if needed, to get it to reconcile while an engineer will declare "Close enough!" and move on.
I reviewed the information on Rich's chart and found that the major differences between his assumptions and my actual station were the ground conductivity and the ground resistance. The ground resistance he used was much higher than that measured for my antenna system and this certainly accounts for my apparently stronger field strength at 1 mile than estimated. Having said this it is important to note that Rich was not modelling my station so what is the problem if the numbers didn't match? My original post was to point out that there are factors affecting the remote and weak field strength which cannot be accounted by the simulation. I had assumed that my installation was "typical" and hadn't noticed that the parameters didn't match and wanted to present reasons which could explain my observations which were at variance with the conclusion reached by using the NEC data.
An engineer, comparing such a prediction with actual range, would not be surprised if they disagree by quite an amount because of the factors I originally posted. When it was noted that the simulation did not match my experience then it was obvious that something was wrong and, in my judgement, the discrepancy was large enough that it was worth pursuing. It came out that it was because the parameters used were not for my station.
A few years ago I reported on the build of my station and had measured the coil RF resistance, the ground resistance, and my transmitter efficiency. Rich kindly used the physical and measured data I gave him to run a NEC simulation and everything except the antenna capacitance and tuning inductance was in very close agreement with my measurements.
I can measure everything about my station except the field strength and this is why it was useful to use Rich' NEC simulation, that is to estimate the field strength from which I expected about a mile range, which I have.
The experience of this thread illustrates that a simulation is only as good as the data input and if these data have to be assumed then the results can be questionable. If the input data are accurate then the results are also accurate, or at least acceptable if they are seen as being close enough.
Neil
I am not questioning the validity of those graphical results or the use of NEC4.
I am questioning putting that particular post in the thread after a post on a range claim (and Rich even quoted the 3 mile claim at the beginning of his post).
If one just looks at that graph quickly, without questioning the assumptions, then you would conclude that a legally compliant Part 15 system would get less than a mile range. Even Neal thought that was the purpose at first, hence this thread.
It appears to me that the post was put there to debunk the 3 mile range claim (otherwise, why bother?). When, in fact, it only shows one possible scenario for a legally compliant Part 15 installation, and in Rich's own words, describes what might be an installation with 'typical' conditions (which may or may not be true).
I personally don't know what the maximum range would be for a legal install under absolutely ideal conditions. Perhaps that could be graphed and posted as well (i.e., what would be the field strengths under ideal conditions at 1 mile, 2 miles, 3 miles, etc.), and then we could all view claims about range on a solid technical footing).
... I personally don't know what the maximum range would be for a legal install under absolutely ideal conditions. Perhaps that could be graphed and posted as well (i.e., what would be the field strengths under ideal conditions at 1 mile, 2 miles, 3 miles, etc.), and then we could all view claims about range on a solid technical footing).
To ArtisanRadio:
If you wish to post the parameters you ascribe to a legal install under absolutely ideal conditions, and assuming that they permit doing so, I will use NEC4 to evaluate your assumptions based on FCC Part 15, and post the results here.
But I wonder how the knowledge of such maximum range would benefit Part 15 AM operators who never could operate under such conditions.
Something to shoot for rather than shoot down.
Might a perfect goal be more frustrating, than realistic?
I see it all the time. Claims of 6 miles range or even more. And, of course, details of the install are never forthcoming.
I would like to know what is possible, legally. Not likely, but possible. Given no obstructions, as close to a perfect ground as possible, multiple radials (64 or 128) - I don't know whether quarter wavelength ones would be better or worse than, say, 20 foot ones, etc.
I think everyone would like to know that if they are serious Part 15 AM practitioners. As MRAM says, it's something to shoot for. I'm actually very happy right now getting a few hundred meters (on BETS-1 FM, still working on the AM indoor install, but that distance, for what I'm doing now, would be fine as well).
The graphic below shows the groundwave fields at 1, 2 and 3 miles from the system described there.
The configuration shown for the transmit system is based on compliance with FCC §15.219, a practical antenna and radials that a Part 15 operator might install, and readily available Part 15 AM transmitters (properly adjusted).
The table shows the fields for earth conductivities ranging from 30 mS/m to 0.5 mS/m, which would include the best conductivity in the U.S to about the worst. But likely few Part 15 AM operators will know the r-f conductivity of the earth at/near their own installation site.
The fields shown in the table can be much greater than shown there by using elevated installs with long, radiating "ground" conductors, due to their greater radiation efficiencies. However those systems are non-compliant with FCC §15.219(b).
Great info, Rich. Thanks for posting!
Everything was going well with the responsive Chart & Graph until, with no relational significance, the section was tacked on showing how to make the installation non-compliant with Part 15.219.
Long gound lead.
Negative. Negative. Negative.