There are many great nations. One of my favorites is procrastination. But another one I frequently visit is explanation.
There are many great nations. One of my favorites is procrastination. But another one I frequently visit is explanation.
On the new Spectrum Analyzer I can see KMOX 1120kHz 50kW all day long registering 20dBm on the screen, picked up by the 13-inch telescope antenna.
At 8 PM, local sunset, the 20 dBm peak from KMOX vanishes, even though the station is still broadcasting and comes in on the radio.
According to radio-locator.com KMOX is non-directional, 2 Patterns, 1 tower.
A comparison of their daytime and nighttime maps look like exactly the same pattern.
How can they change pattern with only 1 tower, yet be non-directional with 50kW. What actually are they changing?
KMOX
The FCC database sometimes reports a 50 kW non-directional, 24/7 station this way.
But KMOX does not have different radiation patterns or different powers between day & night. They use a single tower, and it radiates the same pattern and ERP 24/7.
KMOX may transmit IBOC (HD) during the day, and turn it off at night so as not to interfere with adjacent channels with their nighttime skywave. If that is true, then maybe your daytime SA reading could be different than your nighttime reading.
That Must Be It
Yes, KMOX has been shutting off the IBOC at night, although I think there’s an exception on nights when major sports are in play-by-play.
Some Sites
Some AM sites doing the “hybrid” dance does exactly that, shut off the IBELCH at night.
Thank the maker.
In a way, sort of like how the back and forth fiasco of AM Stereo was.
“Is it this one, or that one, or both, or all 4, or all 1’s and 0’s, or just let it all rot and fade away”.
Maybe in another 25 or so years, it will be worked out. :/
RFB
A Related Lack of Explanation
The main local signal I see on the screen is 850kHz 5kW from 1-mile away, registering 30dBm all day until 1-hour past local sunset when they turn off to protect KOA Denver.
All other local signals show up at lower levels because they are mostly all over in Illinois, 15 or 20 miles away. I see a few stations all day, but at night they all fade into a skim of background noise jiggling across the bottom of the screen.
I was under the impression that station power remained the same at night, in the case of stations that maintain the same day/night power, but that background noise increased at night, but the SA tells me that radio signals fade down into the noise. The noise doesn’t appear to increase.
SA Settings
Carl, you might want to fiddle with the sweep speed and bandwidth settings of that SA to see if its display for KMOX can be made about constant day and night.
You are too close to KMOX for your reception of them to be seriously affected by their nighttime skywave. You should be receiving essentially only their groundwave 24/7, and their analog field intensity should be almost a constant at your location, 24/7.
If it doesn’t appear so on your SA, most likely it is a measurement problem.
Doing
Alright I’ll fiddle with sweep and bandwidth.
Also, something taller than the 13″ antenna might be good, but I like the way this setup shows the very local signals of the part 15 variety.
The manual is a little bit dizzying because it’s a translation from Chinese by a well meaning person who didn’t know their English too well, and perhaps wasn’t actually of technical mind.
The device, in any case, is excellent.
Widths and Sweeps
There are 2 selectable bandwidths, 20kHz and 400kHz. Of course the former is suitable for viewing MW/AM frequencies, and the latter provides the correct view of FM channels.
A series of pushbuttons select a range of Scanwidths, that is, how large a swath of the band is being viewed. For AM use I am looking at a range from 800kHz to 1700kHz, which contains the frequencies of interest.
There is one more useful button, a “Video Filter, so-called” which is actually a 4kHz filter, which eliminates a lot of the noise that tends to conceal weaker signals.
The “Sweepspeed” suggestion is unanswered at this time, as I’m not sure what setting would correspond with sweepspeed.
Fiddling
Carl – Maybe you’ve posted the ID of your SA, but I don’t remember where or when.
Most professional-level SAs allow setting the sweep rate (speed), which, with suitably available/selected bandwidths and spans can show even the sideband components of a single AM waveform.
Trying to make such measurements in the presence of relatively strong, local signals such as radiated by your physically nearby Part 15 AMs might be a factor, also.
Spectrum Analyzer Analyzed
The SA we have is from ATTEN Instruments, Model AT5005, manufactured by Shenzhen Atten Electronics Co., Ltd. over in China.
Before any local part 15 equipment is turned on, the local AM band is no different than after part 15 transmitters are added, one at a time.
There is a bank of attenuators for those cases when input level is increased, such as with a better receive antenna.
Oh, and I have just learned that the sweep speed is fixed at 23ms.
Filters
“Trying to make such measurements in the presence of relatively strong, local signals such as radiated by your physically nearby Part 15 AMs might be a factor, also.”
This is something I pointed out when Carl asked about the various filters used in the field to prevent such circumstances from causing measurement problems.
I’m not sure he would need any with the power levels involved here, but filters can be of the home made variant. I believe Carl has already gone through the steps of turning his transmitters on one at a time while observing the spec an looking for any problems.
Noting something else, don’t consider any test gear any less professional or any less useful just because it may or may not have a feature than the next piece of test gear.
At this point, Carl is light years ahead of the curve with that spec-an compared to what was being used before..a simple radio with an S meter.
When the opportunity comes around to add to one’s arsenal of tools and do so at a reasonable cost, why not!
RFB