Bruce,
Here is the suggested updated o/p circuit for the c.cuff tx, the output is to match in to a 3 metre antenna.
Variable capacitor is low loss 0-10pf or similar.
L1 inductor,i suggest you wind inductor first to give approx 100uh from end to first tap, then make successive taps to make each additional tap say 40uh, so it would test as 100uh on first tap, 140uh on second tap, and 180uh on third tap etc.
I was thinking you could rewind the cigar coil to suit.
If you were to use a larger variable capacitor, say 0-20pf, you could increase the tapps on the coil to say 60-70uh each, but the capacitor would be slightly more lossy, but would work, so not fatal.
Whilst this o/p stage is to match in to a 3 metre antenna, you can load this in to a 50 ohm dummy load by feeding the output of above in to a series capacitor of 33pf, and the other side of this cap in to the 50 ohm dummy load.
The 33pf cap and 50 ohm simulates a 3m antenna and radial ground quite nicely !
BOARDMAKER looking at the Chris Cuff schematics, at which point would you recommend cutting into the circuit and adding your drawn circuit from above? In other words, after which component would this circuit be best tied in?
Second question is, after that componet, should the rest of the original output stage be removed all together?
Bruce.
On the c cuff board, i would lift the 47n (marked 473), on it's output end, this is the side opposite it's input end, which is connected to both 4.7 ohm resistors of the previous pnp/npn emitter followers.
This lifted connection of the 47n capacitor would connect to the base of my circuit.
So the c.cuff board capacitor (47n), becomes the capacitor on my diagram.
Regarding the o/p stage, i am guessing the c.cuff's design is the same as mine, with a series blocking cap from the collector, then the cigar inductor's series connected to the collectors blocking cap, and then the cigar inductor o/p connected to antenna out, with the variable cap connected between the output and ground ?
If this is the case, the existing variable cap would likely be in the order of 60pf, and would be quite lossy.
You will have to rewind the cigar inductor to give you the tapps, to get the inductance range.
The final output transistor will need a small heatsink attached, or if you can fix it to the aluminium enclosure, that would be fine, as long as it is insulated, the BD135 has an insulated case, so can be bolted directly.
Assuming you have not started any modification yet, can you try just connecting the simulated dummy load on the exiters output, and see what happens,does it tune up with a peak on the 50 ohm dummy load, when series connecting the tx output to the dummy load with a 33pf cap ?
Paul.
BOARDMAKER Said:
Regarding the o/p stage, i am guessing the c.cuff's design is the same as mine, with a series blocking cap from the collector, then the cigar inductor's series connected to the collectors blocking cap, and then the cigar inductor o/p connected to antenna out, with the variable cap connected between the output and ground ?
MrBruce Said:
No actually, the Chris Cuff design was the rice bulb lamp and collector of the final choosen output transistor fed to the tuning capacitor first.
On the opposite side of the tuning capacitor is two wires, one goes directly to the antenna, the other wire goes through the coil on a cigar plastic tube holder, 100 turns of coated wire, then connects directly to the antenna output along with the other wire that comes from the tuning capacitor.
The transmitter PC ground is used as the antenna ground connection, it is on a separate external terminal when the kit is built in a plastic box. That terminal has no physical connection to the ATU or tuning capacitor, it is simply soldered to PC board DC (-).
The PC ground does not have any connection at all to the coil, or tuning capacitor.
When I see antenna tuning systems, such as ATUs etcetera, I believe, they usually have some connection to ground or Earth ground. With the Chris Cuff, the PC board (-) trace is the only connection to ground when the kit tuning capacitor and coil are used.
Refer to my photos showing my transmitter in this topic, in post #1 it shows the coil and tuning capacitor hooked up. (See link below)
The screws on back right cover are the ouputs, the one on back right is PC (-), the one on left, is the RF output to antenna.
Bruce.