1.2 ghz antenna swr through the roof

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KD2TYN, Nov 11, 2020.

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  1. KD2TYN

    KD2TYN Ham Member QRZ Page

    hi thanks for looking. my question is as follows ill provide as many details that i can think of . the radio is an icom 9700 with a triplexer connected to a comet desconne antenna. i unscrewed the 1.2 connector bought a gp-21 ran lmr 400 about 30 feet and connected it directly to the rear of the radio . i then ran it through a comet cn-801 watt/swr meter and its horrible watts are reading 1-2 watts and the swr reads between 2.5 and to infinity. the antenna its self is sitting on a wood mast and reaches about 35 feet in the air.
  2. W1TRY

    W1TRY XML Subscriber QRZ Page

    I didn't know what a GP-21 was, so when I Googled, it came up on DX Engineering. There is only 1 review and it's only 1 star:
    "The GP15 is my best performing vhf/uhf antenna. So I figured why not get the GP21 for 1.2ghz. Am using lmr600 with my IC9700, there are only a few places where the swr is 1.5 or less. I know it's 60mhz wide, but this antenna can't be tuned. I think I'm done with GP antennas, Pete"
    Looks like the antenna might be the problem.
    I suggest finding bringing the antenna down, testing everything except the antenna with a dummy load at the far end of the coax and see what you get for SWR.
    If it's good, the antenna is the culprit.
  3. KD2TYN

    KD2TYN Ham Member QRZ Page

    what would be a good antenna ? ill do teh testing as well but if thats the case i might start looking elsewhere ?
    W1TRY likes this.
  4. NM7G

    NM7G Ham Member QRZ Page

    I'd remove the SWR/power meter all other components from the ANT-RADIO path. Connect the GP-21 as direct as possible to the radio. Tell us what you hear. Do you have a repeater nearby; maybe a few simplex stations? It will tell you a lot, particularly if you hear nothing on 3-4 occasions when the band should be active. With nearly 15 dBi gain on the GP-21, you should hear sigs. After determining you can hear, move to the VSWR issue.

    We now don't have a clue about how valid the VSWR and power numbers are. Do you have a quality dummy load for 1.2GHz? You may want to build one you believe in. It will help you proceed with testing.
    Last edited: Nov 11, 2020
  5. NM7G

    NM7G Ham Member QRZ Page

    remove the SWR/power meter and all other components
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    Even if the antenna would fall off entirely, the SWR at the end of 35 ft properly connected LMR-400 on 1300 MHz cannot exceed about 2:1 when measured with a quality instrument.

    My suspicion is that the wattmeter either is faulty, there is a connector fault near the transmitter end or that the wattmeter is not intended for use at 1300 MHz.

    Very few "ordinary" wattmeters work properly on 1300 MHz.

    Last edited: Nov 11, 2020
    WA9SVD likes this.
  7. NM7G

    NM7G Ham Member QRZ Page


    Standard LMR-400 (no suffix) cable assemblies have near 1.6 dB of loss with quality connectors at 1.2 GHz. Thus you cannot know GP-21 VSWR unless the measurement is made directly at the antenna base.
  8. KC9ONY

    KC9ONY Ham Member QRZ Page

    Which Daiwa CN-801 are you using?
  9. W1TRY

    W1TRY XML Subscriber QRZ Page

    If you have a Type N female flange connector, you can make a ground plane. Basically you use 70mm (2.76") of wire as the vertical element from the center conductor and 4 61mm (2.4") wires off the 4 corners bent to a 45 degree angle.
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    CN-801 is only rated to operate up to 200 MHz. There was a model CN-801G rated for 1.2 GHz but it's discontinued and I've never seen one.

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