End Fed Half Wave Antenna

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by LB0TH, Feb 26, 2021.

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  1. LB0TH

    LB0TH Premium Subscriber QRZ Page

    I have built an half-wave end fed antenna whit 1:64 transformer and an 110uH shortening coil at the end (2,4m from the end).

    The antenna are not resonant and I need to cut it. Shall I cut at feed end or behind the coil?
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Hello Frode,

    What frequencies are you trying for?

    Describe how the antenna is installed? Straight? Bent? How high? Coax length? and Grounds?

    At what frequency does the fundamental (lowest design band) SWR minimum occur ?

    At what frequency does the second harmonic minimum SWR occur?

    What instrumentation do you have? SWR meter? Antenna Analyzer? Nanovna?

    I explain what the series loading coil is supposed to do in a slide show I showed at a Hamfest. The movie linked to that slide show is here.
     
    Last edited: Feb 26, 2021
  3. KC3RN

    KC3RN Ham Member QRZ Page

    The transformer might be the problem. You're using a 64:1. Most EFHW's us a 49:1 ratio...
     
  4. LB0TH

    LB0TH Premium Subscriber QRZ Page

    I prefer 40 and 80 meters. Antenna are configured as an inverted V. The Coax are short as I am using the MFJ Analyzer. No earthing. Lowest SWR (1:1,2) at 6,9 MHz at the.

    I have got a bit closer. Changed the transformer to a 1:49 and cut the front part wire until I got resonant at 7,1. Then I change to 80 meter and cut the wire behind the coil to the best SWR at 3,65 MHz. Then I got SWR 1:1,3. Connecting again the 1:64 transformer ang get “bad result”, the antenna is to long.

    Why act the antenna to be longer whit 1:64 comparing to 1:49 transformer?
     
  5. AI3V

    AI3V Ham Member QRZ Page

  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    How "short" is the coax?

    The minimum length of coax that "completes" this antenna so that it can resonate is about 2.5m

    How close to earth is the transformer? It must be quite close if you can read the MFJ with a "short coax" between it and the transformer?

    Tuning any 80/40m dipole with the ends of the dipole very close to earth means you will get different results when you elevate the antenna.

    To work well, the center of an 80m inverted-V should be >10m, and the ends should be >2 to 3m agl.
     
    Last edited: Feb 27, 2021
    WB2UAQ likes this.
  7. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Can you expect the EFHW feed point Z to be the same with a loading coil inserted vs an actual 1/2 wave of wire? I know you have modeled this stuff before and might have done the loaded case. I can't imagine the radiation resistance remaining unchanged and of course capacitance to earth will be different (less). I measured the feed point Z a 67 ft wire going from the ground to a high tree branch at a steep angle (maybe 70 deg with respect to horizontal) at 7 MHz as a reference (about 3800 ohms shunted by 8 pF). We finally are seeing the ground again after weeks or more of snow cover but I am not going out to experiment for a few months.

    110 uH of inductance? Wonder how that was measured and determined by the op.
     
  8. KU3X

    KU3X Ham Member QRZ Page

    A lot of good suggestions above so I won't repeat them.

    What I will say is my first shortened HWEF40 used the 34uh coil as shown in the following link.

    http://pa-11019.blogspot.com/2012/04/149-transformer-for-endfed-antennas-35.html

    I fed it with a 64 to 1 UnUn. It had a very good SWR on 10, 20 and 40 meters, but I lost 15 meters.

    I gave to a friend of mine a shortened HWEF80 using the 110 uh coil. I supplied him with a 49 to 1 UnUn to feed it. I can say it had a low SWR at
    its resonant point on 20, 40 and 80 meters where he uses it.

    I only post this to verify that the design does in fact work and works well.

    Want to test your UnUn to see if it works? Build two of them, put them back to back, put a dummy load on one end and put power through it from the other end. It's kind of a ball park test but will get you in the game.

    Good luck,
    Barry, KU3X
     
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    Damn! I just lost a post that it took me a couple of hours to create. Maybe tommorrow...
     

    Attached Files:

  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    Here is what I tried to post:

    Start with a free-space dipole made of un-insulated copper wire. Looks like this:
    [​IMG]

    For that off-center fed dipole, the feedpoint R is high when fed near the end. The ratio of K to L effects R. K cannot ever be zero!

    The wire can be brought to resonance (jX=0) by changing the sum of K and L.

    To be useful in a 50 Ohm system, the leakage inductance of the transformer primary must be high compared to 50 Ohms. With available ferrites, it requires about 3turns for the primary. It is easy to build wide-band, ferrite-core RF transformers that have turns ratios from 1:2 to 1:9 (3:6 to 3:27, considering a three turn primary). This produces an impedance transformation proportional to the (turns ratio)^2. For example, a 3:21 turns ratio makes a practical impedance ratio of 50 Ohms at the primary to 50*(21/3)^2 = 50*49 = 2450 Ohms at the secondary, which is the turns ratio most commonly used for these antennas; but not necessarily the only turns ratio that can be used.

    Using AutoEz/EzNec, I solve for the required K and L lengths that make the lowest Swr at the transformer primary for different attainable turns ratios T:
    [​IMG]

    So, you see that if you pick the turns ratio T first, that changes K and L. It is somewhat counterintuitive that the sum of K and L is larger for high ratios. That is one reason that ratios above 1:7 are rarely used.
     
    Last edited: Feb 28, 2021

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