160m from a WS6BKW or new antenna?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W0AZZ, Jan 1, 2019.

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  1. W0AZZ

    W0AZZ Ham Member QRZ Page

    I have a WS6BKW version of the G5RV hanging in a tall tree in an inverted V. I cannot tune 160m on it with an MFJ 941E external tuner. I have 50' of RG-8 running to it but the SWR is thru the roof on 160 (and 6 meters for that matter).

    This isn't my daily use antenna so I thought about modifying to determine if I can actually bring down the SWR on 160m. I have read that adding an ugly balun helps with SWR on these antennas. Also, turning the antenna into a doublet by eliminating the coax and running straight ladder line to the tuner may help but are either of these methods enough to lower the SWR on 90 something feet of wire? I have a small, city lot so I don't have many options to run long lengths of wire.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I assume you're talking about a ZS6BKW at roughly 93.8 feet end to end length for the radiating elements. If so, it is much too short to load as a dipole on 160m. To run this as a center fed antenna you'd need to roughly double the length of the elements but then it would no longer be a ZS6BKW nor provide a reasonable match on the bands normally supported by the ZS6BKW.

    You could use an old trick and run the antenna on 160m as a T-top vertical. You basically tie the coax shield to center pin at the shack end of the coax line and connect that to the longwire terminal of your antenna tuner. Then run some kind of ground radials/counterpoise on the tuner's ground lug. You're basically turning a center fed antenna into a top loaded vertical where the coax and ZS6BKW section of matching line is your vertical and the normal dipole sections provide capacitive top loading to allow you to tune it on 160m. If you take a look at the MFJ-941E's manual it will basically advise this when trying to tune an antenna much less than half wavelength long on a given band.

    From page 4 of the MFJ-941E manual:
     
    KA0HCP likes this.
  3. AA7QQ

    AA7QQ Ham Member QRZ Page

    As above, but with a relay at the base to short the feedline up to the T. Relay also adds in base loading coil.
    So, where the feedline comes to ground level, you have:
    Rig coax
    Ant coax
    Radials
    Coil
    Relay (s)

    During normal use, Rig coax center & braid go to Ant coax center & braid.
    During 160 use, relay changes Rig coax center to Ant coax center & braid through loading coil. Rig coax braid to radials.
    In this way, you can have it switchable from normal use to 160 base fed T.
    You may find that the loading coil might need to be a tapped fed arrangement. Eg. Bottom of coil to ground radials, 5 turns up, tapped for feed & the rest of the loading coil feeding the T.

    Ed
     
  4. W0AZZ

    W0AZZ Ham Member QRZ Page

    Yes, it's the ZS6BKW. I found an QST article where K0BXB made a box with a switch that turns the G5RV into a T-top a you describe : https://www.huyettm.net/ham-radio---k0bxb.html

    I have most everything to make the switch but I'll have to figure out a counterpoise. I have my doubts this will work for DX but may get me out a few hundred miles.
     
  5. WA7ARK

    WA7ARK Ham Member QRZ Page

    It is all about the counterpoise. Sad reality about vertical antennas (especially short ones), it takes about 10,000ft of wire to make an effective 160m antenna. 5% of that wire is above ground, 95% of the wire is in/on/just-above ground!
     
    AF4RK likes this.
  6. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It may actually work ok for DX. The T-top is basically a top loaded vertical, the primary radiating element is your feed line so the longer and higher the feed line goes the taller the effective vertical. The original doublet elements are really capacitive top loading to the 'vertical' portion of the antenna. So if you've got a reasonably tall vertical run of feed line you may have a decent DX antenna with the low takeoff angle benefits of a vertical. The typical issue is that it's a very short top loaded vertical unless the feed line runs really high and like any shortened vertical there are performance tradeoffs.
     
    WB5YUZ likes this.
  7. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    To elaborate: with a typical "T" antenna, almost all the radiation from the flat top part is cancelled out. This amounts to loss. Only the vertical portion radiates effectively. So, if the horizontal part is twice as long as the vertical section, that represents a %66 percent loss!

    Even so, some order of compromise is almost always necessary on 160m, and you can make plenty of contacts on 160m with 30W or so ERP.

    The advantage of this situation, at least for DX, is that radiation from the horizontal section in most typical installations would be very high angle; on receive, this translates to a great deal of regional interference when trying to dig the DX out of the noise. With the "T", most of the high-angle received radiation is cancelled out, also! This is not true of the typical inverted "L." This is why the "T" is often preferred as a purely DX antenna, while those who want both DX and regional contacts prefer the "L".
     
    Last edited: Jan 16, 2019
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Yes!
    No!. The capacitance of the two T-wires serves to increase the current in the vertical section. That increase in current creates a stronger vertical radiation field than would otherwise exist without the T-wires being there, and it moves the peak current in the vertical section higher than it would otherwise be. The presence of the T-wires makes the antenna a more efficient radiator, so is never a "loss".
     
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  9. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Okay, Mike. I stand corrected. Thanks for the clarification.
     
  10. N8CMQ

    N8CMQ Ham Member QRZ Page

    Thank you for correcting that.

    Otherwise every cap hat would be a lossy part and not a benefit of use.

    I have a 123 foot length of wire sloping at 45 degrees into a nearby tree attached to my 4btv working against the 144+ radials and it works very well on 160M. I use a tuner with it, but the antenna SWR is low enough at 1.9 MHz and up to where I do not need a tuner there, only below 1.9MHz.

    Now if I could only get WY confirmed I would have WAS...
     
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