Signal propagation under 15.219 gets better the higher the frequency, with 1700 being the best choice.
But around here 1700 is filled with loud buzz caused by the 2nd harmonic of licensed 850kHz, 5kW less than a mile away. On an S-meter the strength of the buzz is less than 1, down near zero, but it overflows to 1690 and a little bit to my channel, 1680, because it contains IBOC sidebands.
WHAT IF I put a well installed transmitter/antenna on the air on top of the buzz, right at 1700? The buzz would go away in the immediate area, being covered by silence.
AS LONG AS WE’RE AT IT we could transmit our audio on 1700, and together with the 1680 signal we would appear to have a very wide signal!
What could be wrong with that idea?
2nd Harmonic
The 1700kHz 2nd harmonic of the station at 850kHz is receivable on every radio I have including the auto radio, throughout this part of town.
I think it’s a real harmonic, but with rich’s comment we now leave room for doubt.
My proposed “fix” would no doubt work only very locally, not over much of a distance.
Part 15 radio is only part realistic, the other part imaginary.
2nd Harmonic Field Analyzed
Carl – Doing some number crunching on your scenario with the 850 kHz broadcast station close to you shows:
– Their radiator has 5.05 dBi h-plane gain at 850 kHz (92-degree monopole), with 2 ohm loss in the r-f ground connection
– Their radiator has 7.13 dBi h-plane gain at 1700 kHz (184-degree monopole), with 2 ohm loss in the r-f ground connection
– Their 850 kHz groundwave field at 1 km is about 686 mV/m
When their antenna system is resonant on 850 kHz (1:1 SWR w.r.t. 50 ohms), it has an input SWR of about 14:1 on their 1700 kHz second harmonic. So, much of whatever 1700 kHz power was applied to that antenna would be reflected back to the transmitter, and eventually dissipated in circuit losses.
But assuming all of the 1700 kHz power from the transmitter was radiated, and that the transmitter just met its FCC spec for 80 dB attenuation of its harmonics, then the 1700 kHz field at 1 km would be 86 µV/m.
That 86 µV/m field most likely is below the 1700 kHz noise floor in an open environment.
Probably nearby receivers hearing KFUO on 1700 kHz are generating that signal internally, due to overloading by the very high field on 850 kHz.
These numbers are based on FCC data for that station, NEC4, and a perfect ground plane.
Either Way
The description of how the 1700kHz “2nd harmonic” is a phantom of a radio’s overload caused by the excessive local power from the main station frequency ends up being the same thing: interference caused by a particular radio station.
Now that we have been informed the problem may not exist according to my original description, perhaps my proposed very local solution might work after all?
Try it..
…and see.
I’ve never found a reason to
I’ve never found a reason to get my signal up as high as I could go in frequency. Theoretically, yes, it should go as high as possible. But reality, at least for me so far, has been that I’ve never been able to detect any difference in range. listenability or anything else between, say, 1570 and 1700. I’ve had to move off 1570 because of the appearance of another station that shows up in some places in my listening area, but the only reason I moved to 1650 was that it is largely clear, even in the evenings).
Differences So Slight
You are right, Artisan, that upper frequency differences are very small from, say, 1500 to 1700.
Probably so tiny that it would be hard to observe the difference on receiving equipment.
To go with that line of thought, I once tried using a 20-foot antenna in place of the usual 10-footer, and I could not observe any improvement in range whatsoever.
Some observable differences are noticed at close range by adjusting input to the final stage of an AM transmitter from way below 100mW to perhaps 300mW, but we’re talking feet, not miles.
1680 & 1700 Together
Problem is, if the IBOC buzz on the 2nd harmonic of 850 kHz is being heard on a receiver tuned to 1680 kHz, this means that the buzz is within the r-f bandwidth of the signals present within that 1680 kHz receiver.
The lower digital sideband of the 2nd harmonic of the IBOC station on 850 kHz extends down to 1685 kHz, and is only 28 dB below its analog modulation centered on 1700 kHz. Other digital hash extends below 1685 kHz, but should be suppressed more than 28 dB.
If you add another local, analog-modulated AM signal on 1700 kHz as you suggest, normally its transmitted r-f spectrum would not cover that hash you are hearing from the IBOC broadcast station on 850 kHz.
The FCC requires AM broadcast stations operating at 5 kW power and above to suppress their harmonics to at least 80 dB below the unmodulated carrier. So probably you are not receiving this interference as direct radiation from the broadcast station. More likely it is being produced by non-linearities within the receiver tuned to 1680 kHz, due to the very strong field on 850 kHz.
But not to discourage experiment…