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Thread: Need a really short antenna?

  1. #1
    4x4 Ham Member Supporting Member KF7ORO's Avatar
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    Need a really short antenna?

    My FJ barely fits in the 7' tall garage door without an antenna on top so I had this idea to use a 1/4 wave 70cm antenna. It's pretty easy to find an NMO mount antenna you can cut down to the right length. I've been running this way for about a year and it's working really well. I just have to make sure not to tx on 2m . I do have a dual band antenna stored in the back that I can throw on the mount when I'm out on the trail too. If you need a really short antenna, this ends up being under seven inches off the roof and works surprisingly well.

    Brian

    IMG_20160207_142639.jpg
    Brian - KF7ORO

  2. #2
    4x4 Ham Member
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    I have been using two Comet B-10NMO antennas on the top of my Expedition. They rub a bit on entry but it is not that bad. The stand about 11 inches off the top of my truck. Use one on 2M and 440 and the other for either my UHF DMR or my dual band APRS HT. About the East Valley I get great results. Most of the time I am putting out 5w for the mountain top repeaters as well as some others. Last time I was at HRO they had a few in stock.

    Douglas
    W7ADD

  3. #3
    Administrator Supporting Member K7VZ's Avatar
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    FYI, here's another low profile antenna that I've seen used to clear garages. It'd allow for VHF transmit. https://www.tessco.com/products/disp...28&eventPage=1
    Virgil - K7VZ
    Offroad and on the air.

  4. #4
    New 4x4 Ham Member
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    On my TJ Jeep I have vhf/uhf plus HF antennas, Both are made by Diamond and both lift up and tilt over so I can get the Jeep in the garage, then there's always the option to unscrew it from the base, takes just as long to unscrew or tilt over,

  5. #5
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    It would be cool if they came out with a decent U/V phantom antenna. I have seen them on eBay but not sure as they ship out of China.

    Douglas
    W7ADD

  6. #6
    4x4 Ham Member Supporting Member KF7ORO's Avatar
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    Quote Originally Posted by SA4MDN View Post
    On my TJ Jeep I have vhf/uhf plus HF antennas, Both are made by Diamond and both lift up and tilt over so I can get the Jeep in the garage, then there's always the option to unscrew it from the base, takes just as long to unscrew or tilt over,
    I ran with a tilt over sort of antenna for a while but I found it a bit of a hassle to stop after pulling out and put it up and remember to put it down before pulling in. So I often just didn't put it up and that kept me off the air. With this setup I'm limited to 70cm but lots of the repeaters here are linked to 2m repeaters so it's not a huge problem around town. Plus it's more convenient so I use the radio more.
    Brian - KF7ORO

  7. #7
    4x4 Ham Member Supporting Member KF7ORO's Avatar
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    Quote Originally Posted by K7VZ View Post
    FYI, here's another low profile antenna that I've seen used to clear garages. It'd allow for VHF transmit. https://www.tessco.com/products/disp...28&eventPage=1
    One of our work trucks had an antenna like that and I was never really all that impressed with it. I guess it was ok but the radios in the other trucks with a longer whip antenna worked better.
    Brian - KF7ORO

  8. #8
    4x4 Ham Member Supporting Member WB9YZU's Avatar
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    I know this is an old thread...

    When I had the Volvo XC90 I built one of these:
    https://www.antennex.com/preview/2mddrr.htm

    The one I built is essentially a 1/4 wave loop on the bottom with a 1/4 wave radiator bent into a loop vertically spaced by 3"(Figure 2)

    Instead of a coax feedline, I soldered a PL259 to the lower loop at the braid side feed point, and a piece of 14 awg solid for a center conductor that is soldered at the other feed point. Build material is refrigerator waterline tubing (1/8" Copper). To stiffen up the assembly, I used (3) 3/8" fiberglass dowel epoxied to the loops (evenly spaced). I filled in the holes with epoxy and gave it a coat of black non-metallic paint - which I oven baked on.

    The PL259 acts as a mounting point and the antenna screws on to a SO239 Mag mount. Total height off the vehicle is about 5".

    The antenna is Vertically polarized despite it's appearance and is fairly broad banded at 1.5:1 on the repeater portions of 2M and 440.

    Because of a car wash incident (Hint: Home brew copper antennas do NOT like brush type car washes!) I had to built a replacement. It now sits on top of my Commander which gets parked in the garage every night without incident.
    73's Ron

  9. #9
    4x4 Ham Member W5PKY's Avatar
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    Personal opinion, use a Sti-Co Flexiwhip cut for 146 MHz. It's 3/4 wave on 70 cm and therefore, a natural 50 ohm match. Won't radiate perfectly but for city use it will work adequately.

  10. #10
    4x4 Ham Member AC0VH's Avatar
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    Quote Originally Posted by W5PKY View Post
    Personal opinion, use a Sti-Co Flexiwhip cut for 146 MHz. It's 3/4 wave on 70 cm and therefore, a natural 50 ohm match. Won't radiate perfectly but for city use it will work adequately.
    For a number of years this is what I ran, a trimmed slightly long Flexiwhip. Actually got around to measuring it eventually (see original post here). It was mounted middle of the cab roof on my '91 XtraCab Toyota pickup, plain Jane RG-58 cut to length.

    I actually have a couple of Flexiwhips because I eventually drilled a second NMO and ran a duplexer with a pair of tuned whips, so one shows a more ideal 2m whip compared to one I left a little longer for better match as a dual band.

    Eventually put a dummy load on the NMO and figured out that some of the mismatch on 70cm was actually due to the coax and/or SO239 (probably) rather than the antennas. Still, though the ~19" whip didn't seem to have the bandwidth to cover all of 70cm it would certainly work acceptably for regular mobile FM repeater duty.

    combined_measured.jpg

    Even modeled the whip in NEC to see what it might look like in elevation on 70cm. That's the weird 3/4 wavelength radiation pattern to which you're referring.

    Over perfect ground:
    perfect_ground.jpg

    Over average ground:
    average_ground.jpg
    Last edited by AC0VH; 12-03-2017 at 11:12 AM.

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