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Slotted Line Measurements

Question asked by magnatom on Jun 14, 2006
Latest reply on Jul 30, 2007 by Frank_BR
Using a HP 415E with a X810B slotted line in a 809B carriage shouldn't the distance between the standing wave minimums equal 1/2 the wavelength of the signal? I am doing some infrequent tests on some old X-Band radar antennas where the tests are for measuring the SWR using a HP 415E with a slotted line. My signal frequency is around 9.35 Ghz. which is an electrical wavelength of 3.2cm. According to the theory of operation manual which I have of both the HP 415E and the 809B if you measure the distance between 2 minima and multiply by 2 that equals the wavelength of the signal source. When using the X810B slotted line I get a distance between 2 minima of about 2.2cm which when multiplied by 2 equals 4.4cm which when computed into frequency ends up around 6.7 Ghz. It seems that the X810B slotted line electrically lengthens the signal. Is this or should I say was this a rather normal occurence when engineers did these type of measurements? If I use the 806B slotted coaxial section my measurement of the 2 minima equals 1.6cm which when multiplied by 2 equals 3.2cm which is the correct wavelength for 9.3 Ghz. I know this is rather an old and crude method here but, is there an engineer there from the old HP days that can tell me what the problem is? Is there a propagation factor/velocity factor to consider when using waveguide sections? A different signal source did the same thing. I am using a HP620B with square wave modulation. I tried 2 other radar test sets and I get the same results. Just for my own personal interest I would like to know what someone did years ago to correct this problem, or maybe this is normal. Thanks for any help.