Carl,
Just got caught up on the postings of late. I leave for a few days and there is progress....hmmmm...there might be a lesson here. Congratulations on getting on the air.
Now to add somethings. Your strong harmonics are a problem. From the FCC rules:
§ 15.225 Operation within the band
13.553–13.567 MHz.
(a) The field strength of any emissions
within this band shall not exceed
10,000 microvolts/meter at 30 meters.
(b) The field strength of any emissions
appearing outside of this band
shall not exceed the general radiated
emission limits shown in § 15.209.
And so on.....
Section b) is of concern here so we look now at 15.209 for out of band emissions and find:
(c) The level of any unwanted emissions
from an intentional radiator operating
under these general provisions
shall not exceed the level of the fundamental
emission. For intentional radiators
which operate under the provisions
of other sections within this part
and which are required to reduce their
unwanted emissions to the limits specified
in this table, the limits in this
table are based on the frequency of the
unwanted emission and not the fundamental
frequency. However, the level
of any unwanted emissions shall not
exceed the level of the fundamental
frequency
So, it would appear that one has to look at the radiation limits at the harmonics and out of band frequencies and ensure they are less than stipulated. I didn't research this further but the statement about However, the level of any unwanted emissions shall not
exceed the level of the fundamental
frequency doesn't quite cover it since the emission limits at the "unwanted" frequency needs to comply with the restrictions for this frequency.
On the other hand, what if the emissions at the out of band frequency are wanted (that is not "unwanted") as suggested in your two transmitters in one comment? Think the jury would buy this?
Neil
Hi Neil, I hope you had a good trip
on Thanksgiving.
Hi Carl and everybody else.
You know, it's funny, but I have seen two
different figures for 13.560 MHz Part 15
field strength. I have seen 10,000 microvolts
per meter measured at 30 meters, and then
I have also seen a different figure in some
other places. I can't seem to find it right now,
but it is something like 15,848 microvolts
per meter measured at 30 meters. I have
seen both figures quoted on this board over
the years.
Neil, knowing your knowledge, I'm willing to go
with the 10,000 figure. I wonder when it
changed? And why?
I imagine in the real world there isn't really
any difference. You guys know the equations,
but I really don't (although I could look it up.)
So from a practical standpoint, the two figures
are pretty much the same, aren't they?
Best Wishes,
Bruce, MICRO1690/1700
Hi Carl!
I forgot to thank you for the info on
the Velleman amp driving the transmitter.
I'm going to have to read everything you
and Neil wrote a few more times and think
about it all.
Bruce, MICRO1690/1700
The concentration here is on the output filter, based on the circuit referenced by SCWIS in his 9/18 posting, which consists of 3 inductors in series with the RF, tapped at two junctures by 270 pF caps. The filter I built had "surface-mount" capacitors because those were the only 270 pF I could find, but they are so tiny it was impossible to solder them properly, so I don't know if they were "in-circuit" or not. Point is my 2nd harmonic at 27.12 mHz was strong.
Upon delivery I'll try 330 pF, but have a question about the circuit. The diagram states it is 50 ohms in/out. Let's talk about the input to the filter. How does the RF output need to be set for 50 ohms? Should a resistor be installed at that stage?
Of course the output of the filter either goes to a dummy load or an antenna designed to present a 50 ohm impedance, which is beyond the boundary of ignorance.
I just did a sweep of the VHF TV band on a TV radio and the 3rd harmonic is there at 54.24 mHz. It'll be a great advantage to be able to hear these harmonic frequencies for knowing when the suppression kicks in.
This is a special Part 15 Brain Puzzle to see if you can be tricked the same as I was.
The 4th harmonic above 13.56 mHz turns out to be 108.48 mHz. What a convenient spot! But when I tuned the FM radio to the top while playing audio on the SW transmitter, all I heard was a blank carrier.
Why couldn't I hear the program?
"The 4th harmonic above 13.56 mHz turns out to be 108.48 mHz. What a convenient spot! But when I tuned the FM radio to the top while playing audio on the SW transmitter, all I heard was a blank carrier."
Quite Simple Actually
Shortwave Broadcasts Are Made In Amplitude Modulation.
The FM Broadcast Bands Are transmitted in Frequency Modulation.
Therefor The Radio Cannot Decode The 108.48 mHz Broadcast because Its In Amplitude Modulation!
Yes, Mighty 1650, you are correct! But it took me a minute or two to figure it out. Now here's one that may stump you.
At 54.24 mHz on a TV Radio I heard the 3rd harmonic, including the audio! But TV has FM audio. Why did I hear it?
New Mystery of the Night: a strong but distorted image from the transmitter shows up just on the edge of NOA Weather Radio at 162.55 mHz, but that isn't a harmonic! Why is it there?
"At 54.24 mHz on a TV Radio I heard the 3rd harmonic, including the audio! But TV has FM audio. Why did I hear it?"
that Is an interesting question, could it be the Radio in the TV band decodes both? Or The Harmonic Is Strong Enough For it To Decode somewhat?
"New Mystery of the Night: a strong but distorted image from the transmitter shows up just on the edge of NOA[A] Weather Radio at 162.55 mHz, but that isn't a harmonic! Why is it there?"
Could Be That The Transmitter Is Overloading Your Receiver Causing Images Of Your Station Showing Up multiple Frequencies. If You Get far Enough Away (usually a few feet) They Should Go Away
I found a pack of 220 pF capacitors, and the circuit calls for 270 pF, so it's a good start. But with two of them in place I could still hear every harmonic clearly. I have no way to measure their relative signal strength.
So I doubled up and put 440 pF on the two legs of the filter. Perceivable results! The 4th harmonic was gone! The goofy signal on weather band was weak, the 3rd harmonic on tv radio was weak, and the CB band 2nd harmonic still registered with strength. We're in the neighborhood!
Hi Carl!
Thank you for doing this experimentation.
You are making 13.560 MHz Part 15 workable.
Best Wishes
Bruce, MICRO1690/1700
Filter analysis continues as this morning I reviewed our latest setup, the filter posted here by SCWIS 11/18, with 400 pF caps in place of the original 270 pF caps. I've already reported a positive reduction of much of the harmonic energy, but in the light of day I pursued an impression from last night, where I noticed that reception of 13.56 mHz down at the end of the driveway was much weaker than previously and the audio sounded duller.
For comparison I returned to using 200 pF caps, because I have them, and indeed the 13.56 signal became stronger and the audio returned to sounding crisp and full bodied. From this we concluded that 400 pF is too much filtering and cuts not only into the main signal but rolls off the audio response.
It is beginning to look like the filter is correct exactly as first published.
The caps I used were 220 pF
With two of them in parallel there was 440 pF
Hi Guys:
I'm talking the oscillator stage for the
transmitter off of the protoboard and
putting it onto some perf board. Once
it is running on the perf board, I will
add the rest of the circuitry.
Best Wishes
Bruce, MICRO1690/1700
Online talk tends to be short and to the point, so I will hurry. Today I am truly enjoying the fruit of the past month, building a shortwave transmitter with the help of numerous online contributors, whose ideas led to a working model. I am streaming late tonight on KDX1 so as to keep hearing the splendid sound coming from my Sangean shortwave at 13.56. Who knows at this point how far it goes, as it's only an indoor antenna, but I alerted two people to look for it on their radios.
Because of ordering mixups I ended up with two crystals, so I'm already planning a second transmitter, I guess it'll be a backup.
What I'll do once this is wrapped up is print the schematic of the final version and upload it to this thread.
Still unknown is whether this transmitter is below or above the allowable radiation limits.
Hey Carl,
Glad ur having fun with your project. 🙂 I would not stress to much about your signal level being over or under the emission limits. You could take a working model with the antenna you intend to use, to an Open Area Test Site near your home and have a quick scan to measure your emissions.