The slotted line apparatus
for the precision measurement of complex impedance at VHF and UHF frequencies.
A practical and precision instrument for the serious VHF UHF experimenter

by Ralph Klimek 1982
copyleft: you are free to copy and use this article and images as you see fit

This instrument was biult by me to permit the measurement of actual complex impedance for antenna  arrays that I was atempting to biuld for the 2 Meter amd 70 CM amateur band. It can be used with a few other accesories to measure the true SWR on any line. It is the true measure of whether or not your VHF antenna is or is not a good match.  The length of this instrument even permits measurements down to the 6M meter band, however to be meaningfull at this low frequency you must make some precision stub cables. The other required accessory is a high impedance 10Mohm voltmeter.

The unit is made from off the shelf aluminum extrusions. The formulas from assorted engineering handbooks gives a characteristic impedance of about 52 ohms for the stock materials and ignoring the effect of the slot for which there is no formula.  As the slot opening is less then 10 % of the perimeter of the overall line, the effect of the slot should be negligible.  I have yet to evaluate this with an excellent arbitary line calculator tool  called  atlc.

The only real construction difficulties arise where the inner conductor is joined to the coax connectors. In the ideal scenario there would be a tapered transistion to minimise the inevitable reflection arising from the change in dimension of the conductors.  I had to use hollow tube for the inner conductor.  This, and the fact that it was aluminum made it hard to terminate to real world type N or PL259 connectors. The image shows PL259 connectors which are horrible at UHF and only just barely acceptable at VHF. I have since replaced them with type N and have no regrets. When I made this in 1982  Type N connectors were very expensive (for the hobbyist).

The connection to the coaxial connectors was originally made with horrible grounding lugs. Later when I had access to a lathe I machined a nice transistion piece which has proved very satisfactory. If I was biulding this instrument again from scratch I would use hard drawn copper tube to permit direct attachment to the coaxial connectors, or alternatively,  solid aluminum rod with a drilled out hole at the end with a threaded boss to permit a solid connection to the coaxial connector.

There is only one element of precision required in the assembly. This is the spacing of the inner conductor. Its absolute spacing, as it turns out, has very little bearing on the characteristic impedance. However, the distance between the probe and the line must be kept as uniform as possible because the probe pickup voltage is quite  sensitive to the spacing between it and the centre conductor.  Make sure that your inner tube is not bent, if it is, reject it and buy one that isnt bent! I have made two perspex spacers which mitigate sagging.  File some slots in the four mounting screws of the Type N connectors to allow for about 1mm of vertical adjustment. Use the depth gauge of vernier calipers to ensure that the conductor is trully parallel with the slot. You can see from the pictures that I have allowd the end plates to be slightly adjustable so as to permit precision alignment.

Even if you cannot understand the very difficult to master Smith Chart or the complex equations or transmission line theory ( as I havent yet! )  this apparatus still permits quick and dirty antenna matching alignments because you get a very readily accessible indication of SWR by directly observing the standing waves !  A "flat" line or well matched load will present little or no variation as you move the probe. It becomes obvious if the load impedance is above or below Z0 from the position of nodes and the relative amplitude of the nodes quickly gives you a qualitative idea of the size of the resistive component.  It is the ultimate tool by which SWR meters can be calibrated. I have found that half a watt is sufficient power to energise the line and load to get meaningfull readings from the probe.  The line should be fed from a 6dB pad to minimise spurious reflections from the transmitter end.

The probe diode must be a germanium point contact for best results. They have the lowest forward voltage drop , minimum junction capacitance and will maintain maximum acccuracy and superior sensitivity. The old faithfull OA91 diode is still readily available and still rectifies at microwave frequencies.

Copper tube inner, used silver plated Type N connectors, perhaps 3 meters was too long to be really practical. Approximate methods work well enough at 6 Meters ( 50Mhz ) . The probe presents some difficulty.  The DC output voltage of the simple diode rectifier is not a simple linear function of the the line voltage. It has frequency dependency because we are sampling the line voltage field with a capacitive probe, and the indicated DC probe voltage is a non linear function of the line voltage and voltmeter loading. True measurements require you to calibrate the transfer function of the diode probe , construct a table, and from the measurement infer the line voltage. As a first order approximation the probe can be calibrated from a variable DC voltage source, and this should be sufficient precision  for amateur use.  Perhaps the next probe will have a broadband MMIC amplifier and an INDUCTIVE probe.  Select metal stock on the basis of first having simulated the lines with  ATLC the arbitary transmission line calculator so as to get the nearest fit to 50Ohms Z0

 

     

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