I already discussed this on the virtual forum since part15.irg has been down for two days, but here's a truncated version of the discussion that took place which I think you might find interesting..
I began:
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Hey Spock, It's a given the AM broadcast signals degrade at night due to the ionosphere, but considering that Part 15 AM signals (power) are so low and don't even reach the upper atmosphere at night any differently in the day... But it seems often part 15 broadcasters report their range is best in the day.
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Spock replied:
... .. 👉 Their signals never reach the ionosphere, day or night.
So there’s nothing up there to “degrade” your signal.
So yes — strictly speaking, the upper atmosphere is irrelevant to Part 15 AM performance.. ... ..
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. What doesn’t happen to Part 15 AM at night
❌ No ionospheric absorption issues
❌ No “nighttime fading” caused by skywave cancellation
❌ No meaningful change in propagation physics
The signal leaving your antenna is the same at noon and midnight.
... ... The signal leaving your antenna is the same at noon and midnight.
One subtle exception (worth mentioning)
Sometimes Part 15 AM can actually sound slightly better very close in at night because:
Man-made electrical noise can drop in some neighborhoods
Fewer appliances, solar inverters, LED drivers active
But this is hit-or-miss and very location-dependent.
... ..
Bottom line
You’re right to question the common assumption.
Part 15 AM doesn’t degrade at night because of the upper atmosphere.
It appears worse because the band fills up with skywave interference from licensed stations, raising the noise floor and masking your signal.
--Me:---
Many, perhaps a majority Part 15 broadcasters choose 1700, as it provides the greatest range potential.. How does that factor play into this discussion?
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Great follow-up, Rich — and this is where the 1700 kHz choice really ties the whole thing together.
You’re right again: 1700 isn’t chosen by accident, and its behavior at night actually reinforces your earlier point rather than contradicting it. ..
1700 kHz usually has a lower baseline noise floor
This is true day and night, but it’s most noticeable at night, when distant stations otherwise raise the noise floor.
Here’s the key point:
At 1700 kHz, skywave interference is weaker and sparser
Fewer high-power legacy AM stations above 1600 .... ..
So while skywave exists, there’s usually less of it landing on 1700 than on mid-band channels.
That’s why many Part 15 operators report:
“1700 seems to hold up better at night.”
And they’re not imagining it.
The quiet irony
The very reason licensed AM stations historically avoided the top of the band (lower coverage, poorer skywave) is exactly why Part 15 broadcasters love it.
For them: .. Weak skywave = bonus ...
Bottom line
Part 15 AM stations favor 1700 kHz not because it propagates better at night, but because it avoids the worst nighttime interference, allowing their already-weak groundwave signal to remain intelligible.
-Me--
You said "many Part 15 operators report: 1700 seems to hold up better at night.” - From what did you derive that?
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Short, honest answer
That statement ... comes from long-standing anecdotal operator reports, repeated over many years in specific communities where Part 15 operation is discussed.
So it’s observational, not authoritative.
Over the years, comments like “1700 holds up better at night” or “1700 is quieter after dark” appear repeatedly ... .. — they’re patterned anecdote.
Bottom line
That statement was based on accumulated informal operator experience, not a citable authority.
-Me----
Ok, well the primary question that began all this is: Is it to be expected that a part 15 AM station will have less range at night than it does in the day?
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Great — this brings it back to the core issue, and we can answer it cleanly and defensibly.
Short answer
No.
It is not to be expected that a Part 15 AM station will have less actual range at night than during the day.
Slightly longer, precise answer
A Part 15 AM station’s propagated signal strength and physical groundwave range are essentially the same day and night. The transmitter, antenna, and groundwave physics do not change with darkness, and the signal does not interact with the ionosphere at any time.
What does change at night is the usability of that range, because the AM band fills with skywave signals from licensed stations, raising the noise floor and masking the Part 15 signal. ...
One narrow caveat (for completeness)
In some locations, local noise (switching power supplies, lighting, solar inverters) may be lower at night, which can partially offset skywave interference very close to the transmitter. That’s why some operators report “no change” or even slight improvement within a few hundred feet.
But again — that’s reception environment, not range.
Final bottom line
A Part 15 AM station should not be expected to have less range at night — only less usable range due to interference.
Why the common belief persists
People say “range is worse at night” because they’re really describing this:
“I can’t hear it as far away at night as I can in the daytime.”
That’s a reception problem, not a transmission one.
The Part 15 signal is still there — it’s just being buried.
I think the entire discussion is missing out on the point that it's other station interference from skywave that reduces a Part15 station's range at night. They go further, so what may seem an open frequency during the day could be filled with interference at night.
That's why the X band is popular - there are a few frequencies that don't have many, if any, North American stations. Some only have Mexican stations (but they tend to be strong).
I think the entire discussion is missing out on the point that it's other station interference from skywave that reduces a Part15 station's range at night.
Actually it emphasized that point quite clearly, the above was not the whole discussion, just some key excerpts from it, but even the excerpts shown above acknowledge your point.
"... strictly speaking, the upper atmosphere is irrelevant to Part 15 AM performance ... What does change at night is the usability of that range, because the AM band fills with skywave signals from licensed stations, raising the noise floor and masking the Part 15 signal. ..."
The point is that it has no effect on the range of your part 15 AM signal, there should be no difference in range based on day or night broadcast.
Yes, distant high power stations will introduce increased interference, but, considering that 1700 is a popular part 15 band, those utilizing it are unlikely to be affected by night time interference because there are so few licenced AM stations broadcasting that high.
"...because it avoids the worst nighttime interference, allowing their already-weak groundwave signal to remain intelligible..."
@ artisan, by the X band I assume you mean the top 1600 to 1700? Not just less interference but best signal.
@ Rich Powers, saying that a part 15 signal has less range at night is correct as the cause of the reduced range isn't the point, range can be defined as the area the station can be received listenable, irregardless of the cause. And at night the range, for whatever cause, is less. On any receiver.
"The very reason licensed AM stations historically avoided the top of the band (lower coverage, poorer skywave) is exactly why Part 15 broadcasters love it".
Not necessarily. It's mostly because the way the transmitters are designed with the short antennas and ATU/loading coil the best signal is in that range.
It's not the best place to be as any radio made before 1990 doesn't cover that part.