A Low Cost RF Impedance Analyzer
TLD Calculated
Frequency R + jX
5.0 MHz 6. 8 + j110
6.0 MHz 29. 4 + j234
8.0 MHz 27. 7 – j211
10.0 MHz 4. 3 – j63
12.0 MHz 2. 3 – j23.9
14.0 MHz 2.1 + j1.2
16.0 MHz 2. 8 + j26.7
18.0 MHz 6. 4 + j68.8
20.0 MHz 63.0 + j251
Z
110
237
213
63
24
2. 3
27
69
259
■ FIGURE 13. Coaxial cable test
results of shorted cable.
This data along with selected, actual
measured values obtained with the
analyzer are shown in Figure 13.
As shown, the values are reasonably
close. Don’t assume all of the differences
are due to errors of measurement of the
analyzer. The theoretical calculations
don’t take into account connectors, the
effect of splices (my cable had one), and
age factors of the cable. In this case, I
appreciate the confirmation of the closeness of the data but tend to lean to the
actual measurements as being more true.
As an interesting experiment, if you
connect an antenna to the coax as a
load (replacing the short), you can
again measure the input impedance at
some frequencies of interest. Using
TLD (in reverse), should then enable
you to find the impedance at the antenna. This obviates the need to go directly to the antenna for measurement.
Final Comments
The straightforwardness of the
impedance analyzer makes it attractive
as a low cost measuring tool. But, belying this simplicity, there are limitations
on its accuracy at low impedance
angles and at high SWR. It certainly
won’t replace a good network analyzer, so don’t sell your stock in Agilent!
On the other hand, it can be very
CONTACT THE
AUTHOR
■ George R. Steber Ph.D. can be
contacted via email at steber@execpc.
com with “TVM” in the subject line
and email mode set to text.
TVM Analyzer
R+jX Z
6. 8 + j115 115
30. 2 + j240 242
38. 5 – j213 217
8. 9 – j63 64
5. 5 – j23.5 24
3. 3 + j1.7 3. 5
4. 6 + j28.1 28
6.1 + j67.9 68
52.0 + j216 222
useful in many situations where a more
sophisticated instrument is not available.
Accuracy is good over a reasonable
impedance range of about 5 to 1,000
ohms, and better between 10 and 250
ohms. Angles near zero degrees (
corresponding to a pure resistive load) and
angles near 90 degrees (corresponding
to a pure reactive load) are the least
reliable. Making manual voltage
measurements and entering them into a
program may seem like a drawback
to some, but I have found it to be
surprisingly easy to do and very inform-
REFERENCES
■ [1] George Steber, “A Low Cost
Automatic Impedance Bridge” QST,
October 2005, pp 36-39.
■ [ 2] George Steber, “LMS Impedance
Bridge” QEX, Sep/Oct 2005, pp 41-47.
■ [ 3] Transmission Line Details, Dan
Maguire AC6LA, available free as a
download at www.qsl.net/ac6la/tl
details.html.
ative in many cases. I was often amazed
by the analyzer’s accuracy and seldom
dissatisfied with it. Overall, it works well
if you understand its limitations. If you
find yourself making lots of measurements or needing more portability, you
may want to consider moving on to a
commercial impedance analyzer.
It was fun and instructive working
with this impedance analyzer. I hope
you will find that to be the case, too.
So angle for some time, analyze your
resources, and don’t be impeded in
building your own version! NV
February 2008 47
