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Old 18th Nov 2005, 1:31 pm   #12
YC-156's Avatar
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
Default Re: Homebrew transmitter project (Frank vs. Oskar :) )

Originally Posted by Oskar.B
Ok, I will try winding my own RFC this weekend and do some measurements to see if I will get troubles with self-resonances and such.
If you have access to a grid dip meter, the technique is to temporarily connect a short piece of wire from terminal to terminal, and then search for resonances near the operating frequency and a few of the harmonics. This will reveal the troublesome series resonances.

If you dip the choke without the short, you will find the parallel resonances, and those happens to actually increase the effectiveness of the component.

Originally Posted by Oskar.B
If I can get away with it, I'd rather use my standard hard-paper rolls for coil form. I like them better than plastic.

Paper and wood are quite lossy at RF frequencies, as those materials absorb moisture from the air. I wouldn't even waste time trying to use those for coil forms at any significant power level. In the oscillator a paper coil form would be way too unstable as well. Even PVC tubing would be a better choice, and that is saying something.

In the other thread I mentioned that the oscillator in particular should be built like the proverbial battleship, and I wasn't joking. If either the chassis or the components can be deformed or deplaced by pushing with a finger, then it isn't strong enough. You want to build something, which is mechanically stable to within a tiny fraction of a percent.

Below 28MHz the bandwidth used for amateur transmissions on AM is less than 6KHz (3KHz sidebands), so your transmitter oscillator must stay put to within a fraction of this if you want it to be practical for real communication with other amateur stations.

Edit: To illustrate just how sensitive the oscillator components are, then imagine you secured the oscillator coil with some type of glue. If you were unlucky, then the presence of the glue itself could be enough to render the oscillator useless, simply because the electric or mechanical properties of the glue might have some odd temperature dependence.

Originally Posted by Oskar.B
By the way, I was glad to become a Tetrode ...
Oh! OK, congrats! You had totally lost me there for a moment.

Frank N.

Last edited by YC-156; 18th Nov 2005 at 1:37 pm.
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