- September 5, 2018 at 5:52 pm #106182
One of the visitors to the museum surprised the heck out of us by funding the purchase of one of Sean’s C-QUAM transmitters from eBAY.
So with as little fanfare as possible, CDL will be going stereo on 980 KHz.
BTW, if you are wondering why the lower TX frequency, our other LPAM transmitter in the radio museum is on 1080 KHz.
- September 6, 2018 at 8:40 pm #106197
Total posts : 203
Best decision you have ever made. You’re going to love the Sean Cuthbert transmitter. What type of antenna are you planning on using? I’m not sure your range is going to be all that good down there on the lower frequencies like that. You never know though let us know how it comes out.
- September 7, 2018 at 3:01 am #106201
re the antenna, not sure at this point. There’s a Marine HF antenna floating around (Shakespeare 390-1) we can use, however it’s 23 feet long. The feed point is about 28 inches from the base, so the electrical length is closer to 20 feet.
If we go that route, we still need to meet the 2400 / f in kHz field strength rule.
Playing with the spreadsheet from https://sites.google.com/site/lowpowerradio2/antennacalc to see what we can do to load it correctly with the coils and parts that are on hand.
- September 7, 2018 at 7:08 am #106206
Operating a 980 kHz unlicensed AM transmit system in the U.S. in compliance with FCC §15.209 would limit its groundwave field intensity at 30 meters from the radiator to about 24.5 µV/m — which is below the ambient r-f noise level at most receive locations.
Below is a calculation for an unlicensed 980 kHz AM system operating in compliance with FCC §15.219a,b&c. The field it can produce at a distance of 30 meters from the radiator might be about 80X greater than under §15.209.
- September 7, 2018 at 9:09 am #106209
Total posts : 441
Even if originally unintended by the FCC, using Part 15.219 produces a much greater field strength than Part 15.209.
- September 7, 2018 at 9:28 am #106211
Total posts : 1541
Idea for spareparts
A linear RF amplifier can (theoretically) be used to drive a carrier current system from the C-QUAM transmitter, allowing far better coverage under FCC Part 15.221.
At the moment I do not know a suitable RF amplifier to suggest, but in a moment I’ll name someone who possibly can offer a solution.
Also needed would be a CP-15 Tuning Unit to match the RF amp to the power lines.
For further assistance on exploring this idea contact Bill Baker at ISS (Information System Specialists). I do not know his email but one of our other members knows.
- September 7, 2018 at 10:28 am #106215
Total posts : 268
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- September 7, 2018 at 3:23 pm #106225
Total posts : 402
Reply to the above post
- September 7, 2018 at 3:36 pm #106227
- September 8, 2018 at 4:41 am #106229
RE: … see what we can do to load it correctly …
Below is a summary with a head start at describing a loading coil for a 3-meter vertical monopole system with its base a few inches above Earth, that might produce the inductive reactance needed to resonate the 980 kHz antenna system defined in my earlier post. “YMMV.”
- September 8, 2018 at 6:10 am #106231
Wll be on site later today to discuss the antenna situation with management. External choices are pretty limited, and the absolute best I can hope for is a mono pole that does not require digging or pouring concrete.
The other option is cloning the Procaster housing and antenna system, and mounting it on the fence.
Question: What is the Z of the output on the transmitter should I want to build a coaxial or open wire feedline to the actual antenna?
- September 8, 2018 at 7:10 am #106233
Sometimes simple questions can’t (accurately) be answered very simply.
… What is the Z of the output on the transmitter
Most Part 15 AM transmitters are designed to drive a load impedance whose real term might range from maybe 10 Ω to 100 Ω, or so. Assuming that the transmitter load is resonant at the operating frequency, maximum r-f output power versus the d-c input power to the final amplifier usual peaks somewhere within that range.
The feedpoint impedance of a resonant antenna system for Part 15 AM use typically might be around 50 ±j 0 Ω. The first term (50 Ω in this case) is the “real” resistance and the only parameter that can dissipate power. The ±j term is related to the reactance of the load, and is zero ohms at resonance.
The real resistance term of the antenna system at the operating frequency consists of the following …
- radiation resistance
- r-f loss of the radiating conductors (usually negligible)
- r-f loss of the loading coil, at system resonance
- r-f loss in the connection to r-f ground (typically, Earth)
Of those elements, only the radiation resistance produces the e-m field useful for “broadcasting.” The others in that list simply produce heat.
… should I want to build a coaxial or open wire feedline to the actual antenna?
Any/all feedline length must be included in the 3-meter maximum length permitted by the FCC under 47 CFR §15.219(b). This is the reason that many commercially-made transmitters for Part 15 AM connect the transmitter r-f output terminal directly to the base of the ~3-meter monopole, or to the input of a loading coil installed directly at its base.
Sorry that this wasn’t stated more succinctly, but I ran out of time 🙂
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