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Comet SBB2 NMO with high SWR on 440

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5PB, Jul 1, 2020.

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  1. W5PB

    W5PB Ham Member QRZ Page

    I have a Comet SBB2 NMO mounted on the roof of my car. It's attached to a Breedlove 504 Mount and 15 feet of RG 8X running to the front of the car. The coax connects to the mount with ring terminals. The leads on the coax are as short as I could make them. About the same length as a small ring terminal.
    2 meters is fine, but I'm still getting high SWR on 440. It's at or just above 2:1 between 440-450 MHz. It dips to 1.8:1 around 445 but quickly goes back up. I've made every adjustment possible with no effect.

    Ironically, when I used a lip mount on the hatch, I had the opposite problem. 440 was great, 2 meters was not.

    At this point I'm not sure where the problem lies. Continuity between the pl 259 and the mount are good. Since the Breedlove mount doesn't have "teeth" on the nut, I sanded the underside of the roof to get a better ground connection. Im using the Breedlove mount because rather than drilling, I'm using the factory hold for the OEM sharkfin antenna. It's a square hole that I thought would be much bigger than it actually is, so I bought this mount just in case.

    Any ideas are appreciated.
  2. K7JEM

    K7JEM Ham Member QRZ Page

    My guess is that the added length of the ring terminals is messing with SWR. Almost all NMO mounts are constructed differently than this, with very short exposed center conductor, maybe 1/8" or so. At UHF, it doesn't take a lot of added length to change SWR.

    You could find an NMO to UHF adapter, and screw a dummy load on it to check out the mount, or maybe install a known good QW antenna and see how it works.
    W5PB likes this.
  3. W5PB

    W5PB Ham Member QRZ Page

    I fear you are correct. I don't have a dummy load for VHF, or another antenna. My best bet might be to get a regular NMO mount and use that instead, now that I know the hole is smaller than anticipated. I suppose I can sell the Breedlove and make a few bucks back. I like the low profile of the antenna, and I'm in a large metro area so I don't really need the extra gain.
  4. W5PB

    W5PB Ham Member QRZ Page

    Yup turns out the Breedlove mount was the issue. I probably shouldn't have jumped the gun and assumed that's what I needed. Turns out I can use a 3/8 NMO in my car's factory antenna hole. Problem solved.
    K7JEM likes this.

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