On 1 Jul 98, WE9V Chad Kurszewski <Chad_Kurszewski@csg.mot.com>
wrote:
> >Following Autek's instructions, I tuned for minimum Z. At 28 Mhz,
> >according to their formula, my loss was 1.3 db and at 7 Mhz it was 0.7
> >db! I used the formula 0.17 x Z = loss in db (for 50 ohm coax). This
> >seems awfully high, or does it?
>
> Well, doing some back calculating, this almost sounds correct, except you
> didn't include the Z you measured.
>
> There is a rule of thumb that Insertion Loss is roughly equal to one half
> the Return Loss that you would measure with an open (or shorted) line.
>
> If for example, you measured a 10 ohm Z (10+j0), it would have a return loss
> of 3.54 dB. Half of that would be 1.77 dB, which work work out to the
> same answer as Autek's 0.17 x Z = 1.7 dB.
>
> If you measured 2 ohms, it would have a return loss of ~0.72 dB, or an
> insertion loss of 0.362 dB. Using 0.17 * 2 = 0.34 dB is pretty darn
> close, eh?
>
> One thing that I cannot answer is if the "tuning for minimum Z" all works
> out. What one would assume is that if you tune for minimum Z, that you
> are operating at j=0. If so, then the above calculations work out.
> You just cannot pick a frequency, measure the Z, and convert it to loss
> by multiplying by 0.17. It would appear that you have to choose a freq,
> such that the coax would be a half wave, such that you are at the
> Zreal < 50 and j=0. As you move away from that frequency that the coax
> is a half wavelength, you will get more inductive (as you go higher in freq)
> and the Z will increase, even though the Return Loss (and thus insertion loss)
> remains relatively constant.
>
Yup, that's what I did. Put the Autek into the Z mode, where the freq
alternates with Z reading, and tuned it until hit minimum Z in the freq
range I wanted. As it turned out, I kept the coax open and minimum Z
was near 28 MHz and 7 MHz plus/minus a bit...
Guess it time to check some of the other old jumpers in the shack...
--
Barry Kutner, W2UP Internet: w2up@itw.com
Newtown, PA Frankford Radio Club
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