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Author Topic: antenna analyizer methods  (Read 354 times)
KB9YOJ
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Posts: 15




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« on: January 22, 2001, 12:52:07 PM »

Is there anyway possible to make antenna anaylisis without an SWR meter or MFJ antenna anaylizer?  Could you use a DMM?  How about a o-scope?  Are there mathmatical calculations you can use?  Please fill me in.

Matt
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WB2WIK
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Posts: 20540




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« Reply #1 on: January 22, 2001, 02:21:44 PM »

Any test equipment used would have to be impedance-analysis specific.  These instruments are not really "antenna analyzers," nor is the MFJ or similar products, in that they really measure nothing more than antenna impedance, which is one very tiny factor in determining antenna performance.  But commercial test equipment that can be used, besides the MFJ or similar "analyzer" instruments, include: (1) Rx meter, such as Boonton Radio and HP; (2) Admittance bridges, such as General Radio; (3) L-C-R bridges, such as Boonton Electronics; (4) Combination signal generator/detector and stub tuner or calibrated line sections, such as General Radio, HP, Narda, et al. (5) Dual directional coupler and vector voltmeter, such as HP; or (6) dual directional coupler and spectrum analyzer with tracking generator, such as HP, Tektronix, AIL, et al.  In any case, the TME (test & measurement equipment) required is far more costly and complex than an SWR bridge and/or MFJ analyzer, which is why these instruments are so popular with hams.  There is no way that a DMM or oscilloscope, without radio frequency test equipment attached, can be useful in determining impedance, SWR, return loss or other parameters relating to antennas.
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WB6BYU
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Posts: 12985




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« Reply #2 on: January 23, 2001, 03:52:51 PM »

If you have an RF voltmeter (which could be a DMM with a diode
detector) you can, in fact, determine the SWR by taking enough
measurements along a length of transmission line.  This is not too
difficult to do with open wire line, but more awkward to do with
standard coax, so you will need a number of lengths that you can
add in different increments.

This makes a good theoretical demonstration of the principles of
standing waves, but is not particularly efficient (especially if you
are trying to adjust a tuner for minimum SWR).  Much easier to build
a simple resistive SWR bridge, perhaps using 4 each 100 ohm
resistors, a couple connectors, a meter, a bypass capacitor, and
a pot for sensitivity adjustment.  (You can use your DMM for the
meter part of the circuit if you wish.)
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N1TWY
Member

Posts: 7




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« Reply #3 on: April 19, 2001, 12:57:29 PM »

OK - dumb question while we are on the subject of SWR bridges.

I have an ancient and venerable SWR bridge from a 30-years-ago flirtation with  CB. This thing is
supposedly good to 30 Mhz or thereabouts.

Could this be used as a VHF meter? Seems to me if I can put enough power into it to set the dial to the calibrate point, the reflective power reading should be adequate for rough tuning of antennas.

I just hate to pull my good meter from the shack for quick checks of mobile antennas and the like...

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