G5RV - 1:1 "coax" balun - 1:1 ferrite feed thru balun?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KV4JW, Nov 20, 2018.

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  1. KV4JW

    KV4JW Ham Member QRZ Page

    As most of you know I'm installing a G5RV in an inverted V configuration at approximately 35' AGL.

    I will be feeding the G5RV with RG-8X coax. I also have a MFJ-915 RF isolator "balun" that I plan to use at the feedpoint of the G5RV ladder line. This MFJ-915 isolator is constructed of a 13" length of RG-303 with 50 ferrite beads on it, all encased in a PVC enclosure.

    I've been reading that at high SWR levels (which the G5RV is famous for on all bands but 20M) can cause heating of the 1:1 RF isolator. Unadilla's website also advises of this with their model of the 1:1 isolator, however they also state that many hams seem to not care and do it anyway.

    I do plan to utilize the MFJ 1:1 isolator in my installation, however, I'm curious about whether it would be wise to do any/none of the following in the name of RF supression..

    1. Construct a quick 1:1 coax balun (a few wraps of excess coax around a 6" former) immediately following the MFJ 1:1 isolator? (Transceiver - coax - 1:1 coax balun - MFJ isolator - G5RV feedpoint)

    2. Simply cut off unneeded coax, feed the MFJ 1:1 isolator normally and connect the G5RV feedpoint?

    3. Construct a quick 1:1 coax balun at the G5RV feedpoint, and place the MFJ 1:1 isolator INSIDE at the connection to the transceiver? (Tranceiver - MFJ isolator - coax - 1:1 coax balun - G5RV feedpoint)

    4. Simply cut off uneeded coax, feed the G5RV directly to the feedpoint, and install the MFJ isolator inside at the connection to the transceiver? (Transceiver - MFJ isolator - coax - G5RV feedpoint)

    I'm sure there's lots of reactance/inductance variables at play here. I just want to do what's best for performance and RFI common-mode supression. Also, let's please not get into a politically correct discussion about what a balun really is vs a choke. Tomato, Potato, whatever. You guys know what I'm talking about here. :)
     
    Last edited: Nov 20, 2018
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    High power and especially high power coupled with high common mode currents from an unbalanced antenna can easily lead to choke balun heating but high SWR isn't typically the root of the trouble. The G5RV when installed away from other objects is nominally a well balanced antenna though any antenna can have some imbalance.

    If you're worried about balun heating how about building a big multi-core 1:1 choke balun that sits at the junction of the feed coax and the G5RV matching twin lead and then use your line isolator where the coax enters the shack?

    Some good info on sizing baluns for higher power operation in this guide: http://audiosystemsgroup.com/RFI-Ham.pdf
     
    AA7QQ and KV4JW like this.
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    The SWR caused by the antenna having other than 50 + j0 looking into the twin-lead does not cause the common-mode current on the coax to increase. The common-mode current is due to asymmetry in the antenna legs (primarily due to capacitance to surrounding objects), and knows nothing about SWR.

    If the isolator (better name is Common-Mode-Choke) is worthy of its name, then it will have a choking impedance of ~1000 + j5000, in which case it will dissipate a few mW of power, and not even get warm. Unadilla is trying to sell their stuff...

    Ugly baluns (coils of coax) work at best only on one band. The best advice about how to deploy a CMC is here ( post #20).

    A G5RV is a really crappy antenna. Why bother with it? Build a ZS6BKW instead.
     
    Last edited: Nov 20, 2018
    KD6RF likes this.
  4. KV4JW

    KV4JW Ham Member QRZ Page

    I probably should have mentioned my power.. Less than 100W. I have no plans to ever use an amplifier.
     
  5. KV4JW

    KV4JW Ham Member QRZ Page

    Because it was free. /thread
     
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    You'll be fine with the string of beads balun at the coax to matching section junction.

    FWIW, I wouldn't say the G5RV is terrible, but the ZS6BKW is basically just a small tweak on the same idea and typically presents a better match on more bands than the G5RV. All you do to change one into the other is alter the element and twin lead matching section lengths a bit.

    http://www.nc4fb.org/wordpress/zs6bkw-multi-band-antenna/
     
    KV4JW likes this.
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    At 100W, you will not overheat the ferrite cores in that isolator.

    They have to surpass the Curie point to be damaged, and 100W can't do that.

    Just use it.
     
    KV4JW likes this.

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