Calculating Antenna Length on an Inverted L

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W3KDK, Jul 7, 2016.

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  1. W3KDK

    W3KDK Ham Member QRZ Page

    I am using a remote tuner to feed a Inverted L antenna. The manual fro the tuner (FC-40) indicates that 1/2 wavelength lengths should be avoided.
    When I measure the antenna, is it only the horizontal element? Do I measure the twisted part that attaches to the insulators? I am able to tune everything but 160, and 17.
  2. N8AFT

    N8AFT Ham Member QRZ Page

    No. & No.
    Measure all the wire from start to end. Eyelet of insulator to eyelet, not rocket science critical since it is a non-resonant wire.
    Mine is 90' and good for 80-10 but not 12.
    Enjoy the experimenting in good weather friend... VY 73 lane n8aft.
    W3KDK and K7TRF like this.
  3. N8AFT

    N8AFT Ham Member QRZ Page

    PS... 135 foot should give you 160 as well.
    WA7F likes this.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    But...135' may create an impossible to match load on 80m, and maybe 40m. 135' is 1/2-WL at 3.47 MHz.

    I'd try to avoid all integers of 1/2-WL. Normally made my inverted-L's 165' long and could match 160, 80 and 40m with them pretty easily using a single-ended tuner. Never cared about the higher bands where I had beams.

    A lossy ground system will generally improve the match but reduce performance. Mine had 20 radials, about 60' long each and seemed "okay," although more radials and some longer ones may have helped.
    WA8FOZ and NL7W like this.
  5. KQ0J

    KQ0J XML Subscriber QRZ Page

  6. K8JD

    K8JD Ham Member QRZ Page

    I would calculate an inv "L" for 1/4 wave total length for a band you are SERIOUS about using. Then you don't need a tuner.
    KN6FWT, AK5B, K2CAJ and 1 other person like this.
  7. WR2E

    WR2E Ham Member QRZ Page

    Reason being that it presents a very high impedance to the tuner and will probably be outside the range that it can tune. Also, because the impedance is high, so will the voltage and the tuner could potentially be damaged... IMHO it SHOULD be able to handle the voltage by design, but you know how they cut corners these days.

    How long is the wire?
  8. KH6AQ

    KH6AQ XML Subscriber QRZ Page

    An inverted-L often (always?) ends up longer than the formula for a quarterwave vertical. Using EZNEC to find the length of a #14 wire vertical and inverted-L we get:

    Resonant vertical 133'
    Inverted-L 65' horizontal, 77' vertical, length 142'

    The formula for this vertical is 239/F
    The formula for this inverted-L is 256/F

    I would cut it long - like 270/F and trim it to resonance.
    WB5YUZ likes this.
  9. WB4MDX

    WB4MDX Ham Member QRZ Page

    I agree with giving a little extra so you can trim or fold and wrap wire on itself at the feed point. My inv L has 2 pulleys with a weight attached to a rope to the far end so I can easily move its resonance from the 1.820 region to 1.865 or shorter wavelength. To get 80 or other bands attach a second wire as shown on my QRZ page. You can make a 7.150 COAXIAL trap using 213 coax and a 4" dia pvc pipe. Now you can get 160-30 easily (and other bands). The yahoo EZNEC group ran this a few years ago. Hope to hear you on 160.

    I'm more worried about a near or direct lightening strike to any remote tuner. The way I feed the wires is easily made and MAY offer some protection to a remote tuner and the coax. The homemade 'tubular' capacitor and Jacob's ladder lightening arrestors add about 22pf in series and helps tune a slightly too long antenna.
    AI3V likes this.
  10. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    This could be a useful feature for an inverted "L." Radiation resistance for inverted "L" antennas is often lower than 35 ohms, and lengthening the antenna to raise radiation resistance and then using a series capacitor to tune out the resulting inductance is one common solution.

    (Later) This is of course only an issue if the inverted "L" is being operated resonantly. Reading the post again, I remembered OP is operating a non-resonant "L" with a tuner at the base, so please disregard.

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