Homemade Antenna Help

Discussion in 'CB Radio Forum' started by Fudz073, Apr 26, 2012.

  1. Fudz073

    Fudz073 Bobtail Member

    18
    2
    Apr 4, 2012
    Pennsylvania
    0
    I've decided to try my hand at building my own whip antenna. I realize they're not really expensive to buy, but this is mainly for my own enjoyment of building a working antenna for my new Mirage MX-36hp (modified to operate on the 11m band CB channels. Needless to say, I've ran into a little problem. So lets start with some pics

    [​IMG][/url] Picture0425122234_1 by Fudz0732011, on Flickr[/IMG]

    [​IMG][/url] Picture0425122237_1 by Fudz0732011, on Flickr[/IMG]

    The first pic is the antenna stud. It's a machine threaded 3/8" 24 bolt shank brazed into an extended hex nut (total length of about 4"). The actual whip structure is three 4' fiberglass rods epoxied together with JB Weld and PVC sleeves. The antenna is 12 guage copper wire brazed into the extended hex nut. I have the copper wire temporarily attached to the fiberglass via electrical tape about every eight inches. Overall, the antenna is currently 108" long (not including the 4" stud)

    So here's the problem: When I test the SWR I'm getting a 3:1. On a similar rig using 8 guage steel wire, I was getting 1.5:1. It is my understanding that copper, being more conductive than steel, should work at least the same as the steel. I shortened the rig already but it had no effect on the SWR readings. Having no actual experience in building antennas (i've studied a few designs on the internet...) I ask that what is wrong with my rig and how can/if I modify it work properly with my antenna? Here is a short list of what I think may be the problem (for what it's worth)

    1. Wire to thin/thick
    2. Electrical tape may be interfering with broadcast pattern

    As an interesting side note, the SWR meter on the radio reads a 1.1:1 with the antenna mounted. I doubled checked my Radio Shack meter to make sure it still worked and got good readings with my Firestik, so I don't think the meter is at fault.

    If any one has any suggestions, please drop a line. Thanks for the help!
     
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  3. ghz24

    ghz24 Bobtail Member

    43
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    Mar 25, 2012
    peoria, IL
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    IMHO wire thickness is not the problem (and the tape is insignificant RF wise) try cutting about 3 inches off ( or more even but a little at a time) 12 gauge dipoles I've modeled get best SWR (~1.4 ) at about 104-105 inches for each pole/element. Increasing the models poles to 112 gives a swr of 3.4

    You have 2 more from the romax you stripped anyway right so clip away.
     
  4. Xcis

    Xcis Medium Load Member

    492
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    Jul 9, 2008
    Bridgeport, Pa
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    So "When I test the SWR I'm getting a 3:1. On a similar rig using 8 guage steel wire, I was getting 1.5:1."
    .
    By testing on a similar rig do you mean that was a different vehicle? If it was tested on a different vehicle, that SWR reading of 3 would indicate that to much of your signal is reflected back to your radio in the current vehicle. So what can causes your signal to be reflected back to the radio?
    .
    Basic troubleshooting of the excessive SWR: Let us do the easy things first.
    .1. check that the antenna is not located close to any structure or especially any metal that could reflect the signal.
    .2. Check the SWR with an external SWR meter with the vehicle doors closed.
    .3. substitute a known good antenna and recheck your SWR reading with an external SWR meter.
    .4. substitute a length of known good coax cable from your antenna mount to the CB radio and recheck your SWR reading with an external SWR meter.
    .5. With a multimeter check for no resistance between the
    .....A antenna mount and the door
    .....B the bolts that mount the door to the door hinge and the bolts that mount the hinge to the cab.
    .....C the bolts that mount the hinge to the cab and the chassis frame.
    .
    .
    Something is not right with the CB installation in the current vehicle.
     
  5. ghz24

    ghz24 Bobtail Member

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    Mar 25, 2012
    peoria, IL
    0
    By all means double check the system as Xcis suggests. (it wont hurt, but I think the element is just to long)

    How much of the 4" stub sticks above the nearest gounded/shielded surface?
    That is what is the total height once the center conductor leaves the shield?

    I'm assuming you have a plastic piece to isolate the whip from the shield/ground(plane).

    It sounds like your element is 112" long (108 + 4) where best SWR is usually around 104" total length (for 12 gauge wire dipoles and ground planes)
     
  6. Fudz073

    Fudz073 Bobtail Member

    18
    2
    Apr 4, 2012
    Pennsylvania
    0
    By referring to a "different rig" i was talking about an antenna I built from stainless steel wire (It wasn't pretty or sturdy enough for road use which is why I'm building a better one). The current mount on my vehicle (Jeep) is properly grounded and I get fantastic SWR results (1.0:1) with my firestik antenna.

    As for the stud, It screws easily into the existing door jam mount I use for my current antenna. It is properly shielded from the ground plate. If anything is wrong in this system, it is the antenna. I think, as ghz24 mentioned, that the antenna is too long and needs to be shortened.

    I appreciate the help guys and hope to post some results soon!
     
  7. Happy_Hamer

    Happy_Hamer Light Load Member

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    Aug 28, 2011
    Illinois
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    Last edited: Apr 29, 2012
  8. Xcis

    Xcis Medium Load Member

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    Jul 9, 2008
    Bridgeport, Pa
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    I apologize for my earlier post. I was under the impression that you had switched from one vehicle to another. I don't know where I got that obviously incorrect idea.
    .
    If you don't mind, I will not delete my earlier post. I hope that post would eventually be useful to someone else at a later date. Good luck with that homemade antenna.
     
  9. Fudz073

    Fudz073 Bobtail Member

    18
    2
    Apr 4, 2012
    Pennsylvania
    0
    Hey, don't worry about it! I misread posts all the time. Regardless, ANY information about antennas is helpful. I'm a starting amateur hobbyist and any input is huge help.
     
    Last edited: Apr 29, 2012
  10. WA4GCH

    WA4GCH Road Train Member

    3,324
    577
    Aug 12, 2009
    Seminole Florida
    0

    You can buy dipole parts to make hamsticks into dipoles ...Lakeview makes a mount I used them for a groundplane on this 12avq vertical.
     

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  11. Fudz073

    Fudz073 Bobtail Member

    18
    2
    Apr 4, 2012
    Pennsylvania
    0
    Once again, thanks everyone for the help with my build. I cut the antenna to size (104") and I managed to get an SWR of 1.4, Just as ghz24 predicted. So, it works!...

    However, the actual structure holding the wire up is not as sound as I would like. Since the pole is three fiberglass poles jacketed together with PVC sleeves, i will have to rethink the build a little to provide a lighter, more durable structure. Happy_Hamer and WA4GHC have provided options for the poles which I will research further. So, I have a few questions for the next build

    1. What's the best pole for a mobile mount (one that is less likely to break while I'm cruising down the highway)?

    2. Is there a way I can further reduce my SWR?

    3. Is there a website that caters to building homemade MOBILE antennas?

    Again, I thank everyone for their help and I hope to post some more results soon!
     
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