I will too. I would have been on a long time ago but couldn't find anything pre-built until now, and building is not my thing. You guys with the quiet reception locations are truly blessed! Maybe you'll get one of my first QSL cards!
I have always felt that 5 mW
ERP (I suppose that's what you'd
call it) can propagate via the
F layer on 13 MHz. Hams have worked
all over the place on 7 and 14 MHz
with less power.
However, that is just with Morse code
and digital modes. Even on the
best receiving set-ups, I don't
think the receiving operator could
ever hear the amplitude modulation
on the signal.
Just my opinion, of course.
Bruce, From the MICRO1700 and
Dog Radio group - GNAT 90.9,
SLUG 88.3, etc.
Suppose a 5mW shortwave transmission at 13.560mHz employed both code and amplitude modulation?
Let's talk about the code signal.
Morse code is usually a tone, although I think it can also be done with carrier on off, I'm not sure.
For tone sounded code, which I think is also called CW (continuous wave), what is the audio frequency of the tone? How much can it vary? How is it typically modulated?
Is tone sent by amplitude modulation the same as code sent by whatever other method?
If program audio included occasional Morse code tone transmissions, would the code part reach farther than the audio portion?
This is a chance for us to learn from our HAM brethren.
The most efficient means of using Morse code is by on/off keying of the carrier. Receivers using envelope detectors produce a tone for the time that the carrier is present by injecting a locally-generated r-f signal (a "BFO"*) into the passband of the i-f amplifier. The pitch of the audio tone produced at the receiver output is the difference in frequency between the down-converted carrier and the local signal present in the i-f strip.
Another means is to amplitude modulate the transmitted carrier with an audio tone, where the tone is comprised of the Morse code characters. This is less efficient, as only the a-m sidebands carry the transmitted intelligence, rather than the full power of the carrier.
*Beat frequency oscillator
I appreciate the explanation, as my understanding of how it works has been murky for a long time.
Perhaps CW stands for "carrier wave", and not "continuous wave".
If CW is the proper term for keying the carrier off/on, then the former would seem correct. This is a question without a question mark.
Trying to get to some understanding of the potential for skywave coverage by a compliant Part 15 system using 13.560 MHz...
As a baseline, a center-fed 1/2-wave dipole in free space needs about 4.6 mW at its feedpoint to generate the maximum legal field of 15,848 uV/m at a distance of 30 meters from the antenna.
Suppose that this antenna is installed near and parallel to the surface of the earth. If the earth had perfect conductivity, the total power radiated in some vertical direction would be twice the free space value, and for the same applied power, the field in that direction would increase by SQRT(2).
Considering a skywave path length totaling 100 miles (including the trip to the ionosphere and back), and a 100% reflection at the ionosphere,* then the skywave field arriving at the end of that path would be about 4 uV/m. There could be some enhancement to that by reflections near the receive antenna.
It seems possible at least for these conditions that a good receive setup including an antenna with sufficient gain in the direction of the arriving wave would have a reasonable chance of detecting that transmission, assuming lack of interfering signals and local r-f noise.
*However at the angle such radiation reaches the ionosphere for a skywave path totaling 100 miles, probably the wave will not be 100% reflected.
Rich has presented good information about CW and propagation signal strengths and here's some more communication theory which could perhaps stimulate some thoughts.
There are two equations which can apply to the situation of getting information through a system. One states that the noise power at a receiver is proportional to the square root of the bandwidth of the signal (and also the temperature and input resistance). The smaller the bandwidth, the less the noise. The bandwidth required depends on how much information is sent per second. Video and stereo music are examples which require a high bandwidth and the receiver will need a wide bandwidth thus will receive more noise. To overcome this high transmit powers are used so the signal to noise ratio is acceptable.
Voice is lower bandwidth so less noise is heard and lower powers will be acceptable. CW requires a very low bandwidth since the information rate is low and the S/N ratio can be acceptable if the receiver bandwidth is very narrow. In short, if you leave the receiver door open enough to receive a wide bandwidth it will also be open to receive more noise.
To take advantage of CW the receiver needs to have a very narrow bandwidth.
Another equation is Shannon's equation which is more theoretical than practical but essentially states that the maximum rate of digital data transmission depends on the bandwidth and the signal to noise ratio. Web search it if you are interested.
An accurate estimate of the possible range would need to consider the noise figures of the receiver, the bandwidth, the signal strength, and the modulation method. Some QRP DXers use very slow frequency shift keying at bit rates of 0.2 per second for example. The Voyager space craft used a bit rate of 10 bps to send back pictures in order to overcome the signal loss over the distances in space and the compromise was good quality at the expense of transmission speed.
Carl speculated about combined modulation so here's another story. During my campus CC days I hung out with broadcast techs, one of whom was a ham and worked at WLW. He said a couple of them experimented with shifting the 700 kHz carrier a few Hz. to send morse code. With the right receiver it could be copied.
Most digital systems in use today, such as your internet connection, use QAM coding which is a combination of amplitude and phase modulation. C-QUAM for AM stereo is such a system.
Part 15 rules do not restrict the modulation method per se but the problem is to get the receiver to match the modulation mode for best range.
Neil
There was a military data communication system called BROFICON (for BROadcast FIghter CONtrol) lasting into the 1960s. It used frequency-shift keying of the carrier of most high-power, clear channel AM broadcast stations to transmit data both between the AM stations, and to aircraft. The frequency shift and data rates were chosen for tolerable effects to the AM programs being broadcast. In the mid 1960s such hardware was still installed, but disused, at the transmitter site of one such AM broadcast station (not WLW).
BROFICON was superseded by dedicated VHF/UHF radio links having much higher data rates.
"The FCC rule 15.255 was updated at some point to allow more power at 13.560mHz.
15.225 (a) The field strength of any emission within the band 13.553-13.567mHz shall not exceed 15,848 microvolts/meter at 30 meters.
When did this change take place? Spring/Summer 2011?
Hi Ken Norris, My data sheet RE: 13.560mHz (FCC 15.225) shows a Revision Date of Oct. 1, 2009, printed from
http://hallikainen.com/FccRules/2009.
I Wikipedia'd the term "CW" and it means "continuous wave", to answer my earlier question. This triggers memory on a post a year or more ago in which we exchanged notes about the seeming paradox of using the term "continuous wave" to describe the Morse code technique of triggering the carrier on/off.
And, in the hunting around, I found a fascinating post from Mark Amos W8XR on the subject of CW Bandwidth...
http://www.w8ji.com/cw_bandwidth_described.htm
I Wikipedia'd the term "CW" and it means "continuous wave", to answer my earlier question. This triggers memory on a post a year or more ago in which we exchanged notes about the seeming paradox of using the term "continuous wave" to describe the Morse code technique of triggering the carrier on/off.
It is a bit confusing, but the r-f waveform transmitted during the "on" parts of each Morse code character are comprised of energy having continuous characteristics (frequency, and amplitude) -- except for the rise and fall times of transitions between the on and off states.
Rich - Right On! Your post #21 mirrors my theory I tried to explain in post #13. Because of the ground reflection's tendency to boost the signal at good skywave radiation angles, I feel the dipole has a definite advantage over a vertical. While we're measuring a legal field strength at ground level, the signal leaving the antenna at it's most efficient angle of radiation should be considerably stronger. Strong enough to make a 5mw AM signal readable via a skywave? Ah! That's where the fun of the experiment comes in! See you on the air!
Got mine 2/21. Anyone else get their's yet? Nice carrier -- seems to be about 100mw -- but the audio is terrible. I've tried 3 different audio sources, 2 at line level and 1 headphones output -- all of them create major gross distortion no matter where the audio gain control is set. I must give credit to the builder in Turkey, however....he's very willing to help me work out this problem. If we can clear up the distortion, I'll be ready to test it on the air in about a week. If we can't -- back it goes. The distortion is BAD. Worst I've heard in any transnmitter. (Yes, I changed over to battery power in both the transmitter and the audio sources in case it was a power supply problem --- distortion still persists. Also couldn't find any ground loop problem.) Please post your experiences ---- I will keep the forum updated.
Haven't got mine yet. Have you tried attenuating the input signal?
Yes! 2 of my audio sources are mixers. The audio level can be brought down from about +10 dbV all the way to NOTHING simply by bringing down the faders. When I bring the level down, the transmitter's audio level comes down, but the distortion does NOT! The distortion persists no matter where the levels are. If there's any audio at all going into the transmitter, even if it's so low you can barely hear it, it's distorted. Grossly! Should I have to further attenuate audio that's already so low? I don't understand that!