At 09:06 9/15/98 +1000, Peter VK3APN wrote:
>This is the effective capacitance, averaged over the whole winding. If each
>turn contributes equally to the capacitance, the maximum allowable total
>capacitance is twice the above or 3.08 pF. However practical windings are
>less than uniform, so the maximum allowable capacitance will be (say) 2 pF.
My rather poor first pass at solving this problem (FT50-43 core with 7
turns of #29 AWG for the primary, and 28 turns of #29 AWG for the
secondary, with the primary and
secondary windings separated as much as possible on the core) resulted in a
measured primary to secondary total capacitance of 1pf. I wound a
transformer similar to that W8JI suggested (not the same core material, but
same dimensions) and measured 5pf.
I have not tried this one yet as I am waiting on the 73 material cores to
arrive. I had some doubts as to the 5pf number, but was gonna try it. Maybe
I won't bother now.
I made the measurement in a bench setup similar to that which I actually
use to launch the 75 ohm coax to the xfmr and then to the pennant. Having
the winding leads extend radially from opposite sides of the core
circumference allows the inter-winding capacitance to be dominant. I have
no clue what the capacitance of the antenna to the coax would be as I have
no equipment capable of making such a measurement on field deployed systems.
And, based on my limited experiences with this type of transformer, I
believe the #43 core will probably be best. However, I'm not ready to give
up on W8JI's original 73 material just yet. (Mostly 'cause I bought some!)
I intend to experiment with those cores some.
Next order, I will also get some of those "solid gold" 140 size cores in 43
Peter, thanks for the analysis. I had not even attempted to do that
rigorous of an analysis of the problem. Being as impatient as I am, once I
identified the problem, I quickly went for a solution. It's nice to see
someone independently verify what I had mostly just guessed at.
Larry - W7IUV
FAQ on WWW: http://www.contesting.com/topband.html
Administrative requests: topband-REQUEST@contesting.com