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Whip Length.

  • 09-09-2013 5:11pm
    #1
    ardle1
    Registered Users, Registered Users 2 Posts: 1,521 ✭✭✭


    Hi everyone, I'm just wondering, if I post my taxi base frequency would anyone be able to tell me the length I should cut my whip to for my car!!, is there a formula for working this out? sorry if this is a silly question, just not sure, apparently we need a 'long' aerial/antenna....


Comments

  • #2
    Quaderno
    Registered Users, Registered Users 2 Posts: 817 ✭✭✭Quaderno


    ardle1 wrote: »
    Hi everyone, I'm just wondering, if I post my taxi base frequency would anyone be able to tell me the length I should cut my whip to for my car!!, is there a formula for working this out? sorry if this is a silly question, just not sure, apparently we need a 'long' aerial/antenna....

    First you need to calculate the wavelength λ for the frequency the radio is used on. You get that by dividing 300 by the frequency in MHz. So if your radio is tuned to 160MHz you would get 300/160 = 1.875m. If you transmit somewhere lower, say on 75MHz, the corresponding wave would be longer. For the example: 300/75 = 4.0m. That's sounds like quite a whip, but you are not going to use the full length. A very efficient length would be λ/2 (or 0.94m resp. 2m for the examples), but depending on the frequency this would still be quite long. To adjust the antenna perfectly (especially if you need to cut it shorter) you would want to use an swr-meter, because a mismatched antenna can cause expensive damage to the amplifier in your radio.


  • #3
    watty
    Registered Users, Registered Users 2 Posts: 32,417 ✭✭✭✭watty


    There may be base loading for 5/8ths.
    You need a correction factor.

    It's nearly impossible to get right without a professional SWR meter. The aerial needs to be mounted too as a the car metalwork is part of the aerial.

    Best to copy an identical model for the same frequency or buy a new one.

    1/2 wave is very hard to match. Aerials are usually 1/4 or 5/8ths. But actual length is a affected by rod diameter and other factors. Aerials can have a coil at any point from base to about 1/3 below tip to reduce physical length too.


  • #4
    Capt'n Midnight
    Moderators, Recreation & Hobbies Moderators, Science, Health & Environment Moderators, Technology & Internet Moderators Posts: 93,567 Mod ✭✭✭✭Capt'n Midnight


    your antenna will be slightly shorter than you calculate because of end effects

    in general you make an antenna longer than predicted and slowly trim it back as it's easier to make it shorter than longer

    and if it's already got a coil, then all bets are off unless you can measure the SWR

    and yeah you have to do it in place as the car roof affects it


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