# Why Coaxial Collinear Antenna Gives Strange Graph of Frequency Sweep vs SWR?

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

Photo 2 of 4: The Quad's SWR

Photo 3 of 4: The Quad's Frequency Sweep vs SWR

Photo 4 of 4: The 12 element Coxial Collinear Antenna's Frequency Sweep vs SWR.

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

• 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. Apr 29, 2017 at 21:58
• With such a wide sweep, is the analyzer taking a sufficient number of samples to accurately show what is really happening? Apr 29, 2017 at 22:47
• 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, Apr 30, 2017 at 1:36
• 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? May 1, 2017 at 20:27