Total posts : 45366
“I still think I must be doing something wrong. When I have this TH pushed right up against a window and the wire antenna running straight up, I get lousy range compared to the previous TH I was playing with last week (in the same configuration). By the time I drive to the end of my 300′ driveway, it is almost unlistenable.”
Usually with the antenna up next to a window that alone will affect performance as the window frame will seriously affect the radiated signal from the wire. However if you did notice a significant difference between one unit vs another in that same configuration, perhaps the current TH unit has a strange problem going on inside..probably in the modulator circuit would be my first guess.
The modulator feeds the two final transistors with B+ voltage (aprox half of supply voltage) and amplified audio current through a single transistor being fed by a current amplifier op amp IC. If there is any DC offset taking place in that circuit, it will seriously affect the way those final RF transistors will work, as they are entirely dependent upon that modulator circuit to deliver the proper voltage and current under both unmodulated and modulated situations.
“On the plus side, when I tune it to 540, I can pick up the signal at three places on the AM Dial!! (;>))”
Indeed! The unit has a broad band output tank so it can work across the entire AM band. There is a broad band filter after the two final transistors, but it is not a very good one but does notch down harmonics to their required -20db point per 15.209. The reason why you pick up 540 on different points on the AM band is 540’s first harmonic is “in-band” at 1080, and the 3rd signal heard is most likely due to “ghost” signal produced by a mixture within the receiver’s IF circuit from a nearby station. A spectrum analyzer will show the 1st harmonic when connected directly to the TH output, but it won’t show the 3rd “ghost” image carrier because that is generated by a receiver and it’s “front end” and IF circuits. The receiver still sees other frequencies whereas the spectrum analyzer connected directly to the TH output will be isolated from external signals, thus it’s front end will not be affected.
Most measuring done in high power transmitter sites are done with very tight band pass notch filters to prevent false readings that will be produced by co-located high powered transmitters. Though not the case here, the regular radio receiver will “mimick” this effect even with very low power signals intermixing in the radio itself..thus the term “ghost image” signal.