Hi,

I am wanting to check the match efficiency or return loss
of a C-Tap matching network that matches 50 Ohms in to 4500 Ohm load
with 2.5pF in parallel. Match at single Freqency = 45 MHz.

Using a 134nH inductor with Q of approx. 85
My calculations result in Cap to ground = 864pF, Cap to inductor = 102pF

I tried the match on the front end of my IF circuit and it appears to
tune smoothly and peak ok at 45MHz.

I then wanted to measure the return loss to see how good the match is.
Using the MJF-269, I get a lousey reading of 0.9dB. (almost everything
reflected).

Either my calculations are way off base, or I am not using the MJF-269
correctly. I also get lousey SWR (16.5), match efficiency below 5%.

I checked your numbers and they seem correct except that you
appear to have ignored the shunting effect of the tuned circuit's
resonant impedance. For an inductor Q of 85 I work this out at
around 3K (so then you have 3K || 4.5K). Further, was the Q
specified at 45MHz? If not it can only be a guide as to the
relative merits of the inductor since it is very frequency
dependent.

My advice is to use the 469 to measure what you have and then
work out what the inductor is really doing from there. If you have
a couple of suitable variable capacitors, put them in place of the
fixed ones and trim for the correct resonance/match, measure them
and then re-do the numbers.


Cheers - Joe (G3LLV)