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GP Antenna Help

Discussion in 'General CB Services Discussion' started by Joey Migs, Apr 27, 2009.

  1. Joey Migs

    Joey Migs Member

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    Hey Now,

    I have been experimenting with a 1/4 ground-plane w/ 4 drooping radials. After three attempts I am puzzled as how to achieve a reasonably flat VSWR across the frequencies I frequent (~ 11 - 10 meter). I'm trying to perfect it in anticipation of a skip-full dog day nights of summer. I prefer to work bare-foot or QRP so SWR is critical in making efficient use of my rig's RF output.

    Question one

    What length should I cut the radiator (driven element) for lowest SWR at a frequency of, let's say, 27.375 MHz. Using the 234/F(MHz) formula yields 102.6 inches. This produces a SWR ranging from 2.5 ~ 3. The Antenna resonates a 25.650 MHz with an SWR of 1.1 : 1. Moreover, I have always thought the 1/4 wave length of the 11 meter band was 108 inches, i.e on a cars 102 inch whip is attached to a 6 inch coil base. Please help, if you can, to clarify this to me - I am clearly missing something.

    Question Two

    I know that running the radials of the aforementioned Drooping 1/4 GP will lower the feed-point impedance from about 36 ohms (horizontal) to approximately 50 ohms at a droop angle of about 45 degrees from horizontal. I am using pretty good coax (RG-8/U - solid center, 95% braid). Does this antenna/transmission line set-up seem reasonably sound to you?

    Also, I have heard conflicting "facts" within ARRL books and other sources regarding the length of the 1/4 GP drooping radials. Some say to cut the length 5% greater than that of the length of the radial (driven} at operating frequency and some sources say that this is merely a myth.

    Thanks, in advance, for any light you may shed my way.


     

  2. Wire Weasel

    Wire Weasel Senior Moment

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    The radiator is too short not too long. You need to be using 246 rather than 234. And whatever the numbers, they always end up still too short when you stick 'em up in the air. Radiator will probably need to be somewhere between 106~109" inches long.

    Had to take the 102" whip out to 109" with a shaft extender to achieve 1:1 in the CB range on my truck.

    The manufactured Stardusters and other 1/4 wave groundplanes achieve better bandwidths as they were using "fatter" aluminum tubing rather than a skinny radiator like a whip. The larger the diameter of the radiating material, the more bandwidth they will create.

    Ya know....you can still buy a Starduster new. They're called an M 400. Easy Google search will find them for you.

    Good luck

    [​IMG]
     
  3. i made and use a wire version of what youre talking about . mine has four 108 inch grounds at about a 33 degree angle . i made the top radiator a lil over 9 feet because i wanted to be sure that i had enough to tune the antenna because different installations can affect an antennas tuning length . i have no idea what my finished length was but i got it down to where my astatic 700 meter barely moves on channel 1 , moves just a lil on channel 20 and is still below 1.1 1/2 at channel 40 .

    you need to allow enough extra wire/length to the radiator (i did 9 ft 6 inches to start) so you can tune the antenna to your particular installation .
    you may also find better performance isolating the antena from its mast and not using a ground wire .

    btw . i used 12 gauge stranded speaker wire .
     
  4. W5LZ

    W5LZ Crotchety Old Bastard

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    First, don't expect to get a 'flat' SWR over a Mhz of frequency range. An SWR meter can show you that, but it can't tell you why it's that 'flat', which is not a good thing. The better the antenna, the more efficient it is,the higher it's 'Q', the less bandwidth it will have. Antennas that have a 'good' SWR over a very wide bandwidth are just not very efficient antennas. SWR is NOT a very good way to judge an antenna's worth.

    There are always two 'parts' to tuning an antenna. Resonance, and impedance matching. Resonance refers to getting rid of any reactances, which do not produce radiated power (resonance is defined as '0' reactances). The most common method of finding resonant length is by formula. That formula changes with the size of the radiating conductor. For wire conductors that 'magic' number is '234'. If you really want to do the math, find a ham Handbook, it'll tell you how, I'm too lazy. SWR meters can't tell you squat about resonance. A field strength meter can get you ball-park close. An antenna analyzer can get you much closer.
    The second part of tuning an antenna is to make it's input impedance the same as the rest of the antenna system. Get it as close to 50 ohms as you can. A GP antenna with 90 degree radials has an input impedance something close to 35 ohms give or take some. A vertical dipole has an input impedance somewhere close to 70-75 ohms. That vert.dipole is the same thing as a GP with straight down radials. So, finding the right 'angle of the dangle' for those radials is the most common way to adjust a GP's input impedance. something close to about 45 degrees of 'dangle' will get you close. And then it's just a matter of changing that 'dangle' till you get it as 'right' as you can. That 'right' changes with surroundings/environment, so it isn't a 'constant' thing.

    How long should those radials be? Same length as the vertical element, maybe a little longer if you feel like it.

    The only good reason to 'ground' any antenna is for safety. A 'ground' wire will make very little difference RF wise if the antenna is right to start with. RF wise, those radials are ground, not the dirt under the antenna.

    Really want to make that 's.o.b.' better? Paint it pink, that always works, always! All the rest is variable...
    - 'Doc
     
  5. PAINT IT PINK !!

    U DA MAN DOC !!!!!!
     
  6. Joey Migs

    Joey Migs Member

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    Thx

    Thanks, I appreciate the SWR antenna transmission line edification, as does my back. Humping that thing up and down is a real bear and wreaking havoc on the ole' spine. When I say a relatively 'flat' VSWR, I mean 1.1 : 1 (or less) on the tuned frequency but not exceeding 1.5~1.6 : 1 on the fringe frequencies at either end of band in question. I have been using the magic '234/freq' formula albeit the driven element as well as the four radials are 12 AWG stranded . I think that the issue of diameter to length ratio is the crux of the problem. I'm going to cut the radiator to ~ 108.5 inches so I still have decent RF transmission on the open-band toward 10 meters. Your thoughts?

    Thx,
    Joey
     
    #6 Joey Migs, Apr 27, 2009
    Last edited: Apr 27, 2009
  7. Joey Migs

    Joey Migs Member

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    So it's the paint, eh?

    I think you're on to something there. You see I painted it w/gold spray and evidently gold spray paint is an excellent conductor; I might as well have used the street lamp in front of the house as the antenna and some zip cord as a transmission line to achieve the same results. Got to go, I have a date with a can of paint stripper and sleeve of steel wool; looks like it's gonna be a long night.

    I would paint it pink but I'm afraid that might be tantamount to putting up a salt-lick to attract every fag in the neighborhood, not that there's anything wrong with that (vis-a-vis Seinfeld)

    Later, and thanks for your invaluable advice, now that meth lab is not gonna run itself so you better go get to that.

    Later,
    Joe

    PS I'll be in my garage - Joe's Garage.
     
    #7 Joey Migs, Apr 27, 2009
    Last edited: Apr 28, 2009
  8. Joey Migs

    Joey Migs Member

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    Sounds like some sound advice and I didn't know you were the 'paint it pink' dude so disregard my 'meth lab' remark; I'm a new yorker and having a sharp tongue goes with the territory.

    Gonna try 107.5 ~108.5 inches for the radiator and see what happens.

    Thx again,
    Joe

    PS Watch out where the huskies go.... (you know the rest)
     
    #8 Joey Migs, Apr 27, 2009
    Last edited: Apr 28, 2009
  9. Joey Migs

    Joey Migs Member

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    I am tinning the radiator and 8.5 inch extension segment as we speak - actually I have delegated that task to my semi-retarded son and I just hope he keeps the business end of the Weller gun out of his eyes (and ears for that matter).

    I think the 'Merlin' used the same design as well. I would buy a pre-built antenna but that would be no fun at all. It's like growing killer home-grown; there is a sense of satisfaction associated with it.

    Thanks for your help,
    Joe
     

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