WEAK-AM

I’m running a classical music format with my SSTRAN. Now that it’s been up and running for awhile, I’ve noticed that on softer passages, there’s kind of a “gating”effect on the audio. Soft sounds are greatly attenuated below a certain point, above which they increase rapidly in volume. I’ve played with the gain and compression settings, but to no avail. I doubt if anyone else out there is running classical music on AM, but I wonder if anyone has noticed this effect at all, and what you think might be the cause? I haven’t reverse-engineered the compressor circuit to see if that is a side effect of the circuit design.

I am getting closer to putting up the new stick!

I am getting closer to putting up the new stick! After two trips apiece to the hardware store and the home center, I think all of the pieces now fit together properly. 😉 Thanks to Antenna Guy for doing the hard part and making the coil for me! Oh by the way, after doing quite a bit of testing, I have moved up the dial to 1560. It’s much clearer in the daytime at my location, and only slightly worse at night than 1500. The efficiency is also a little better.

One hint: I found that it’s not really necessary to use 1-1/2″ rigid conduit for the base. You can’t buy that in anything less than a 10′ length at a home center store, which will set you back about $35 or so. Hardware stores don’t carry it. They told me to use galvanized water pipe, which they will sell in whatever length you want, and they will also thread it for you. I tried, and the threads fit into the PVC adapter just fine. They are a little different than the threading used on the rigid conduit, but they work.

When I first started investigating the theory of operation of the SSTRAN transmitter with the base loaded antenna, the explanations I received from several people did not seem credible. For the benefit of anyone else that might be cudgeling their brains over this issue, I recommend the following thread: www.part15.us/node/883

These gentlemen have done an excellent job of determining how this works, and I highly recommend reading their analysis if you are at all curious.

I would like to modify the base loaded antenna so that it presents a DC ground to help discharge static electricity. I live in a part of the country where lightning storms are very common in the summertime and I think having a direct connection to ground through the coil (e.g. by grounding the lower side and tapping up on it) would be helpful.

I have a vintage Knight Wireless Broadcaster, as I mentioned in an earlier post. It has been the subject of fairly extensive modifications, entirely in the audio department to date. I use it with an isolation transformer so that I can directly ground the chassis to earth ground. 🙂 It will deliver 100V p-p of RF directly to a 10′ wire antenna without any attempt at matching.

Where it falls down in comparison to today’s LPAM equipment is in the use of an unstabilized carrier frequency. It’s hard to enjoy listening to your station when there’s always a whistle or hum in the background. In the past, I’ve seen some circuits for modifying the Knight to use crystal control, but haven’t run across any lately. If you modified your unit to use a crystal, I’d like to hear what you did, and how it worked out.

My SSTRAN is just fantastic in this regard. There’s no carrier frequency adjustment, yet the unit is right on frequency and drift is imperceptible. I’m never going back to using a free-running oscillator…

Has anyone ever thought about using a Wi-Fi access point to broadcast to other computers directly so that you could have your own “Wi-Fi” radio station, like we do on the AM and FM bands?

Has anyone ever thought about using a Wi-Fi access point to broadcast to other computers directly so that you could have your own “Wi-Fi” radio station, like we do on the AM and FM bands?

I’ve heard that some hobbyists have “hacked” router software to modify the transmission characteristics. Has anyone thought about how to do this for a relatively narrowband application like audio broadcasting? We don’t need to take up a 20 MHz wide chunk of spectrum like 802.11 to transmit an audio program.

I’m curious to know whether anyone has considered this, and what kind of range you could get. Wouldn’t it be cool if there was a way to kick a router into a “radio receiver” mode that would allow you to tune in local broadcasts in your area?

OK, I could not resist… I ordered a Ramsey AM25C and put it together over three lunch hour sessions. There were no major problems assembling the kit. The coils were easier to wind than I expected.

When I turned it on, there was no smoke, but there was no output either… until I found I had made an error setting the DIP switches, and corrected that. Then I got a strong carrier on 1500, accompanied by a massive amount of hum! Keeping the unit powered on for very short intervals, I made all of the adjustments according to the manual. When I was finished, the hum was still very strong, and the output power measured 23.9 dBm into 50 Ohms (over 200 mW)! I also noticed that the regulator transistor (Q9) and the driver transistor (Q5) were both hot.

Thinking to reduce the power a bit, I padded down the drive into Q5 by putting a 1000 pF capacitor across R16. The power went down a few dB, and the hum dropped significantly. I bumped that up to 1500 pF and the power dropped to 19 dBm. The hum is essentially gone, and Q5 is a lot cooler now. The 2nd harmonic is down just shy of 40 dB. I do notice some close-in spurs at around +/- 250 kHz, which I will have to investigate. They are pretty far down, but I would like to eliminate them. I am guessing that they may be caused by instability in the audio circuitry.

The overpowered ac adapter supplied with the kit appears to be the reason for the hot regulator transistor. Even after padding down the RF drive, I am still measuring almost 18Vdc into the board. The adapter is rated at 1 Amp at 15 Vdc, and since it is unregulated, the voltage goes even higher when the current drain is less. So I think the problem here is not that Ramsey supplied a cheap adapter; rather, it is over-rated for this application.

I like the 50 Ohm output. It simplifies designing the matching network for a base-loaded antenna, and making measurements on the transmitter’s output. The audio section of this unit doesn’t look like much to write home about. I have not made any measurements of it with modulation yet, but have read that it’s difficult to get it to hit 100%. I’ll write with more details after further tests are completed.