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Feedline chokes for mobile operation at 10m, 6m, 2m, 70cm

Discussion in 'Mobile Radio Systems' started by N0IOP, Oct 17, 2019.

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  1. N0IOP

    N0IOP Ham Member QRZ Page

    I'm drilling a hole in the (steel) roof of my car for an NMO mount. NMO mount came with RG8x which I have fished through the headliner and down the rear pillar into the trunk. I have a stack of antennas to experiment with and am ordering a transceiver.

    70cm - 1/4 wave
    2m - 1/4 wave
    6m - base loaded 48" whip (simple series loading coil)
    10m - base loaded 48" whip (loading coil grounded at one end and fed somewhere in the middle)

    I will run separate radios for 70 cm, 2m, and HF. 50-100 watts depending on band.

    I am thinking that I should insert a current balun in the feedline to keep the RF out of the car.

    I have some questions:
    * Is this a good idea? In other words, will it make for a more reliable, trouble-free installation?
    * Will I need separate baluns for each band?
    * What balun designs make sense?
    * Are there any balun designs that can utilize the RG8x or will I have to switch to a more flexible, more heat resistant coax for the balun?
  2. W5LZ

    W5LZ Ham Member QRZ Page

    A balun is not the same thing as a choke!
    A choke seldom ever hurts anything.
    I've never used a balun on any mobile antenna. A choke, yes. Balun, no.
    I'm not aware of any balanced mobile antennas so a balun just isn't needed. An 'unun' maybe, but not really.
    Make that choke for the lowest band of use. It'll 'do' for the other bands.
    A roof mount is fine for VHF/UHF, not too practical for the lower bands. A choke ought to be as close to the antenna feed point as possible and -could- be done above the head liner, but it will show, maybe?
    There is no such thing as a totally 'reliable' RF free way to do all this. Count on there being -some- RF, probably nothing that's 'un-livable' though. If RF is objectionable then you've got something wrong. Check everything.
    Have fun.
    N0IOP likes this.
  3. K7JEM

    K7JEM Ham Member QRZ Page

    I wouldn't worry about it at all, isn't really needed for that type of install.
    N8ZL and N0IOP like this.
  4. WA4SIX

    WA4SIX Ham Member QRZ Page

    A coaxial current choke is the exact same thing as a balun, except for the connectors.

    KU3X and N0IOP like this.
  5. K0BG

    K0BG Ham Member QRZ Page

    Well, it sort of kinda is. If...

    Baluns come is all favors, but in order to act as a choke (offer common mode rejection), it must be a current balun, not a voltage balun. Without exception, mobile installations will always benefit from a proper common mode choke on all of the control and feed lines.
    K0UO and WA8FOZ like this.
  6. W5LZ

    W5LZ Ham Member QRZ Page

    I'm probably missing something in terminology, so exactly how would you apply other connectors to make a balun, or a coaxial current choke out of a common coaxial choke?
  7. K1LKP

    K1LKP Platinum Subscriber Platinum Subscriber QRZ Page

  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    You can certainly use one, no harm done, but nothing likely to be gained either. Just how would you get it to the antenna base anyway? Too much trouble for nothing gained.

    Back in the 80's when electronic ignition and engine control modules first first showed up, A unun was required for low band frequencies especially of Fricking Old Rebuilt Dodge's as the TX frequency interfered with the ECM Clock signal and the engine would shut off or stall when you keyed the mic. An unun would solve the problem most of the time. Law Enforcement and Utilities reamed the auto manufactures and made them clean up the interference problems with both RX and TX.

    So unless your vehicle is experiencing weird problems when you key the mic, adding a unun choke would just be a waste of time and money.
    KU3X likes this.
  9. K0BG

    K0BG Ham Member QRZ Page

    Common Mode Choke?? Read my web page!
  10. KF5LJW

    KF5LJW Ham Member QRZ Page

    Shilling for traffic again I see. There you can learn how to burn up your radio power wiring and generate so much noise the RX is useless with the engine running. I understand why you want folks to go to your blog because there you cannot be challenged and exposed.Around here you get you butt kicked and exposed.

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