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Post by Admin on Jan 31, 2020 22:14:01 GMT
Long ago and far away there was talk of a project to put some Part 15 operations on a small slice of spectrum in the ISM band at 13.560 mHz. Part 15 use is permitted with up to about 14,000 uV/m. There is enough bandwidth centered on 13.560 to permit AM operation. Here's a little kit to rekindle the interest in this project. For $25 bucks, you can buy a CW beacon transmitter. Looks simple enough to put together in a short time. Coupled to a dipole antenna, it would be interesting to see if propagation at that frequency would result in reception reports from afar. Check it out here: The 13.560 mHz Beacon KitIt has a built in CW memory keyer Tell them what message you want to send out and it will be programmed in for you.
I've ordered mine. I'll let you know how it goes... |
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Post by Boomer on Feb 1, 2020 1:08:45 GMT
Congrats on your purchase, Black Cat Systems has been around for a long time making small hobby projects like this, and various apps for signal logging of stations and other things.
I remember that project from a few years ago, several senior members of Part-15 were working on it and I think each added a separate part to the circuit. I didn't see the results and if anyone got theirs on the air, but Tha Dood built a similar rig and had it on the air.
I've been interested in this band, and when I first was interested in getting a crystal for 13.560 to build a small transmitter, they were expensive. Recently a hobbyist who retired sent me some of the small can 13.56 commodity crystals as used in the Black Cat rig, and I was thinking about making a simple modulated oscillator for the band to see what it could do.
You could even experiment with the beacon by adding an AM modulator to it, transistor or even a diode modulator, since it has extra power output headroom.
I like that it has a 5-pole low pass filter, even for 5 milliwatts, and uses the ATTiny micro controller as a keyer, that's all you need in something simple.
Maybe we should suggest that Black Cat make Part-15 mediumwave gear as well, I don't think they're doing it.
Boomer
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Post by Admin on Feb 1, 2020 20:00:51 GMT
I bought a couple 13.560 mHz 4-pin oscillator chips when everyone was talking about getting on 13.560.
They work just fine and you can even modulate them by simply putting an audio source in series with the power lead.
Of course the output wave form is a square wave since they were intended to be a TTL clock signal.
But, passing that through a 5 pole filter would clean it up.
My intent was to build a buffer and final modulated stage to follow the oscillator chip. Some day I'll get around to it.
As it was, the signal could be heard maybe 100 feet away using just a short piece of wire for an antenna.
The Black Cat beacon should be here today so perhaps tonight or tomorrow I'll have it on air!
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Post by thelegacy on Feb 2, 2020 5:02:11 GMT
I think it would be cool to be able to send an AM carrier through it. If you build a dipole antenna for the correct length or build an ATU for it I think you could do some serious distance. I would think you stand a good chance if they ever up the rules to 100 milliwatts down there.
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Post by Admin on Feb 3, 2020 2:47:23 GMT
Ok. It took just a little over an hour to assemble the Black Cat Beacon and tweak the tuning. It's on air now. The kit has 5 extra caps to pad the crystal, to move the frequency a bit. By trial and error, the caps are changed around until the best combination is found. I was able to put it within 500 Hz of 13.560. The built in CW keyer was programmed with "MRAM 1500 CF OH V V V". If you happen to here it, let me know. It sounds like this: Beacon.mp3 (425.37 KB) Since it is CW, your receiver will need to be capable of receiving CW or SSB. Otherwise all you'll here is on/off clicks or the rushing sound of an on/off dead carrier. |
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Post by Admin on Feb 3, 2020 20:38:50 GMT
I drove to a couple locations today and while out I checked for my signal.
I'm hearing it about 3 miles out, not strong but copy-able.
The beacon is connected to a ground mounted Tri-band vertical made for 10-15-20 meters. Not tuned for 13.560 mHz so maybe an antenna for that frequency will perform better.
The receiver is a Tecsun PL-600 using the built in whip antenna.
Surprisingly enough, while driving around and listening I also picked up a few signals. Probably RFID as the signals were heard driving past businesses.
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Post by Boomer on Feb 4, 2020 2:43:49 GMT
Radio Gets Results
Good real-world experience there, and it seems a few miles is already pretty good for just being a test. I'll have to give it a shot, and probably since I'm pretty close to your QTH as HF goes, it might be best to try to hear it in the middle of the day, like high noon.
Maybe those other signals on 13.56 were Flo's price gun from the insurance commercials, that was my first thought.
I've heard other beacons there, and things that sounded like digital modes, and voice communications, but that was on an analog receiver, so it might have been adjacent to the Part-15 band, I wasn't sure.
I found out about it through Popular Communications magazine, when someone wrote about frequencies hidden away on HF, and you could hear spies and other strange signals, so I started checking them out, and 13.56 was on the list.
Boomer
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Post by Boomer on Feb 7, 2020 4:23:03 GMT
Thursday afternoon I listened on a SDR in Southern Pennsylvania, didn't hear MRAM, but zooming in on the display there were 5 or 6 carriers showing as vertical lines clustered around 13560, and an elevated noise level, S5, 10 khz centered on the center frequency, compared to the average noise level of S3 outside of the band. Maybe the noise was all of those RFID systems leaking off microvolt signals from stores and warehouses everywhere.
I heard what I thought was one CW operator, and maybe a second one, but the dots were lengthened to 1-2 seconds, listening in CW mode. I could see it on the display better than actually hearing it.
A friend thought we might be too close together to receive the signal, and maybe he means with our low sunspot numbers? I could check out a propagation predictor to see when it could be possible to have a path between our locations.
Boomer
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Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on Feb 7, 2020 14:04:30 GMT
It took just a little over an hour to assemble the Black Cat Beacon and tweak the tuning. It's on air now. I was able to (Edited by author on Sat, Feb 8 2020)put it within 500 Hz of 13.560. If you happen to hear it, let me know. (Edited by author on Sat, Feb 8 2020)
For some expectations about that, below is a table showing the free-space field vs. distance from a 13.56 MHz transmit system producing 15,848 µV/m along a given direction/bearing at a distance of 30 meters from its radiator.
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Post by Admin on Feb 8, 2020 0:27:41 GMT
Thanks Rich. You are always a wealth of information whether or not it's encouraging. But you are off by a factor of 10 on the field strength. Hope springs eternal though. My 1 watt 20 meter QRP rig has made contacts in excess of 600 miles. The beacon frequency is only 1/2 mHz lower. Propagation should be very similar to 20 meters. Granted QRP 1 watt TPO is a lot more than 4.6 mW TPO but you just never know... And local ground wave, using a ground mounted vertical not cut for the frequency, I have copied my signal at over 3 miles from the antenna. More info on 13.560 mHz Beacons: Hifer2.pdf (87.48 KB) |
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Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on Feb 8, 2020 13:31:49 GMT
Deleted by author.
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Post by Admin on Feb 8, 2020 14:25:33 GMT
Rich I was referring to your stated FS amount of 1,548 uV/m being off by a factor of 10. Per 15.225 the permitted FS @ 30 meters distance is 15,848 uV/m. See 15.225 here: ecfr.io/Title-47/se47.1.15_1225?ampAs for the authors stated power levels, I didn't crunch the numbers as it appeared he did that. I'll have to do that... edit: I suppose if your FS voltage amount was off by a factor of 10 that would account for the difference of a power factor of 100 (squares and all.) edit: Rich deleted his analysis. Perhaps a re-evaluation will prove better. Of course we're not talking much power here anyway. |
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Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on Feb 8, 2020 15:26:51 GMT
Rich I was referring to your stated FS amount of 1,548 uV/m being off by a factor of 10. ... Right you are, and thanks for catching it. My speed reading can be a problem, at times.
I apologize for posting what I did about that, and deleted it so as not to supply bad information.
Rich
PS: Looks like I should edit my post in this thread from yesterday, 2/7/2020 at 8:04am
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Post by Admin on Feb 8, 2020 15:32:34 GMT
Well, on the plus side it made me look twice.
Curious to see another analysis with the correct FS since I don't have the software to do it.
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Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on Feb 8, 2020 16:44:01 GMT
... Curious to see another analysis with the correct FS since I don't have the software to do it. See below, which is based on the 4.6 mW input power that was mentioned/referenced in your earlier post, along with a set of ~ typical installation conditions.
Its maximum radiated field is a bit greater than the FCC specifies, but fairly close.
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