Re: Thursday Evening (kinda long, grumpy)


Date: Fri Feb 10 1995 - 22:26:16 EST

Bob, you wrote:
> Joe - go over for me once again how you used the Autek RF Analyzer to
> tell how your final tank circuit was going to work and be at 50 ohms? Is
> this something I can do with my MFJ 249 - now that I think of it probably
> not since it does not read the reactive resistance. Anyway tell me what
> you did - maybe this is the excuse I need to by the Autek - hi
> 72 bob VO1DRB/WA6ERB
Well, glad you asked. Like an ornery teacher, I stuck a few tidbits in my
rambling narrative just to see if anyone was paying attention. You get a
gold star!
I alluded to this capability of the Autek RF Analyst in my recent review
distributed on qrp-l. In addition to measuring and displaying SWR, the RF-1
also measures rf impedance. (I may write this up further in the future.) It
will display SWR, impedance, inductance or capacitance, depending on what
you tell it to show.
The actual steps were:
1. Turn instrument on.
2. Connect component across measuring terminals.
3. Push button to read frequency.
4. Tune internal oscillator to about 3.5 MHz
5. Push button to read impedance.
6. Adjust oscillator to get impedance reading of 50 ohms.
7. Push button to read frequency.
Elapsed time: less than 20 seconds or less per component!
In fact fewer button pushes are needed. If you press the Frequency and
Impedance buttons, the display alternates between frequency and
impedance twice a second.
A "trick" was invloved here, too. The output network I used is a simple,
Q-of-one pi network. In it, each component is equal to to transmission
line characteristic impedance. (See my Joe's Quickie in the latest QRP
Quarterly for more info.) In this case the impedance is 50 ohms.
All I had to do was measure each component independently.

The RF-1 measures impedance directly, but the inductance or capacitance
displayed is calculated assuming that the impedance is completely reactive.
It doesn't "know" what the resistive and reactive components of the
impedance are, nor whether the reactive component is inductive or
capacitive. You have to figure this out yourself. Guidelines for doing
so are given in the instruction manual. Impedance values less than about
4 ohms or greater than 2000 ohms give an under- or over-range indication.
Accuracy is stated at a few percent near 150 ohms, degrading to 20 percent
at the measurement extremes. All in all this is a very handy benchtop
instrument, not high quality lab grade, but super for a home rf bench.
BTW, the MFJ-249 can't do this (I have use of one of them, too). But the
MFJ-259 measures SWR and "RF Resistance." I think this really is RF
IMPEDANCE. On the plus side the -251 covers HF and VHF and lets you use
its counter with an external source. On the negative side, compared to
the Autek Analyzer, it is much larger, much heavier, about 75% more
expensive and doesn't have a digital display or direct capacitance or
inductance readings.
I took the liberty of cc'ing the qrp list on this 'cause I think others
may be interested, too.
Joe E. N2CX

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