Radio Face Plate

Discussion in 'Early Jeep Restoration and Research' started by johnny and sandy, Aug 1, 2017.

  1. Sep 12, 2017
    58 willys

    58 willys Sponsor Sponsor

    Millsboro, Delaware
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    This antenna is the AMC/Jeep era they were mounted on the right side cowl area they came with a template that you tape to the cowl using the fender and cowl seams for proper positioning drilled many of those antenna holes during my days at the dealership. Tim you are correct that was the only way you were going to get a radio in an early CJ was under the dash as you described threw away a lot of those early radio boxes when we moved the dealership into the new building.
     
    Last edited: Sep 24, 2017
  2. Sep 23, 2017
    Hellion

    Hellion Exciting New Member

    Eastern TN
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    Dec 25, 2016
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    115
    The Firestik is a helical antenna.
    I have not found documentation, yet, to support your presumption that the ideal antenna length for FM is 30 inches... In fact longer or taller as it were is optimal since FM radio waves are propagated line-of-sight.
    However, it is my understanding that where the smallest of antennas is desired, the helical is the way to go.

    Been used for years on vehicular Citizen's Band and FM applications...
     
  3. Sep 24, 2017
    Howard Eisenhauer

    Howard Eisenhauer Super Moderator Staff Member Sponsor

    Tantallon, Nova...
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    Indeed it is. Built one when I was in high school out od an old fishing rod & some magnet wire. Probably from the same plans Firestick uses. Worked OK bu not great.

    Antennas are most effective when they are resonant at the frequency you're operating at. Think of two nearby tuning forks cut for the same note- if you tap one it will vibrate & the other one you didn't tap will start vibrating as well. If the tuning forks are cut for different notes tapping one will not get the other one going.

    The "ideal" antenna is a dipole which consists of two quarterwave sections in a line, get a copy of Termon's "Radio Engineering Handbook" if you want to know the theory.

    For CB radios operating around 27 mhz (11 meters) a quarter wave section for an antenna is (.25 x 11m) = 2.75 meters or 108.25 inches. As it's impractical to run dipole antennas on a vehicle for 27 mhz. we go with a single quarter wavelength section & use the vehicle body as the other section of the dipole (counterpoise). As jeeps are not usually resonant at the frequency you want generally there's always some tweaking to be done with the antenna length to get the most signal out of unit.

    The purpose of a helical antenna is to make an antenna shorter than the ideal length of the frequency you're trying to receive, i.e 48" Firestick vs. 108" whip. The helical coil of the antenna causes it to be "inductive", to operate properly with the radio that inductance needs to be counteracted with capacitive reactance, this is why helical antennas may incorporate a tuning control at their base (mine did). So helical antenna can be made resonant to the frequency you're interested in but there's always a performance tradeoff compared to a straight dipole or 1/4 wave element. More wire = more resistance = less signal, the thinner wire used in a helical compared to what is normally used in a whip = more loss to "skin effect" (see Terman). Bottom line is that helical antennas are not as efficient as antennas with 1/4 sections.

    For FM radio @ ~ 100Mhz. it's the same story- ideal 1/4 wavelength = 3 meter wavelength divided by 4 = 75 cm = 29.5" length.

    Now at these lengths a full dipole is a lot more manageable than at cb frequencies so many cars actually have a dipole embedded in the windshield. Those that don't use a whip that is about 30" long. Of course for AM frequencies a 30" whip is way short of the ideal length, ~1 Mhz = 300m wavelength = 75 meter = 246' quarter wave length so you antenna is way short of what it should be. This equals the antenna being more capacitiive than ideal, the radios are built with big loading coils in the AM receiver section to introduce inductance in the circuit to make things "look" right for AM frequency reception. But there's still a tradeoff- you $1000 fancy car stereo system radio will never pick up AM stations that a crystal radio kit will get with a couple of hundred feet of proper antenna wire running out the back 40.

    Many VHF handie talkies (140-175 MHZ) actually use "rubber ducky" antennas- these are helical antennas as well & are used because a 3 or 4 " long antenna is a lot more manageable for these things than a full length 1/4 whip, but again there's a loss of performance- in technical terms a rubber ducky has a gain of around 1 dB compared to the full length whip of around 3 dB (see Terman). If they worked better by making them a bit (or lot) bigger they'd be longer, but longer would actually be worse for performance so they don't.

    Final note- if you're depending on the Firestick website for your information then don't- there's a lot of just plain incorrect information on there. I suspect the marketing staff has a lot more say on the contents than the engineering staff does, assuming they actually have an engineering staff which I do have my doubts about.

    Personal note- I've been involved in testing/qualifying antennas to determine if their performance met the manufacturer's specs & I've had some fairly interesting , & occasionally loud, conversations with reps discussing whether or not their prized creations could violate the laws of physics.


    Even after testing showed they can't.



    Go with the 30".



    H.
     
    Hellion and Focker like this.
  4. Sep 24, 2017
    JamesCoisp

    JamesCoisp Banned

    Bahrain
    Joined:
    Sep 23, 2017
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    Looks like they are keeping a tight lid on the replacement. Any ideas?
     

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