Trying for 75-15 EFHW

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W8EJO, Oct 26, 2021.

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  1. W8EJO

    W8EJO Ham Member QRZ Page

    Loaded EFHW 75-10 .png

    I want 75 meter phone band + 40, 20, 15. Will this get me there?

    #16 wire. Average ground.

    I have everything except loading coil which I need to wind. I do have a 42 uh loading coil.

    I don't have EZNEC or I'd try modeling this myself.
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    A real, resonant 75/40/20/15m EFHW would start out ~130ft long; not ~75ft. The harmonic alignment method would use a series capacitor; not a series inductor.
    W4KJG likes this.
  3. W8EJO

    W8EJO Ham Member QRZ Page

    I understand the length required for 1/2 wl on 80/75. Unfortunately I only have about 65' horizontal to work with and I want the antenna to cover 75 meters for a local net.

    Therefore I'm looking at the option of inductively loading the radiator to fit my lot. Its a common practice with short dipoles, verticals, mobile, etc. The inductor is there to provide 75 meters not for harmonic alignment,

    Adding a series capacitor to align the harmonics would, of course, be required but the value & placement are guesswork without modeling software which I do not have.
  4. WA7ARK

    WA7ARK Ham Member QRZ Page

    I can envision making an inductively loaded EF wire resonant on two non-harmonically related frequencies (75/40m) by changing the wire length before the loading coil, wire length after the loading coil and by varying the coil inductance. I do not see that with only three variables, you could make the system resonant on three or more different frequencies that happen to align with ham bands. I predict that the 15m resonance will be far out of band.

    Where did you get the dimensions you posted?

    What 75m frequency are you trying for?
    Last edited: Oct 26, 2021
  5. W8EJO

    W8EJO Ham Member QRZ Page

    I saw it on multiple online sites including:
    The 1st section (20.5 meters) is ~ 1/2 wl on 40 so that seems reasonable, but probably too long, for the phone portion of 40.

    The net is on 3840 but I also go up to 3880-3890 for AM.
    Last edited: Oct 27, 2021
  6. KF4NU

    KF4NU Ham Member QRZ Page

    With your length limitations, have you considered running an 84' wire with a 17' counterpoise, fed with a 9:1 unun. Of course you'll need a transmatch as well. What I'm describing is the original W3EDP antenna from the 30's. Or you can run 17' of ladder line feeding 67' of wire, and use a 4:1 balun. Either way, it comes out to 84' total length.
  7. WA7F

    WA7F Premium Subscriber QRZ Page

    Maybe a non-resonant Inv-L would do the trick? You will need a few radials and a remote ATU.
    Search “Cebik Straightening Out the Inverted-L“ and after you have read and understand it go to the following URL to determine the best non-resonant length for your situation.

    A good INV-L acts like a dirty vertical. It’s basically a vertical with some horizontal characteristics. Your vertical height is a bit low so increase the vertical if it’s practical and don’t get paranoid about the number of radials. You can always add more later.

    If you are running barefoot the Icom AH-4 is my tuner of choice. A good remote ATU is required for this to work with minimal loss.
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    I took your diagram and put the wires horizontal at 35ft over average earth, and made them co-linear. Looks like this:
    The transformer is in wire4 and the loading coil is in wire 2. The ferrite coax choke is at the open end of wire5. After optimizing for low Swr50 at 3.65 MHz and 7.15 MHz, the variables of the antenna are as follows:

    I know from previous experience that you have to start with a simpler linear model to understand what is going on. Significant findings:

    1. wire and counterpoise lengths

    2. the length of the "counterpoise", which is coax shield between the transformer and a ferrite coax choke placed on the coax 5.3ft from the transformer.

    3. The inductance of the loading coil

    4. a 50:1800 transformer provides a better match. (Lower swr50 on the primary side).

    5. 10% of the applied RF power is lost in loading coil at 3.865 MHz
    Here are the swr50 vs freq plots (looking into the primary side of a 50:1800 Ohm transformer):
    Note that the swr50 bandwidth is not very wide.

    Better on 40m.

    Not included in the optimization, but looks usable on 20m

    Not included in the optimization, but looks usable on 15m

    I next tried optimizing on three frequencies (3.865, 7.15, 14.175 MHz), and relaxed the goal to a Swr50 of 1.3, and tried again. New numbers:

    Swr plot:
    This made the 40m swr worse while improving 20 and 15m. The optimization is now overconstrained.
    Last edited: Oct 27, 2021
  9. K8JD

    K8JD Ham Member QRZ Page

    One Antenna for Everything. NOT !
    The antenna in original post would be a vertical radiator , closer to a QW length on 75M and not real good for high angle regional QSOs. Maybe a high SWR using the 49:1 ratio BalUn.
    You may need more than one antenna to do both DX on high bands and local on low bands !
    You can't do everything well with only one antenna.
    After 60 years of trying different antennas, I now have 8 pieces of coax and one twinlead, coming into the shack to antenna switches, tuners, and different radios.
    I have been on all bands from 160M to 3/4 M !
    Still have a few more designs I want to try !
    KA0HCP likes this.
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    I'm waiting for the day when someone posts about their "End Fed Halfwave Yagi".

    Everyone has gotten EFHW fever on the brain. They bastardize the installation, bend it into odd shapes, intend to use it from DC to daylight and still call it a halfwave.

    The OP should stop the design gymnastics and abuse of terminology.

    Simply install the highest and longest Random Inverted L that fits. Throw down whatever number of radials strike his fancy. Get a good quality tuner of higher power rating and get on the air. :) b.
    WA7F and KA3VID like this.

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