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Antenna Analyzer gives a understandable results for a Quad Antenna (photos 1, 2 & 3), but gives strange results for Coaxial Collinear Antenna (photo 4)

Photo 1 of 4: Test setup for Quad Antenna
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Photo 2 of 4: The Quad's SWR
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Photo 3 of 4: The Quad's Frequency Sweep vs SWR
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Photo 4 of 4: The 12 element Coxial Collinear Antenna's Frequency Sweep vs SWR. enter image description here

I don't understand the sweep graph of Coaxial Collinear Antenna. Can any one explain this strange behavior? Has anyone else done a sweep of a Coaxial Collinear Antenna?

Many thanks

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    $\begingroup$ You're sweeping over 1170 MHz, and see about 117 ripples in SWR, which makes them 10 MHz apart. I'll guess you have about 7 m of coax going to your antenna. There may be nothing wrong with it, having such a low SWR at many frequencies. It will still only work correctly at its design frequency, at other frequencies the beam won't be properly formed. $\endgroup$
    – tomnexus
    Apr 29, 2017 at 21:58
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    $\begingroup$ With such a wide sweep, is the analyzer taking a sufficient number of samples to accurately show what is really happening? $\endgroup$
    – Phil Frost - W8II
    Apr 29, 2017 at 22:47
  • $\begingroup$ SPECS OF ANTENNA ANALYZER USED: Operating Frequency :140MHz ~ 2700MHz, Frequency Step :1kHz, Measurement parameters: impedance : 0.1Ω ~ 1000Ω (impedance absolute value), SWR: 1.000 to 65, s11 (dB): 0dB ~ -60dB, Resolution: impedance : <Impedance absolute value × 3% ± 0.1Ω (Z less than 200Ω, <1.5GHz), SWR : <Valid reading × 10% ± 0.1, s11(DB / °): 0.1 dB / 5 ° (0dB ~ -10dB), 1 dB / 10 ° (-10dB ~ -20dB), 5dB / 20 ° (-20dB ~ -40dB), Directionality :<35dB (140MHz ~ 2GHz after calibration), <30dB (2GHz ~ 2.7GHz), Scan Points: 280, Output Level: -18dBm, $\endgroup$
    – abcd567
    Apr 30, 2017 at 1:36
  • $\begingroup$ I'd suggest you sweep the same range on both antennas if you want to compare the results as I think Phil Frost is probably right on the money vis a vis the resolution of your analyzer. Also, is your "coaxial collinear antenna" the type created by alternating coax shield to center conductor every 1/4 or 1/2 wavelength? $\endgroup$
    – William
    May 1, 2017 at 20:27

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A coaxial collinear antenna will have a usable bandwidth of 1-2 MHz at most. Sweeping a +1000 MHz bandwidth and trying to interpret the result is not a meaningful exercise.

You should narrow the sweep range of your analyzer to perhaps 5 Mhz centered on the design frequency of the antenna in order to get initial data. From there you can further restrict the range in order to observe more details.

Also take care with any coax between the analyzer and the antenna under test. While the SWR will not be significantly altered by the use of a reasonably short length of coax, the complex impedance will be transformed (changed) by even a few inches of coax at these frequencies.

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