antenna

Discussion in 'Wireless & RF Design' started by PackratKing, Jun 29, 2014.

  1. PackratKing

    Thread Starter Well-Known Member

    Jul 13, 2008
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    I ran across one of the long-tall " whip " CB antennas, oft seen in the heyday of CB,

    The guy at the garage sale, told me it had been in 4 successive sales, unsold. and since I displayed an interest he gave it to me just to be rid of it..

    As I understand, it is a " 1/4 wave " ? unit... Any opinion out there as to how efficient these are / were ?

    It only has a wee bit of rust on the mount stud, and where the cable hooks, appears decent..

    Is that large spring at the base just for flexibility, or does it serve also as a *loading coil*...

    Yep... I am a relative amateur at this stuff :D

    Edit : Pictures of my chopped leads on the CB are pending
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. inwo

    Well-Known Member

    Nov 7, 2013
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    Very good antenna. Big is good. I believe they were full quarter wave. A good match to 50ohm feed.

    Problem was mounting. Best on the roof. Not too practical.

    The one I remember had a braided cu. jumper inside the spring.
     
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  4. Papabravo

    Expert

    Feb 24, 2006
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    One quarter wavelength at 11 meters would be 108 inches. It is a very efficient radiator when mounted to the car body as the counterpoise, and is omni-directional.
     
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  5. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    I worked for Radio-Shack part time during the CB craze in the 70's installing and testing antennas. (mainly for truckers) I've seen almost every crazy install of the 1/4 whip including one in the center of the roof of a truck. It worked really well for CB and for breaking every overhead fluorescent light tube at a truck stop roof. :p

    I still keep a radio for hunting in the boonies here, It's got a magnetic mount loaded coil model that I keep in the truck for emergencies when out of cell range. There's still lots of activity on the bands.
    https://flic.kr/p/dYvm98
     
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  6. JMW

    Member

    Nov 21, 2011
    88
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    It is a good antenna, the best part is the rod. If you're not going to use it as antenna, cut into various lengths, it makes a very good center punch, rails for a grille etc. Use a grinder to cut it. It should be mounted to metal for best results. An old satellite dish antenna is great.
     
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  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Huh? Aren't they a fibreglass rod with a wire stuck to it by heatshrink tube?
     
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  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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  9. PackratKing

    Thread Starter Well-Known Member

    Jul 13, 2008
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    Nope... this is the steel one...:D
     
  10. k7elp60

    Senior Member

    Nov 4, 2008
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    I have one of those stainless steel ones also. With an antenna tuner they can be used on a lot of frequencies. A lot of hams( amateur radio operators) use them. I used to use it as a vertical antenna when I lived in a mobile home that had a steel roof. PapaBravo mentioned a quarter wavelength of 108". That the correct length in free space, however even a piece of wire has a velocity factor.
    The velocity factor is the speed of RF in a conductor in relation to free space.
    The CB actual 1/4 quarter wave length for a single conductor is 102". The formula is 1/4λ is ((492)/f in mhz)/2)) for free space. Change the 492 to 462 for a single conductor.
     
  11. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Good article on the origin of that 4xx number. The actual velocity factor/resonance length is a complex equation that takes into account much more than the antenna near-field space boundary (conductor charge movements and the surrounding dielectric that the fields travel in) but for a 'thin/small diameter IRT wavelength' antenna near ground (added capacitance) it's pretty close.

    http://www.eham.net/articles/23802
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    actually the 462 number is for the shortening due to end insulator capacitance of a 1/2 wave wire dipole.
     
  13. PRFGADGET

    Active Member

    Aug 8, 2011
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    If you are handy with "Excel" or a similar spreadsheet program, or just want to play with a calculator, play with the numbers , you will find that there is a "harmonic" relationship as in fractions or multiples of "wave-length (s)" of the design frequency of the lowly 1/4 wave CB whip.
    With a little research (Google) you can also find these "Harmonic" programs or spread sheets.
    It is interesting to note the departure from "Zero Point" (the design frequency) of the harmonics you will find.
    WITH THE ABOVE IN MIND, any departure from the design frequency (in respect to a 40-80 ohm load to the transmitter / Receiver) will require some sort of "matching network" (some call them "Antenna Tuners") in order to bring the antenna / feed-line system back to a point of "Resonance" (or near).
    The more Radical the departure, the more critical the tuning , both in the physical and the electrical aspect's of the antenna.
     
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