[R-390] Impedence Matching Question

[n4buq at knology.net]

While searching for some replacement coax for my PTO output cable (thanks 
again, Roy!), I got to wondering what the correct impedence of a cable 
would be for this application.  I realize the short length cable and the 
relatively low frequency make this not as much of an issue as it might be 
in other situations, but I was still curious.

I calculated the reactances of the output coil and the parallel capacitor 
at 3 Mhz (close to the center point of the PTO range).  Using Ohm's Law, I 
calculated the effective impedence of the three components -- the coil, the 
capacitor, and the resistor (150 ohms) in parallel.  Unless I did something 
wrong, this comes out very close to 40 ohms.  Is this a correct method to 
calculate the output impedence of this circuit (and possibly why the output 
cable is most likely 50 ohm coax)?

Thanks,

Barry - N4BUQ