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parallel circuit for voltage feeding a half-square 20m band endfed

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by DD5XX, Jul 11, 2019 at 12:48 PM.

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  1. DD5XX

    DD5XX XML Subscriber QRZ Page

    Hi all,

    for my holiday QTH I am planning to install a monoband 20m half-square ANT because I had good experience in the past. An end-fed variation is more convenient because I had many mechanical/stabilisation problems feeding my half-square last year at the top corner with coax.

    For that reason I'd like to try a voltage feeding this time and I have no clue what impedance I have to expect. I guess something in the range of 1000-4000 Ohms ? I thought of using a parallel circuit for the matching and use some radials. In my understanding I will not need as much ground as this is a voltage-fed variant and the current is very low.

    How many pF should my variable capacitor have and what about the fixed coil, what inductance to use ? The two vertical elements roughly are 1/4λ and the top horizontal section is 1/2λ, so this is a 1λ antenna, roughly 20m long

    Any thoughts? any experiences which you can share? thanks to anyone in advance.

    73
    Saki
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Hello Saki,

    I have an idea for you. I am busy today, so may not be able to post it till tonight. Keep tuned...

    73,
     
  3. DD5XX

    DD5XX XML Subscriber QRZ Page

    lookin' fwd 2 ur reply
     
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    Some questions:
    How long coax? Type? How routed? (Along earth?)

    How high are you willing to put the horizontal wire? Since the 1/4wl section hangs down, and it will have a high voltage on the end of the wire, the end should be a minimum of 3m above earth so it cannot be touched, which would put the horizontal wire about 3m+5.3m= 8.3m above earth. Is that ok?
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Here's the feeding technique that I use for my half-square. I made the driven vertical element 1/4WL of aluminum tubing attached to a treated 2x4 in the ground. The aluminum tubing acts as both the radiating element and physical support. Run the coax up through the aluminum tubing to feed the half square at the top. Install a good choke where the coax exits the tubing at the bottom.

    HSFeed.png
     
    Last edited: Jul 11, 2019 at 4:29 PM
    WB5YUZ likes this.
  6. DD5XX

    DD5XX XML Subscriber QRZ Page

    Mike, of course. The bottom ends of the vertical sections are about 2m above ground. Nothing to worry about, no people or animals nearby, restricted area no danger at all.

    @W5DXP: Thanks for your effort and the nice graphic. Unfortunately I cannot install aluminium tube, I am using two fiberglass poles for supports. The coax I used last year was about 20m long but it was not possible to go in 90° angle away from the fed corner. Although I installed a good choke it was not optimal and the coax in near distance to the vertical element caused troubles. The coax way (current feed) is the most appropriate one but in my case unfortunately very very difficult to fulfill. That's why I thought to try a voltage-feed this year.

    Any clues how the parallel circuit should look like for the 20m band use ?
     
  7. WA7ARK

    WA7ARK Ham Member QRZ Page

    Have it worked out, but it will take time to post it:
     
  8. K7JEM

    K7JEM Ham Member QRZ Page

    An online calculator shows an L network match using 3.5uH of series L and around 35pF of parallel capacitance at the antenna. That is assuming a 2000 ohm antenna and a frequency of 14.25 MHz. Of course, real world impedance will change this somewhat, but should be a starting point.
     
  9. DD5XX

    DD5XX XML Subscriber QRZ Page

    Thank you so much, will take that into consideration.
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    I have fiberglass support poles and my first thought was to use them for my half-square. I would first form a 1/2WL "sleeve" antenna as described by Kraus in Antennas. This is accomplished by removing 1/4WL of outside insulation from a piece of coax and folding the coax braid back on itself leaving 1/4WL of center conductor (copper or aluminum tubing can be used instead). The schematic is the same as my above graphic except the aluminum tubing is replaced by the 1/4WL of folded back braid. Then just tie-wrap the sleeve antenna to the fiberglass pole. Electrically, it is the same as my graphic but just using the fiberglass pole to support the sleeve antenna. Note that this is not a good approach for QRO operation as arcing can occur at the end of the folded-back coax braid.

    Kraus.png
     

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