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  1. #1
    DF MaSter TFS's Avatar
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    Newbie antenna questions

    Hopefully this is an easy one which I can get an easy answer for.

    I have a mini-magmount whip type antenna for a mobile VHF transceiver (the ultra mini footprint magmount diameter~2cms). I am using the radio at app 169 MHz. The antenna is a very simple whip design, there is no coil or loading in the base simply the aerial whip.

    My questions are several-

    1. What length should I trim the antenna to? (is it simply 1/4 the actual wavelength for this frequency or is it more complicated)

    2. Is it worthwhile trimming the cable to a mulitple of half
    wavelengths?

    3. As the whip is held in place by a small grub screw and can be adjusted up and down, is the ideal whip length what is visible above the mount or the whole length ie including the bit of the whip inside the mount?

    4. There is no end cap for the aerial, does this make a difference?

    5. What should the coax screen be connected to at the mount? (the base of the mag has a non-conductive cover so I take it theres no way of making a direct connection to the car to make a ground plane or am I way off here??)

    6. As the results don't have to be perfect (it's not that critical) is it worthwhile buying a SWR meter for a one-off (my other radios are all handhelds) or can I get reasonable results without one?


    Hope I'm not sounding too stupid here,

    TFS

  2. #2
    DF PlaYa sparkster's Avatar
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    1. 44.5 cm should be resonant on 169

    2. not really worth the bother m8

    3. make it a total length of 44.5 cm inc base,, that should be fine.

    4. none at all.

    5. the none conductive cover is only the to protect you vehicle,, the braid is/should be connected to the base inside the magnet area.

    6. save your money m8, it is not really necessary to buy an swr meter,, the ariel at the above length should be ok.

    Edit:

    to calculate quater wave ariels,, just use this formula ,, 300 / fmhz.. that means 300 shared by the frequency in megs,, then divide the answer by 4
    Last edited by sparkster; 1st October 2002 at 02:37 AM.

  3. #3
    DF MaSter TFS's Avatar
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    Thanks Sparkster, the formula you give for whip length is what I expected but some cutting charts show that the whip length should be smaller than the value given by this formula. Is this because the height of the base/mount when added to this 'chart' value gives the length found from the formula??

    TFS

  4. #4
    DF PlaYa sparkster's Avatar
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    yes m8 they would show a smaller ariel,, the one stated above is the calculation for an ariel in free space,,, that being,, the speed of light is 300,000,000 divided by the frequency then divided by 4 ,, in your case 300,000,000 divided by 169,000,000 then divided by 4 gives us approx 44.378 ,, as i was told getting an ariel in free space is not possible without a vacuum ,, and as there are no such things as s k y hooks (ham joke) , i was given the formula 2808 divided by frequency then multipy it by 2.54 ,, (cos the answer is in inches) is also correct,, and would give you a smaller ariel length,, i have never had any problems using the first math calculation , and have not damaged any radio using an ariel made with them,,
    the mag base is a part of the ariel so that should be included in the size of the ariel when cuttin your whip,, it's been a long time since i had to use these calculations in making 1/4 wave ariels so does anyone else have any input???

    sparkster

  5. #5
    DF MaSter TFS's Avatar
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    Thanks Sparkster, I'll hold back a bit before I trim the whip to see if anyone else has any different input or ideas.

    Cheers,

    TFS

  6. #6
    DF MaSter ABA's Avatar
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    Default TO SPARKSTER

    you say that there is no such thing as sky hooks now you have upset me for only last week i purchaced a sack full of pudlut holes that were made just for sky hooks now what am i going to do with these holes have i waisted my money or can they be used for any thing els yours ABA












    :signs: :signs:

  7. #7
    DF PlaYa sparkster's Avatar
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    If it was me m8,,, i would send them back,,,under the sale of goods act the items purchased have to be fit for the purpose that they were brought for,,, now as you were sold them under false description ie: to use with sky hooks ,,, and as sky hooks do not exsist, i think you have a good case

    sparkster

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