these are broad-band transformers. their inductance not very critical at all. as long as you have about 200 ohms reactance at the lowest frequency of operation (which is 4MHz: the VFO, in this case).
i have made transformers with 10 as well as 25 turns. they worked as well.
The problem with too many turns will be that the stray capacitances between successive turns of the wires will lead to self-resonance somewhere in HF range and lead to bad karma (narrow-band operation). This is the reason why i didnt use the infamous tap-washers for broad-band work.
Interestingly, the PA output transformer is not on TV balun. this is because the stray capacitances across it's windings become a part of the half-wave filter that follows the output transformer (a trick i saw on a VHF boot in 1974 Handbook).
Let me suggest an experiment to determine when turns are enough.
Wire up a transformer with ten turns in the primary and ten in teh secondry.
Next, take a signal source (like a crystal oscillator) with a 50 ohms ouput (just add a 6 db pad to the output of the oscillator). and measure the output with a 50 ohm resistor connected to it.
Next, instead of the 50 ohms resistor, connect one side of the primary and connect the 50 ohms resistor on the secondary side. Measure if you have about the same amount of RF energy on the other side too.
If you have, then 10 turns are enough. If the output is low, add more turns. If the output is higher with steep peaks ('ringing') then maybe the turns are too many and you can do with less.
Central to our endeavours is that we (amateurs) progress not through theoretical designing but experimental verification and rough and ready measurements. design of these transformers is a case in point.
- farhan
toggle quoted message
Show quoted text
On Sat, 10 Sep 2005, Jim Strohm wrote:
Was this turns count number determined experimentally,
calculationally, by cut-and-try, or by "that's how many fit"?
Just wondering ... I found some tiny tiny binocular cores.
Jim N6OTQ
Yahoo! Groups Links
