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We hear about wider-diameter copper pipes provided “better bandwidth” than a thin piece of wire. But AM bandwidth is 10KHz, which isn’t very wide, and exceeding this would be a violation. Yet it would be bad to have an antenna with a bandwidth of, say, 5KHz, but that antenna would have to be exactly resonant at the operating frequency for such small measures to matter. Is a very thin wire really less than 10KHz in its bandwidth?
More – If the bandwidth is extra wide the antenna will receive signals of high power stations at nearby frequencies and there could be trouble with noise on the carrier. I need education.
Total posts : 45366Electrically-short antennas would have very narrow bandwidth if there were no ohmic losses in the antenna circuit, other than the radiation resistance. In that case, the Q would be in the thousands, and the bandwidth would be very narrow. In reality, the losses in the antenna circuit are substantial, primarily because of the resistance in the ground, loading coil, and the source resistance from the transmitter; so the Q will not be very high, and there is unlikely to be any problem with the RF bandwidth for Part 15 AM. The loss resistance reduces range, however.
Even so, it is better to use a large radiator diameter than a narrow diameter. This is because the lager diameter radiator will have lower capacitive reactance, requiring a lower inductance loading coil, which will have less loss than a higher-inductance loading coil.
Total posts : 45366.
Carl,
Ermi has addressed your major question in his reply and I just wanted to add this for clarification:
You wrote “But AM bandwidth is 10KHz, which isn’t very wide, and exceeding this would be a violation.“. Part 15 rules for AM do not restrict the bandwidth used providing out of band emissions are attenuated per the rules. Obviously, using wide bandwidth on the AM band increases the chances of interference with licensed stations which would be prohibited (and rather unwise) but there is no bandwidth restriction per se other than this practical constraint. Damped waves (such as from spark gap transmitters) are not permitted.
Minor point but I thought you would be interested.
Neil
Total posts : 45366Thank you Ermi Roos and Neil for description of the wideband question. Outsiders would wonder at the minutia, but Part 15ers work with every pixel they can think of. Another thing I thought of after submitting my original question is that I talked about two different states of bandwidth: 1.) the width of the transmitted signal; 2.) the spectrum bandwidth over a range of AM frequencies. I think your answers were more specific than my question, which I appreciate.
Total posts : 45366Let me add to the confusion. It’s true that commercial broadcast staions are limit to 10KHZ and some actually operate at 5 KHz. The idea is to have an antenna with enough BW to accomodate a 10KHZ modualted carrier.
The question is: Is a CB whip large anough in diameter to easily pass a 10KHZ AM signal? If not, than maybe the 1/2 inch copper pipe is the solution. Naturally, one will want to make a special mount with a short wire attached to the pipe.
As in the earlier posts, running an audio BW wider than 10KHZ increases the probability for interference, and maybe sound worse on radios that are very limited in BW.
Total posts : 45366I can confirm that during the development of the Procaster I compared a wire antenna with the 3-section aluminum version which comprises 5/8″ – 1/2″ – 3/8″ tubes. The wider antenna requires a lower inductance loading coil for tuning.
Total posts : 45366If you page through Ernie Wilson’s Meduium Frequency Transmitting Antennas you’ll find a list of mathematical formulas useful for calculating the inductance needed to properly load a radiator.
Diameter is a key factor in the calculations, so it would seem that larger in diameter (within practical considerations) would reduce the inductance needed to achieve a good match.
This, in turn, would likely reduce the losses of the system, which in turn could give a slight increase in range, all other things being equal.
Total posts : 45366I’ve become convinced that wider is better and plan to convert to wider-than-wire antennas.
The discussion has included hollow tubes. What if the tubes are solid? Does extra internal mass/density do anything?
If the width became significant, say a foot or more, would the FCC measure vertical plus horizontal for their final “length”?
Total posts : 45366The discussion has included hollow tubes. What if the tubes are solid? Does extra internal mass/density do anything?
As I understand it, radio frequency energy travels on the outside, or skin of a conductor rather than through the conductor like DC current, so in my opinion the solid would just be heavier – there would be no noticeable difference in antenna performance.
If the width became significant, say a foot or more, would the FCC measure vertical plus horizontal for their final “length”?
Not even the FCC seems to be able or willing to really address those kinds of questions. It seems to be very much up to the individual field agents.
One trend that can be observed in the Enforcement Bureau’s Field Notices web site is that those kinds of experiments do not, so far, appear to generate any field notices.
Despite much chatter about the legality of things like capacitive hats, loading coils and even kit-built transmitters, it appears that only excessive ground lead lengths (which result in some kind of intended or unintended dipole) and/or ridiculous power levels really get the enforcement attention.
Total posts : 45366I went to the hardware store pipe section and the largest diameter copper looked like 4″ X 12′ at $145.00 !! Oh, no wonder no one has mentioned anything larger than about 1″ diameter.
Total posts : 45366.
Carl,I wonder if your window frame setup you described previously doesn’t accomplish the same thing as a 4″ copper pipe.
Ermi explained in a previous post the effect of diameter on bandwidth and I hope it was not lost on the readers here that factors other than antenna diameter in a practical part 15 AM antenna system dominate the antenna diameter. I wouldn’t use anything beyond about a 3/4 inch pipe and that is primarily for mechanical considerations.
I recall a principle which says “You spend 80% effort for 20% gain”. Even if true I think the percentages are way off when dealing with part 15 antennas. More like 99% effort for 1% gain once you get past a certain point.
It seems that the most effective things one can do is use a loading coil and install ground radials and put the vertical antenna outside. Beyond this there appears to me to be no legal means to increase coverage.
Neil
Total posts : 45366I’m glad the windowframe antenna came up, because I noticed something new about its excellent performance.
Yesterday I powered-up the SStran that feeds 2.5ft inside wire, attached to the bottom of a 5ft aluminum window frame, attached at the top to a 2.5ft wire outside on the porch, and the power indicater on my CCrane Plus radio was way down in the mud and there was no audio. Exploring further I found the audio cable had pulled out, and when it was reconnected the power level shot to the top and my strong outdoor/indoor coverage was restored. The audio wire is about 12ft long and connects to left-channel output from the computer. The AC power is NOT grounded. There must be a ground/plane thing going on.
Total posts : 45366One change I noticed right away when the windowframe antenna was installed was a sharper emphasis in the high frequency audio region that was almost tinny. My subjective imagination even pictured the window screen adding its own resonance to the signal, but that would be a confusion between acoustic and electical resonance, so it really didn’t make sense. Then it hit me like a proverbial brick. This was the evidence of wider antenna bandwidth and since the SStran was jumpered for pre-emphasis, the high end was actually more than desirable. By removing the jumper the audio became very solid and in every way resembled larger AM signals. What a testimonial this may be for the SStran pre-emphasis, which I now presume actually compensates for low-bandwidth antennas!
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