1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice
Dismiss Notice
This forum does not allow a single user to have more than one username. If anyone wants to change their username contact an admin and it will be done. Multiple accounts belonging to the same member will be deleted without warning.

Tuning 102" Whip?

Discussion in 'CB Antennas' started by buickid, Jun 3, 2013.

  1. Robb

    Robb Yup

    Joined:
    Dec 18, 2008
    Messages:
    11,416
    Likes Received:
    3,488
    Think I would take the amp/jumper out of line and retest the SWR just for giggles . . .


     
    1 person likes this.

  2. Slim

    Slim Active Member

    Joined:
    Apr 2, 2005
    Messages:
    210
    Likes Received:
    5
    Try another 102" whip if possible. I had one several years ago that wouldn't match right & came to find out the base was bad! Where the threaded part adapted to the whip itself. Even tho it was solid as Hell & we couldn't budge it. It even checked OK with a DVM!!! Anyways,stuck another one on & all was great :cool:
     
  3. W5LZ

    W5LZ Crotchety Old Bastard

    Joined:
    Apr 8, 2005
    Messages:
    6,832
    Likes Received:
    813
    I think if you measure it you will find that your antenna is longer than 108". Whip = 102", spring = 4", and that mount appears to be at least 1.5 to 2" tall, plus the extender 2"(?). Adds up to about 110" total. The antenna starts where the coax ends...
    - 'Doc
     
  4. W5LZ

    W5LZ Crotchety Old Bastard

    Joined:
    Apr 8, 2005
    Messages:
    6,832
    Likes Received:
    813
    A bad base on a whip? They are just silver soldered on, so heating them and then letting them cool should 'fix' things.
    - 'Doc
     
  5. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
    You have to factor in the VF for a 1/4 wave 102" SS whip for 11 meters. It's 102" for a SS whip. An extra 1/2 or so inches are added as well for a standard mount.
    Here's something else I just found:
    An antenna that is electrically long is usually inductive. You can put a series capacitor to tune it out. A suitable variable would do well. At 102" the antenna should be great on 27.5MHz.
    P.S. I originally found out about SS whip VF from this guy
    http://youtu.be/bolyErI2888
    I just watched the video and saw that the SS whip including it's mounting nut is just under 102" for a Radio Shack SS whip.
     
    #20 Lil'Yeshua, Jun 4, 2013
    Last edited: Jun 4, 2013
  6. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
  7. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
    Would'nt a bad ground mess with the SWR ratio?
     
  8. W5LZ

    W5LZ Crotchety Old Bastard

    Joined:
    Apr 8, 2005
    Messages:
    6,832
    Likes Received:
    813
    I think most people are under the impression that a low SWR means an antenna is tuned to resonance. SWR has nothing to do with resonance, a low SWR does not mean an antenna is tuned. It only means that the transfer of power from the feed line to the antenna will be good, which does not mean the thing will work well at all. The efficiency of an antenna isn't measured by SWR, it's measured by the lack of reactance present in the antenna. That lack of reactance is also the definition of resonance.
    The idea is to first make that antenna resonant and then match it's input impedance to the rest of the antenna system (meaning the feed line and transmitter's output impedance).
    Depending on how/where a 1/4 wave antenna is mounted it's input impedance is usually something between about 20 ohms and 35 ohms. That means that it's SWR if used with a 50 ohm feed line will be somewhere between about 2.5:1 to 1.4:1 give or take a little. If you see a 1:1 SWR then you'd better start looking for the reason for it, it's not normal.
    There are a lot of reasons why an SWR reading isn't very informative. Using SWR as the 'measure' of a good/bad antenna just isn't very smart, sorry 'bout that.
    - 'Doc
     
    Slowmover likes this.
  9. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
    A 4 incher but has a braided wire in it
    Hustler SSM-3 spring
     
  10. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
    I agree with what you said. What is commercially available to correct the antenna's impedance?
    Plus,the 102" SS whip setup I mentioned also had a Rat Shack 6" spring on it as well as having 18' of RG8X coax(ignorant of VF in those days). My SWR was Higher on channel 1 than 40 on the cb band.
     
    #25 Lil'Yeshua, Jun 4, 2013
    Last edited: Jun 4, 2013
  11. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
    #26 Lil'Yeshua, Jun 4, 2013
    Last edited: Jun 4, 2013
  12. buickid

    buickid Active Member

    Joined:
    Nov 22, 2012
    Messages:
    194
    Likes Received:
    73
    The MFJ readings were taken without the amplifier inline, straight to the feedline.

    On another note, someone told me my feedline was too short, and that I should run an electrical half-wavelength. I did some research and this does seem accurate.
     
  13. The DB

    The DB Sr. Member

    Joined:
    Aug 14, 2011
    Messages:
    1,917
    Likes Received:
    1,305
    Oh god, not that half wavelength feed line myth again...


    The DB
     
  14. Lil'Yeshua

    Lil'Yeshua .......

    Joined:
    Nov 22, 2012
    Messages:
    4,625
    Likes Received:
    299
    :O :whistle:
     
  15. Captain Kilowatt

    Captain Kilowatt Professional Amateur
    Staff Member

    Joined:
    Apr 6, 2005
    Messages:
    16,614
    Likes Received:
    10,410


    :headbang :headbang :headbang

    Your feedline can never be too short unless it won't reach from the antenna to the radio.The ONLY time you need to use a specific length like a half wave or whatever is when dealing with phasing two or more antennas. Note it is NOT called co-phasing but that is another issue.You can use an electrical half wavelength or multiple thereof to accurately repeat the antenna's true impedance back at the shack end to allow for more accurate measuring of the actual impedance as anything other than 50 ohms will be affected by cable length. SWR at the antenna is not affected but what the SWR appears to be back at the radio end of the cable is indeed affected. You do NOT need to run any special length of cable. That is just one of the many things repeated in discussions due to simple parroting of ideas without having the background theory to understand why it is not true.

    Here is food for thought. If that half wavelength theory was true then how can you use one length of coax on a multiband ham antenna? Granted most ham bands are harmonically related so what is a half wave on one band is a full wave on another etc but then we throw 15m into the mix or 12m or 17m or even 30m and that half wave on all bands things goes straight out the window.
     
    1 person likes this.

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!