The problem might be RF hashcomming from your computer, this has been brought up by others but you should try this anyway, try using a ferrite bead, or cable clamp type ferrite RFI device on the audio cable comming from your computer to the transmitter. Put more than one on the audio cable. Wrap the cable in and out of the ferrite choke.
Far as the HLLY transmitters, I re tuned the ones I have for max output, they are never fine tuned at the factory. I havent had problems detecting harmonics using a quality TV band variable tune radio using TV channels 7-13. The transmitters that I use "for Lab use", put out a fair ammout of power at the operating frequency. The second harmonic travles about a fourth block, still to much for me but pretty good as far as im concerned.
Radio Joe,
Your comment
I havent had problems detecting harmonics using a quality TV band variable tune radio using TV channels 7-13. The transmitters that I use "for Lab use", put out a fair ammout of power at the operating frequency. The second harmonic travles about a fourth block, still to much for me but pretty good as far as im concerned."
has me wondering a bit. The second harmonic from the FM band (88 to 108 MHz) could be between 176 and 216 MHz which is withing the channel 7 to 13 band of 175 to 216 MHz so you are probably hearing the second harmonic. If this is heard from about 1/4 block distance (blocks = ????feet) I wonder how far your third harmonic (264 to 324 MHz) is going. Usually in unfiltered transmitter circuits the odd harmonics are stronger than the even harmonics since they result from clipping in the amplifiers or symmetric non-linearities.
If I heard my second harmonic as far away as you report I would immediately take the transmitter off the air. How far is your first harmonic going? (the first harmonic is the fundamental, that is, it is your transmit frequency).
You are correct in your observation that computers generate hash and perhaps vermonter will try what you suggest.
Neil
After pretty much deciding to try a graphic equalizer to solve my clipping problem, I mentioned my situation to a friend who immediately said: "The sound cards in laptops are notoriously crappy. Why don't you buy a good external, USB-connected card?" He suggested this Sound Blaster model: http://tinyurl.com/SBexternalCard
Today at Best Buy, I bought this black beauty for $59.95. Not only did it solve the clipping problem, but with its 24-bit/96kHz specs, I cannot believe how good my audio sounds, whether it's coming from my Altec Lansing PC speaker system or when broadcast to FM receivers. The audio quality makes the hair on the back of my neck stand up! I guess my buddy Ron was right when he said that internal laptop sound cards can be really lousy. I didn't know what I was missing!
Many thanks to everybody who posted to help me out in the last couple of weeks.
Carl
Carl,
That is great news and is a lesson to all of us to add to our bank of knowledge and wisdom. Thanks for reporting.
Now, keep in touch on this board with your further adventures and escapades and enjoy your great audio!
Neil
Despite the frustration that I've had with my new transmitter up until today, I'm now kind of glad that I ran into that clipping problem. The resulting RF and computer education that I got from this forum and elsewhere in a short period of time is invaluable. Part15.us will no doubt continue to serve me well. A few weeks ago, I didn't know what "Part 15" was. But lately I've been studying this aspect of the radio world with great interest; I'm hooked! Now that I've got my tech problem sorted out, I can delve into the subject even more; wonky legal stuff and all!
Also, I've never participated in a more active forum (almost 670 reads on my initial post as I write this), and the quantity and quality of replies has been downright heartwarming!
Cheers, all.
Second harmonic went 90 feet, no way to test for the third. 1/4 GP antenna. undisclosed power level.
I am thinking of calling the FCC have them come out to test...
Progressive Concepts has a nice low pass filter for a hundred bucks, there specs show 170 MHZ down 50 + db... it has to be good, maybe selling at the price of ignorance, but try to build a good filter, I have, and with no spectrum analyzer to see what it does, guess ill give Progressive the money.