75 meter mono-band MOBILE

Discussion in 'Mobile Radio Systems' started by WL7PM, Oct 24, 2019.

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  1. WL7PM

    WL7PM Ham Member QRZ Page

    Calling on the Mobile HF Amateur- Experts:
    Help me build a NO-Compromise 75 meter mobile antenna.
    I don't need wide-band, don't need multi band, and in my mobile world QRP begins at 100 watts.
    My goal is state-wide comms inside 600 miles radius.
    I have around 500 watts to radiate, my mobile ground plane is roughly 150 square feet. Maximum height while rolling should be under 14 feet, but if the antenna is tough enough, or spring mounted and "leans back at highway speed" we can go a little taller.
    For mobile DC power I have installed deepcycle 4D 170 amp-hour flooded lead acid battery, and a fairly high duty cycle 22 SI alternator which can maintain 160 amps output for a few hundred hours.
    The ALS500M amplifier is a potential weak link, but for intermittent SSB use has held up without breakage since 1996.
    Thinking about a top loaded inverted L, with trailing counterpoise ...?
    Anyone old enough to remember fuel tanker trucks trailing ": Ground wires" ?
    .
    Lets' not be afraid to think outside the RF cage here, I have lots of room, and lots of wire to experiment with.
     
    NH7RO likes this.
  2. W5LZ

    W5LZ Ham Member QRZ Page

    I am well aware of those trailing 'ground chains' and I would do some research if I were you. They do not cut down on interference and contribute to it instead. You won't find them on 'big trucks' anymore, they are dangerous especially to those hauling fuel, explosives, or any flammable substances. There are no regulations about private vehicles so if you wanna, be my guest. You really don't want to do that...
    As for a single band 80 meter antenna think big... and ugly. And as you suggested, fairly narrow banded without a tuner.
     
  3. WL7PM

    WL7PM Ham Member QRZ Page

    The trailing 'counterpoise' is meant in joking fashion, but with intent to generate discussion, ideas, in the direction of Ground Coupling, capacitive or 'other means'.
    I am not opposed to a matching network, or antenna coupler. I have used a Stevens SEA1630 at 250 watts with some success. Have a modified HUMMVEE antenna ( 9.5 feet long ) on roof of pickup truck, has hit every bridge, overpass and telphone line between Talkeetna and Homer a thousand times or so. Makes a loud bang in the cab, but still not broken.
     
  4. KX4O

    KX4O Ham Member QRZ Page

    Sounds like you are willing to max out on lots of good attributes. The closest thing I can find that heads in your direction is this recent article...


    Key points:
    • Monoband (well one can manually adjust the roof coil for 40m, but this mobile station was designed specifically for state QSO party use on 75m).
    • Well tweaked to squeeze the most out of a mobile - a lot of Alan's advice is built in.
    • Loud signal using Kenwood 480HX... no problem making QSOs at the "in-state" distances graphed in figures 7 and 8 (Note: these figures don't include the numerous out of state contacts).
    • For the 2018 and 2019 Virginia QSO Parties, there was never any problem closing contacts with this mobile except when 75m dwindled which was rare these years (figures 1 and 4).
    Start with some of these concepts and make it bigger and better.
     
  5. WA8FOZ

    WA8FOZ Premium Subscriber QRZ Page

    upload_2019-10-24_23-44-30.jpeg
    Are you REALLY sure you want to do this? Breedlove mount, 12 inch long by 4 inch wide bug catcher coil, 2 feet of mast, 6 feet of whip. Quarter panel reinforced inside by welded metal plates. Approximately 13 feet high. Quick disconnects and base and top of coil, so that one can park inside. With 100 wants I have a good QSOs, But one is still dependent upon the kindness of strangers. That is to say, on 75 m it helps to have friends listening for you. Good luck!
     
    NH7RO likes this.
  6. W5LZ

    W5LZ Ham Member QRZ Page

    From the picture;
    It seems like using those weights on the front of tractors might be a good idea. It'd keep the thing from tipping over backwards at speed. I'm really surprised that it isn't guyed!
     
  7. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Nothing you can do, and still drive down the highway, will reduce the ground losses. This said, you can use a (properly designed and mounted) cap hat to raise the current node, which will increase the radiation resistance significantly. By properly designed and mounted, I mean at the very top of the antenna, and with enclosing loops. This makes the antenna more susceptible to snagging on errant limbs, but everything is a compromise when operating mobile.

    As for the ALS-500. As long as you keep the drive down, so the PEP output is under 400 watts, the IMD will be within FCC limits. At 500 watts PEP, you can easily hear it.
     
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  8. NH7RO

    NH7RO Ham Member QRZ Page

    caphatzscreen 003QRZ.jpg
    tqandfultonmobile 010QRZ.jpg
    Note that this somewhat massive radiator/top-hat combo is on a fold-over mount as I'm wary of potential damage if I ever tried using this while actually in motion. I suppose it could be done if I had a way to brace it or guy it but I prefer to devote my full attention to to driving instead.

    Note to the OP; if interested in building something like this feel free to search for my thread entitled "Crazy about cap hats!" in the QRZ Antennas Forum (more pix and construction details there).

    I like your idea and wish you the best of luck in getting on 75m mobile in the best possible shape, too---a solid challenge no matter what!

    FWIW, the first three QSOs during the first session above were TX>SC, then TX>HP9 and TX>KH6 on 20m. The two dx stations called me to tell me how loud I was, too!

    73,

    Jeff
     
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  9. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    You would be better off with a much lighter and larger cap hat. With a shorter mast as well, which will reduce the likelihood of an errant snag.

    There are a lot of mathematical hoops to jump through, but it turns out the cap hat needs to be about the same diameter as the length of the mast. The coil position changes with frequency (resonance point, really), but never exceeds the 40% (from the bottom) position.

    The cap hat on my web site has is three loops. Each of the loops are 72 inches long, and made from303 stainless (don't use aluminum of ANY kind!). You could use four loops, but you increase the wind loading by 33%, with an almost immeasurable increase in the Rr.

    It is also interesting to note, that early adoptees of cap hats (circa 1960s), also settled on the three loop set up.
     
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  10. NH7RO

    NH7RO Ham Member QRZ Page

    This project seen above was never intended for mobile operation, only fixed portable as shown in my photo.

    I do have other all-copper cap hats I've built that are both lighter and larger (48" and 60") as well as the stock 36" 3-loop Scorpion that fits the stock 5/8th" stainless mast that I could use. I didn't know about the same diameter as length rule which is really good to know---that explains why my 60" hat and 60" mast combo worked so well when I had my Scorpion mounted on my back fence at home for a long time (was my dedicated 40m antenna then). I'll have to take my five-foot "wheel" with me next time I go to the seaside and try it out as well; all of my DIY hats and radiators have 1/2" pipe fittings so they can be slipped on and off at will.

    I may try making larger loops for my Scorpion cap hat mounting flange, too (#10 wire should be about right and I can stiffen the wire with a vise and drill before bending into loops).

    Thank you for those tidbits of good information, Alan; I'll put them to good use eventually.

    73,

    Jeff
     
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