[ Specified attachment is not available ]Hello Everybody
I have been on the forum a bit, just usually in the repair section. This topic is about a home built broadcast AM transmitter that needs some tuning
, and I figure that the beginners section would be better than the repair section. I know little about how AM transmitters work, so part of the purpose of this project was to learn a few basic things about how they work, but my other goal is to have a device that can transmit music to my old tube radios.
I found the original design on this page;
http://techlib.com/electronics/amxmit.htm . I liked this one because it did not use any special components or transformers, and the designer gave lots of details and tuning tips. With that being said there are still a few unknowns that I was hoping to get some input on, I will mention them later.
Here are the details about the transmitter, My schematic is attached below.
VCC is regulated 15V
The crystal is 1MHz(rather hard to find)
I have a 36 inch extendable metal antenna attached to it
The antenna loading coil (not shown on schematic) is a tunable inductor that I can tune between 1700uH and about 5000uH (I think).
As it sits right now, the transmitter is working and it sounds pretty good, however it is incredibly weak. The signal all but disappears more than 6 feet away from the transmitter, and that's with nothing in between the TX and the radio! I have tried some tuning steps, recommended by the designer on the webpage. First I tuned the tank by adjusting L for the highest output with no antenna attached, but they also talk about antenna loading a bit, which I understand little about. I understand that antenna length and frequency need to be considered for choosing the inductance of the load coil (and location), but beyond that I don't really how it works. The guide says that when attaching the antenna and coil the meter reading should drop significantly, I can't reproduce this effect.
Here are my main questions: 1) is there a good way to verify that the crystal is working as it should?
2) what peak to peak voltage should I expect on the antenna output with nothing attached? currently it is about 10V pk-pk.
3) How does the antenna loading coil work, and what is the best type of coil to use for it?
I hope this was not too much to look at, I appreciate any input