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Is a Choke Balun Needed Under an AH-4 Antenna Tuner for Inverted "L" Antenna?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA3MOJ, Oct 12, 2018.

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  1. WA3MOJ

    WA3MOJ Premium Subscriber QRZ Page

    .
    I will be setting up a 2nd floor station with an Icom 7600 radio. The coax will be running from the 2nd floor down to ground level and connected to a ground mounted Icom AH-4 antenna tuner.

    The radiating element for the antenna portion will be configured as a 43' inverted "L" using Wire Man #26 Silky (for stealth and near invisibility). There will be 10 (#12) buried ground radials below the AH-4 aprox. 20' - 25' in length, and 2- 8' ground rods spaced 8' apart with the AH-4 tuner in the center.

    I've successfully used this AH-4 and 43' inverted "L" configuration in a 1st floor shack and a basement shack with good results. Attached are sample images of work in progress of other similar installations. These AH-4 antenna tuner installations are now complete and performing well.

    I read conflicting information whether a 1:1 Choke Balun is needed to prevent common mode from traveling up the coax and back into the shack. More specifically, is this helpful in my particular installation using the Icom AH-4 remote tuner.

    So my thoughts are to do one of the following:
    1. Wind an Ugly Balun with ~ 20' of coax on a 4" PVC form, and connect it directly under the AH-4 tuner at the antenna feedpoint, or
    2. Purchase a commercially made 1:1 Current Balun, such as Balun Designs #1115u, and install it under the AH-4 tuner, or
    3. Maybe use a few turns of coax around a 2-1/4" FT-240-31 Large Ferrite Toroid. I have several of these lying around, or
    4. Do nothing and forget the balun idea.
    Thoughts and suggestions would be appreciated.

    Thanks in advance.

    George
     

    Attached Files:

    Last edited: Oct 12, 2018
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    My take: installing a CMC costs relatively little $, costs little loss of RF power, and potentially prevents RFI emanating from electronic devices in your own house from travelling up your coax, and coupling inside the coax to get into your receiver, even though it may not do anything for your transmitted signal.

    You want a wide-band CMC; an ugly wound-coax balun is not wide band (enough). Best placement maybe right outside your house (entry panel) or near the tuner.
     
  3. AF7ON

    AF7ON Ham Member QRZ Page

    My 53' long wire antenna is not too different from yours and fed via an AH-4 that is about as well grounded. With an 80-foot coax run and no balun, I have not experienced any effects from common-mode currents.

    Having said that, with any inherently unbalanced antenna, a choke is never a bad idea!

    Mike
    AF7ON
     
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I'd use a ferrite "balun," which in this case is really a unun -- there's nothing balanced anywhere.
     
  5. WA3MOJ

    WA3MOJ Premium Subscriber QRZ Page

    Steve, do you mean winding coax around a ferrite Toroid?
     
  6. N4MTB

    N4MTB Premium Subscriber QRZ Page

    I have used an SGC-235 coupler at 500 watts without any ferrites or unun on either the feed or control/power lines. The coupler was about 75 to 100 feet from the OP.
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd pick one of these options. If you have the 240-31 cores lying around there's no real harm in winding some of your coax through one as a common mode choke (not really acting as a balun in this application but still handy to choke any residual common mode from your coax shield). If you didn't have cores lying around I'd probably just run the coax to the remote tuner and only go out and buy a core or two IF you experienced some RFI issues in the shack or thought your coax was coupling local noise sources (e.g. DSL data line noise) into your received signal. But since you already have the cores....

    BTW, burying the coax run to the remote tuner also creates a pretty effective common mode choke if your installation supports that approach and you use appropriate direct burial coax.
     
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Sure. Or buy a commercially made one that's weatherproof and has connectors on it, which might be easier but should work about the same.
     
  9. KK5JY

    KK5JY Ham Member QRZ Page

    All amateur HF antennas should be choked near the antenna. People tell all kinds of (usually true) stories about getting by without one, but most amateur installations are far from perfect, and done on the cheap with materials and designs that are far from professional-grade (that's why it's called amateur radio). The choke is an easy ounce of prevention for all kinds of potential problems, some of which are subtle, such as feedline pickup of signals during reception.

    Any of these are fine. If you pick the first one, use more than 20' of coaxial cable. A solenoid choke made with 20' of cable will be good for 20m and shorter wavelengths, but it will be inadequate for most antenna designs on 40m or 80m, and practically invisible on 160m.

    If you use a 4" PVC form, make sure you pick coaxial cable that can handle the turn radius. Any quality cable will have a data sheet that describes the minimum bend radius for permanent installation.

    If you wind a choke using a large ferrite toroid, be even more careful to observe the bend radius limitations of the coaxial cable.

    Enjoy your antenna project. :)
     
    WB5YUZ likes this.
  10. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    This.

    It is perhaps better to think in terms of number and diameter of turns rather than overall length of the coax. Then one can use the below handy table, one of the things for which we will be forever indebted to Steve, G3TXQ:[​IMG]

    As you can see, it is very difficult to get adequate choking impedance out of an air-cored inductor on wavelengths longer than 30m; few experienced hams ever try.
     
    KK5JY likes this.

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