Introduction



To provide Satellite Links at Goonhilly Satellite Earth Station it has and does require testing to be performed on components, systems, and paths, of many types of equipment. There are various types of test equipment need to perform these tests. Some tests are done in situ on an antenna, or in an equipment hall. Other tests are done in lab conditions.


The Goonhilly Heritage Society has set up such a Lab Test facility capable of doing RF testing both in the HF and SHF bands. The heart of the “Network Analyzer Test Bench” are two Network Analyzers (fundamental to testing in relation to “Transmission Line Theory”) . One Analyzer is a “Scalar Network Analyzer” capable of doing measurements between 10Mhz and 20GHz. This analyzer can make sure that various RF Components (such as filters), or systems, meet their specification in terms of loss/ gain, or reflected power.


The other analyser is a “Vector Network Analyzer”, this has a more limited frequency range up to 500MHz, but in addition to doing the work of the “Scalar Network Analyzer” it is capable of more detailed analysis of impedances (performance) of RF Components and systems.


We use the equipment for training purposes, in addition to the building and maintenance of various recovered ex Goonhilly Communications equipment, now considered obsolete, or otherwise relating to the past.


The understanding of Transmission Lines as represented at Goonhilly with Waveguide and Coax Cables and the components within them, is fundamental to being able to perform installation and maintenance work at an Earth Station. In providing this facility, we preserve equipment and history of what was done at Goonhilly in the past, while still being relevant to the technology and equipment of today.



The Scalar Network Analyser



Robin Working on the Scalar Network Analyser



Robin Checking A Frequency Response



This Analyser, with a few other non-working units, was passed to us from a store room being cleared for other use, and it was obvious that it hadn’t been used for a considerable time as the special connector cables were missing from it. Fortunately, Robin remembered it from the time he worked on site, so decided that it would be a useful piece of test equipment to try to resurrect. Initial short power-up tests proved it not to be working, and the power supply fan was very noisy. Fortunately another faulty unit has a similar fan, so this was replaced to continue checking. One of the detector heads proved to have a faulty diode in it so had to be replaced, and a regime of short, but increasing, power up periods whilst being closely monitored was implemented. The second time around, the power supply integral mains filter finally gave up and had to be repaired to continue. In time the unit was coaxed into stable operation and Robin successfully made up a complete working test unit.


The diagram below shows the typical setup for using the Scalar Network Analyser. There are variations to this configuration, both more complicated and simpler but this is a good start. In this arrangement it is possible to measure the insertion loss, or gain and the reflected power from the device under test (DUT).


The Analyser itself is divided into two parts. The signal source a sweeper which generates a signal sweeping from one frequency to another (this signal is passed to the display – see HORIZ. INPUT) at a set level, all set by the operator. The display section displays the signal from the sweeper after it has passed through, or been reflected by the DUT. In this way its possible to measure the characteristics of the DUT. The link marked GPIB is the “General Purpose Interface Bus” that ensures that both pieces of equipment are synchronised together.



Scalar Network Analyser Typical Set Up



Scalar Network Analyser Typical Display



The display both in the schematic and the image above shows a typical display for a filter. The red trace shows the insertion loss of the filter clearly showing the pass band in the middle. The blue trace shows the Return Loss (reflected power). You can see the reflected power is high where the filter is not allowing the signal through and lower in the pass band of the filter.


I know that in this case the measurement here was actually done with a Vector Network Analyzer because of the reference to S-Parameters (see below), but could equally have been produced by a Scalar Network Analyzer without the S-Parameter references (S11 and S21 are also S-Parameter references). Which leads us nicely on to the next piece of equipment below.



Vector Network Analyser



Les & Robin Checking a Radio Component



HP Vector Network Analyser



The Vector Network Anlayzer (VNA) is capable of doing all the measurements made by a Scalar Network Anlayzer. Instead of the test arrangement indicated for the Scalar Network Analyzer the DUT is put between the two terminals on a S Parameter Set. The S Parameter set is the unit beneath the VNA in the picture above.


In addition to Insertion and Loss Measurements this equipment is capable of measuring complex impedances of any DUT connected to it. The Display shows something called a “Smiths Chart” which is the preferred way of representing these impedances in a graphical manner. You may be forgiven for thinking the Scalar Network Analyzer is somewhat redundant given the capability of the VNA, but VNA’s are expensive and while the Scalar Network Analyzer is capable of doing measurement up to 20GHz and beyond with other plugin’s, the VNA in the picture is limited to frequencies up to 500MHz.

.



Waveguide Slotted Lines




On the “Network Analyser Testbench” we have two Waveguide Slotted Lines. These devices were commonly used to measure VSWR in microwave systems before the availability of Network Analysers. The two models we have are designed for operation at C-Band ( Marconi Instrument 3.8-4.3GHz) and Ku-Band (Flann Microwave 14-14.5Ghz). Slotted lines can only be used at high frequencies because they need to be longer and longer as frequencies decrease. In addition, W/G would be unmanageably big at sub-microwave frequencies.


The slotted line is composed of a length of transmission line (waveguide, or coaxial) with a slot cut down its length in such a manner as not to radiate. A probe is run along the slot and can make voltage measurements along the line. The voltage measurements are made using a detector in the probe and a highly sensitive microvolt Meter, or dedicated VSWR meter. When a standing wave is present because of a mismatch of impedance between the source at one end of the line and the load at the other, the ratio between the maximum measurement that can be obtained, to the minimum measurement that can be obtained gives you the VSWR present on the line. The distance between minimums represents half a wavelength and if the dielectric constant of the line is known (air is 1), then the frequency of the source signal can be calculated. Other measurements between the minimum measurement and the load can be used to calculate impedance which could be plotted on a Smiths Chart, and corrections made if desired. The slotted line is a useful training aid in the visualisation of what is happening on a transmission line, it can still be used, though Network Analyzers mean for practical purposes it is (apart from training) largely redundant. ​



C-Band Slotted Line



Ku-Band Slotted Line