Halfsquare antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by G5TM, Mar 21, 2019.

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  1. G5TM

    G5TM Premium Subscriber QRZ Page

    A great idea from G3TXQ supported the notion that instead of feeding a half square in the top corner that you could instead use coax as one of the quarter wave vertical legs and place a good choke at the bottom.

    My question is how to attach the coax to the horizontal wire? Is it via the centre conductor only? Is the braid removed at the feedpoint? Is it the opposite?

    Or could the whole antenna be made out of coax? If that's possible does part of the outer sleeve or braid get removed?

    The thread I alluded to is here:
    https://forums.qrz.com/index.php?threads/quick-question-on-feeding-a-half-square.259225/page-2
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I just built a model. Here are the wires:

    w.png

    Wire 1 is the coax side, going from just above dirt to 45ft agl. The choke is ~10ft up from the bottom.

    Wire 2 is the horizontal wire, 70.7ft long. The feedpoint is near Wire 1. The coax center conductor is connected to the horiz wire. The shield is open. See the red circle for placement of the feedpoint.

    Wire 3 is the down wire, 35ft long, ending 10ft agl.

    Wire 4 is the rest of the coax toward the shack.

    The choke is a good one: Z=5000 + j5000. See the red box for placement on the coax.

    The structure resonates at 7.15MHz, where Z=74 + j0

    It would be best fed with 75 Ohm coax (RG6?)

    The pattern is a surprise to me. It radiates best symmetrically broadside to the plane of the wires. I guessed that it would be an end-fire array like a yagi.

    p1.png

    Here is the azimuth pattern @ 18 deg above the horizion. The antenna wires lie along the Y-axis.

    pa1.png

    Here is the elevation pattern. Best gain at 18 degrees.
    Pe1.png
     
    Last edited: Mar 21, 2019
    WB2UAQ likes this.
  3. G5TM

    G5TM Premium Subscriber QRZ Page

    Thank you it looks a very useful antenna for DX
     
  4. W1VT

    W1VT Ham Member QRZ Page

    I used one this morning to work a FK8, VK3, VK4, and VK7 on 40 meters. I prefer to hang them off trees and make sure then ends aren't close to anything. I just feed my with ooax to one of the upper corners. It's not pretty but it works.

    Coincidentally, the distances to Australia from Hartford and London are quite similar.
     
    Last edited: Mar 21, 2019
  5. WR2E

    WR2E Ham Member QRZ Page

    @WA7ARK
    That's cool!
    What happens if you move wire 2 from the top to the bottom and fed it at bottom corner?
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Not much. The resonant point moves down about 100kHz so the lengths would have to be re-optimized. Best elevation moves from 18 to 22 degrees. Ground loss increases slightly because the horizontal wire is closer to dirt.
     
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  7. K2CAJ

    K2CAJ XML Subscriber QRZ Page

    It's broadside because you have a full wave, so the currents of the two half-waves are moving opposite on the horizontal wire and in the same direction on the vertical wires. Essentially it's two verticals in phase, a half wavelength apart.
     
  8. AJ6O

    AJ6O Ham Member QRZ Page

    I have an end fed 80-10 meter wire antenna that is 130' long. Perfect for having two 33' verticals, and a 64' wire at the top between the two. The best part is its fed at the end which would make it very easy to set up. Right now it is run as a flat top at 23'.
     
    Last edited: Mar 22, 2019
    WB5YUZ likes this.
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Just some observations on this antenna. Years ago (>20), I experimented with a scaled down version of the half-square and found that the position of the coax feeding at the corner was not critical at all and no balun was really needed. Also I found that the horizontal wire was best to be close to a free space 1/2 wave. I did the testing at 200 MHz and taped the whole thing flat on a piece of card board. This discussion made me think about using the EFHW high Z ratio transformers (based on the FT240-43 core) to feed it at the bottom of one of the vertical radiators (maybe some one touched on this above?). The drawback is these transformers can have significant loss so maybe the tuned circuit version would be the best method....its a one band antenna anyway.
     
  10. G5TM

    G5TM Premium Subscriber QRZ Page

    Yes I have just such an EFHW coupler at my QTH - might be worth a try. Also … and this is probably just a whacky idea, but if you used twin speaker wire for the antenna and then only split the wires at the very bottom of one of the vertical legs, could you then connect this straight to a GOOD BALANCED tuner and create a multiband antenna I wonder? So for example a 67 foot version might be used on 20-10?
     

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