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Mobile Antenna SWR issues with different lengths antennas

Discussion in 'CB Antennas' started by turborich, Mar 26, 2020.

  1. turborich

    turborich Member

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    Hello folks,

    I've been into CB radio for years, I have a base and 3 mobile set ups. On two of the mobile rigs I have roof mounted antennas and don't have any issues.

    So here's the issue. I have an old jeep wrangler yj. I installed a Uniden 980 USB along with a little RM Italy KL 203 amp. Lowered the dead key on the radio, opened up modulation to 100% and it's a great low power set up. I have the standard 18 ft length of coax, using RG-58 since we're only talking about 175 watts pep. I bought the standard mount for behind the taillight, I have made sure the mount is grounded to the body of the jeep. Running the coax to the antenna without any coils, all has been checked, no shorts, good continuity, etc.



    When using a 4 foot firestick antenna with tunable tip I have no problem achieving a 1.08 swr at 52 ohms. However when I try using a 5 or 6 foot skip shooter antenna with tunable tip I can't get the SWR below 2.8 at 130 ohms. My buddy brought over his rig expert analyzer over and we checked it. This is at the end of the coax hooked into the analyzer. Radio and amp out of the picture.

    He's a long time CB'er and said certain antennas work better with certain vehicles. Now I understand that having the antenna mounted above the taillight isn't ideal and the roof is always the best location, however my roof is fiberglass so it's not an option. What would cause this and is there an easy fix? Basically a 4 foot antenna works great and the numbers are right, anything longer and the numbers jump up so I don't want to risk it. I did ask for a radio check from 4 miles away with the 4 foot antenna and he reported back that my signal was a 9 over 13. I tried the 6 foot skip shooter with the high SWR's and he reported my signal increased to a 10 over 19. Even so, I'm leery to run it until I can get the numbers lower.

    Any ideas? I figured I'd ask before I start experimenting with coax length, etc.

    Thanks! 20200325_111851.jpg
     

  2. Slowmover

    Slowmover Sr. Member

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    (A KL203 does about 70W.)

    Take the amp out of line.

    Change the coax to a shorter length as part of testing.

    Use a 50-ohm dummy load

    .
     
    walterjn and midnight special like this.
  3. M0GVZ

    M0GVZ Sr. Member

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    When you used the antenna analyser did you sweep outside the CB frequencies to see where it was actually tuned to?

    Think I've seen the problem when you zoom in on the photo. That spring isn't an original part of the antenna is it as it looks like the antenna is screwed into the top of it? If that's the case then there's no way in hell there's enough adjustment in the antenna to compensate for several inches of extra electrical length the spring will add. Remove the spring, connect the antenna directly to the mount and I bet it'll tune.

    Springs should never be used with fibreglass antennas unless they come with them as standard and it's only usually because of a poor RF ground or it's a real lossy antenna that any of them do actually SWR in with one in place.
     
  4. 2NC995

    2NC995 DAN

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    My 7’ skipshooter tunes well below the CB band, for some reason.
     
  5. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Welcome!

    Unfortunately, you've only done part of the whole picture...

    You'll need to look at the antenna thru the Analyzer to even see resonance...we can only guess...

    But here's a STRONG CLUE from someone else...and I tend to side with them on this issue with "shock springs" added into a NON-Tunable - read this as, you have to trim it, antenna...

    Did not know if you're aware, but if you've used that spring with the Firestik and have had no issues -- the best advice is to remember that you are using an extension on the lower end of the antenna - at the base, which will changes the length of the antenna to appear LONGER than it needs to be.

    • Skipshooter antennas are of a different load design than a Firestik. They are more like the older Francis Hot Rod and Rooster variety which are closer to physical resonance but their load winds are different internally - not as tight as the Firestiks'.
    • -That may include designing a brace, mount or lift, to raise the antenna
      • So you can better provide for using an antenna that has a difference radiative performance loading design that doesn't equate to equal radiative performance when it comes to reflective objects or any odd-angle mounts that don't use a flat surface - your drop off adds this problem with feed point impedance - not just length to make it resonate.
    • You may need to fashion a mount that brings the antenna HIGHER as well as CENTERED to provide a better "Takeoff" angle or impedance at the feed point. Your Rear Tire mount can provide a better mounting than a side panel.
      • (I know, you're committed - you've drilled the Holes, But that doesn't mean that you can't fashion a roof rear rack support rail (U-shaped) as part of a crash bar system - and mount the antenna on it)
    • You don't need to "put it on top" but if you had a roof rack or light bar, they (antennas) do a lot better up there than behind and below the roof on a steel panel that angles off straight down.
    • I have fashioned mounts for both SUV and Pickups similar to light bars just to offer a way to raise the mount and the antenna away from the lower reflections the more reactive antennas see when you use the tailgates or even the doors You don't need feet of height, just get to a position you and the antenna can live with.
    • - the panels aren't the fault, it's both the EXPECTED (By the Antenna) and your PROVIDED (By Location ON The Vehicle) are not the best.
    • Remember the antenna and the vehicle have to work together - else you won't be getting out anything - and when you're "out there" you'll need all the help you can get.
    The other side to the spring though, are you keeping an eye on the Springs own INTERNAL connection?

    The Spring for an antenna would otherwise be too inductive, to fix that they use a means to feed signal thru the INSDIE of the spring to make the connection more of a pass-thru.
    • So you need to "Bend over" the spring to look inside and inspect the braid. Yes, they use Braided strap to make a low-inductance path thru the coil so the spring does not add extra inductive length to your setup.
    • It is not uncommon to see SWR issues after only a few days of use - the spring itself survives the whacks from low tree limbs, but the Braid can come loose - offering you a very different and frustrating problem of intermittent SWR problems the whole system exhibits.
    I've known of antennas that use ferrules that don't go all the way down into the body of the mounting - this is so they can cheapen the ferrule and reduce the mass of metal at the mount - which would be fine if you had a typical mount that connected to the antenna. However, springs are not as they seem, some are "Soldrered" to the WASHER that acts as the tensioner and feed thru from the antenna into the mount and same for the base of the spring - others are crimped.. The washer/Braid combo may need to be investigated and see if the Ferrule needs to make contact with that "washer to braid to strap combo" - if the Ferrule of the Firestik is of a different Thread-depth length - you may need to use a spacer washer to change how the ferrule bottoms out at the coil, or try another spring.
     
  6. turborich

    turborich Member

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    The amp was never inline, I mentioned that in my original post. A KL-203 will do 150+ watts pep all day long.
     
  7. turborich

    turborich Member

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    Thanks for your suggestions. I removed the spring and my SWR'S dropped to the 1.1 range. It's difficult for me to understand how the spring didn't effect the 4 foot antenna, but threw the 6 foot antenna way off? It definitely is well within specs now!
     
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  8. turborich

    turborich Member

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    Thank you for your suggestions and advice. I do appreciate it! It's working great now without the spring.
     
  9. turborich

    turborich Member

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    So one more thing to mention. I had that spring on there which measures 3" and I also had an antenna quick disconnect as well which measured 2.5". So 5.5 inches total of extra length. I sure do like the quick disconnect, the SWR's are livable with the quick disconnect, but even lower and nearly perfect without it.

    It's a matter of convenience when I pull the Jeep into my garage.

    Once again, I appreciate everyone's help. I really need to buy an antenna analyzer myself. Has anyone used the sub $100 analyzers from Ebay/Amazon?
     
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  10. M0GVZ

    M0GVZ Sr. Member

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    Antennas have three main characteristics, efficency, bandwidth and size. You can have any two of them and the third is what you'll lose out on.

    A small antenna that covers a wide range of frequencies you have to have a lossy, low efficiency antenna. The ultimate example of this is a dummy load. Because it was lossy it was able to cover a very wide bandwidth below 2:1, sufficient enough that adding the length of the spring still meant it was low. For that antenna to still be 4ft and be very efficient it would only be able to have a bandwidth of maybe 2 channels at the most below 2:1 SWR. Magnet loop antennas are very small compared to a full size dipole but have almost the same performance however if you were using them with a CB the bandwidth is so narrow (typically a few kHz), that you'd need to retune them every time you changed channel.
     
  11. 2NC995

    2NC995 DAN

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    I have a NanoVNA. Love it.
     
  12. turborich

    turborich Member

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    Alrighty, so I picked up a used MFJ 259b for a good deal and went checking my other 2 mobiles and my base. I know for a fact when I installed the Wilson 1000 through roof antenna that my SWR read 1.1 on my SWR meter. I had no reason to suspect that anything had changed. After hooking up the MFJ I was shocked to see an SWR of 3.5 and the "R" read 150. After seeing this I unscrewed the antenna inspected it and tried again with the same results. Then I remembered that when it rained about 6 months ago the plastic housing had somehow filled up with water. I used compressed to blow the housing out, sprayed electrical cleaner in it and repeated. I thought the problem had been resolved. I'm now thinking perhaps the water caused corrosion inside? I checked the continuity with my multimeter directly at the bottom of the plastic housing of the antenna where the coil is and it showed full continuity between the center conductor and the threaded outer ring. Not sure if it.should or not, but I'm thinking no.

    I then swapped it out for another Wilson 5000 load and whip I had in the shed. SWR's read 1.1 and the "X" read 0, but the "R" read 70. This is with using the supplied coax with mount supplied by Wilson. Is this acceptable? Thank you all!
     

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  13. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Only thing we need to see is the vehicle and the antenna placement.

    Because by your efforts, you helped a lot of people in demonstrating a known issue of loading coils, placements and the feed point and how it changes with location and even the COAX in lengths - your R value - SWR? PERFECT by the way...
    • (I mean the law of diminishing returns applies here - you can't get much more out if it than what you've done already.)
    Sorry to hear about that Wilson load and water entry - because the 'Oxides of March - keep marching on...

    And...

    Thank you - for what you've done - let's many others whom need this information, to know they can sleep better at night.

    ...It's a long story...
    .
     
    Slowmover and turborich like this.

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