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Radio Works R.F. Consulting
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Antenna Testing and transmission line sweeping for the broadcasting industry is a specialized occupation and skill set that involves some very interesting test equipment, adaptors and background skills that are honed over years of diagnostics and troubleshooting of new and damaged systems. Testing involves several types of equipment. First and most importantly is a Spectrum Analyzer/Power Meter. This combination device allows the testing Engineer to determine whether a transmission system is safe to test for both the operator and the test equipment. Typically the maximum level desired on any testing line is +20dBm. This is a very large signal level and will corrupt or damage most pieces of testing apparatus. Once it is determined that the system is safe to work on for all men and machinery, the choices for testing become application specific. In the simplified world, a simple TDR, ( Time Domain Reflectometer ) is utilized to ascertain the basic electrical and mechanical health of a coaxial or rigid line system. The TDR domes in two basic types. These are the ½ sine type and the square step pulse type. Half sine models have been out for over fifty years and are the industry standard for many types of testing. They operate on a basic level of literally "pinging" the line system and developing a graph of what is reflected back. These waves are displayed on a screen of some type and determinations are made from the displayed datum, Half sine units are typically used for very long range application that may extend upwards of 50,000 feet. The stepped square pulse types are usually operable up to 2,000 feet but deliver a dramatic improvement in the accuracy of the system to actually determine the integrity of the system under test. The accuracy of the half sine type TDR to determine multiple adjacent faults is only about 6 feet. The ability of the Stepped square type TDR to determine faults is as low as 1 foot. In the big leagues of testing the machine of choice is the Vector Network Analyzer. For those of you who are familiar with computer and data networks, shame on you. This Network analyzer does not look at data on a RJ-45 or other data type terminal. This unit looks at the complex relationships in impedance, phase angle, magnitude and returned power levels in circuits under test. This device is literally a shop in a single box. The VNA as they are called in many machines rolled in to one. The TDR functions, and many other types of measurements are available. Some of the types of charts are, Return Loss, Smith, Phase, Time Domain, Admittance, Polar, and Insertion Loss. These charts are very important for the correct diagnosis of problems in the transmission systems. As a full Vector Network Analyzer, other various modes of operation are encompassable in to these listed charts. These are, Forward reflection measurements. In new models such as the one utilized by my group, these measurements are easily accomplished via 2 simple ports. The accessory networking box and switching box is eliminated by modern circuitry. One simple connection is made for any S-1 measurements with one other simple connection for insertion loss testing.
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With accessory keyboards, externally available VGA monitor jacks and other controller functioning available on GPIB, these machines are very user friendly and helpful. The Anritsu 37347D is the machine of our choice. This is a VNA capable of testing systems, circuits, filters, antennae and any R.F. related device from 40 MHz to 20 GHz. This unit is also able to work well with flexible wave guide systems and large scale WR series television wave guide with suitable adaptors where many other machines can not. This wide range of frequencies and medium types allows the user to make significant diagnostics in very short time periods. The printer interface supports 3 types of printers for black and white output. The built in hard drive and floppy drive, along with the GPIB interface allow for color transfer of data stored internally. For demonstration purposes at Meetings, We like to bring a small assortment of communications type antennae, some various types and conditions of transmission line or coaxial cable, and a filter cavity or two. These items allow us to demonstrate the tremendous capabilities of the VNA when in the hands of a trained appliance operator. ( ham radio humor ) Our demonstration includes some very informative tests. We determine the tuning characteristics of the various antennae that we bring. This includes a return loss measurement of the antenna. It is developed in the following graphs. Log Mag, V.S.W.R., Smith chart, and Polar chart. These three graphs clearly show the ability of the machine to analyze the antenna in the most popular information styles. Testing of a piece of coaxial transmission line is done typically in the domain mode with distance markers, in a normalized mathematic windowing mode. This data is taken in Log Mag and is generally used to determine the health of the line as a graphic display of the line in linear form in distance. This mode clearly shows the return loss value for each item, connector group, elbow or other mechanical or electrical item in the system that would normally be transparent to the signal. ( active or amplified units excluded ) A filter cavity is brought so that the additional test of insertion loss and shaping of the filter can be seen. This is a graphic display of the band pass or notch characteristics of the filter can. A loaded return loss characteristic can also be made with a precision load on the out put of the filter can. Simulations can be made of dents in the transmission line. The proximity effects of foreign objects in the aperture of the antennae can be clearly seen. The demonstration and education takes about an hour and is always fun and informative to see. Guests are encouraged to bring a small antenna of their choice or to book maintenance sweeping or system diagnostics when ever the need arises. For more information or to arrange a demonstration please feel free to call at any time. Thank you very much, |
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