The End Fed Half Wave.

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M6LVC, Aug 9, 2020.

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  1. KN6CSB

    KN6CSB Ham Member QRZ Page

    interesting. I have always wondered given the idea of 1/2 wave verticals being "good for Dx" and requiring less of optimal ground (supposedly) why one cannot erect their EFHW vertically and see results? But if the EFHW is essentially a Hertzian antenna (horizontal dipole) I imagine like all dipoles it depends greatly on it's height above ground for performance.

    I still can't quit grasp how an antenna "knows" if it is vertical or not.. I mean I guess it's obvious looking at it visually, but why does a Vertical dipole not behave like a vertical with one radial? that said, does a vertical dipole tilted horizontally (i.e. typical dipole) behave differently? I think from what I have learned it all comes down to losses that happen to "lossy ground/earth", a term that still seems amorphous to me. Is this to say that if one were to install a grid of copper or lay down as much copper under a vertical AND a dipole, they would be "the same" in terms of radiation...?

    sorry OP, more questions than answers.
     
  2. W9KEY

    W9KEY XML Subscriber QRZ Page

    Spoken like a person who has never tried one. They are basically just a dipole - which work pretty well without radials, I believe.
     
  3. KD6RF

    KD6RF Premium Subscriber QRZ Page

    Yes, that is the entire answer :D Vertical polarization is when the current flow (current flow is responsible for radiation) is flowing vertically. (Easy to see for straight wires, maybe a bit tricky for different shapes and current distributions...)


    There is a fundamental difference between how vertically polarized waves are "reflected" from lossy ground, and how horizontally waves are "reflected". It's just one of those pesky math things that sometimes crop up...... but the bottom line result is the ages old adage: "For HF DX work, use Vertical Polarization for the low bands, Horizontal Polarization for the high bands".

    This is a generalization based upon:
    * DX signals typically (but not always - 160 M seems to have the most surprises here - just ask Carl!) are seen at low elevation angles
    * Horizontal antennas need to be up at least 3/8 wavelength in order to bring the radiation angle down low enough to outperform vertical antennas

    IOW, 3/8 wavelength is around 48 ft or so on 40 M - if you can't get your horizontal antenna up higher than this, then vertical polarization will typically outperform the low horizontal antenna for low angle DX work.


    Have fun! - the low bands have been quite active. :)
     
    Last edited: Aug 10, 2020
    WB5YUZ likes this.
  4. N3HGB

    N3HGB Ham Member QRZ Page

    Easy way to visualize:
    1. Take a center-fed dipole. Install it vertically with the coax going horizontally some distance from the antenna. It works exactly like a horizontal dipole, except it radiates equally in the horizontal plane and the pattern now applies to the vertical plane. These actually work pretty well if you have room to do it. I used to have a 10 meter dipole strung up this way that worked well.
    2. Now imagine some "tricks" to make this a different way. You can make a dipole out of tubing and feed the coax up inside the tube to the halfway point to feed it. I have an antenna made that way for my AIS system on my boat and it works well. (AIS is on 162 MHz).
    3. Back in the day - and maybe now - marine CB antennas were a 1/4 wave whip with the coax feed having an extra outer sleeve that was also a 1/4 wave. That worked well enough as long as no one cut the coax to shorten it. One could assume some rather odd radiation patterns depending on how the cable was led.
    4. An EFHW is like a dipole but fed at the end. Since the end points of a dipole are the high voltage ends of the antenna, once could assume this or any other vertical dipole on HF might have issues if one end is near the ground. Not entirely sure, but it is the total opposite of a vertical 1/4 wave with a ground connection as far as current and voltage at the feedpoint.
     
  5. N1LOU

    N1LOU XML Subscriber QRZ Page

    I own several and they really don't perform very well at all. And a dipole has two poles...so where is the second pole on a EFHW? Your statement is self defeating. A true dipole requires no ground radials. In an EFHW, the coax IS the other half of the antenna. Granted at primary frequency there is little or no need for the other half of the antenna due to the balancing of currents at that precise frequency. But at all other operating frequencies, the coax is for sure a critical part of the antenna. Many of these EFHW antennas come with instructions that at least 25 ft of coax must be in contact with the earth. Why is that?
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    It sounds like you are confusing these two different types of antennas:

    1. A resonant dipole antenna that is fed about 8% from its "end" of the dipole through a 1:49 transformer (called an EFHW). This is a self-completed Hertz antenna. It does not need a connection to any external ground-plane, radials, or "counterpoise". In fact, it works best if you place a Common-mode choke on the coax feeding it to separate the radiating part (the dipole) from what should be non-radiating (the feedline). It also works best if it is as far away from the earth as possible.

    Deploying it as a Marconi (vertical, inverted-L, sloper) works poorly compared to deploying as you would a "dipole", exactly because a Hertzian dipole naturally has high voltage nodes at both ends. If you bring the fed-end to near ground level, there is capacitive coupling to the earth, the current through that capacitance is driven by the high voltage at the end of the "dipole". This changes the current distribution on the wire and the feedpoint impedance. Current in the earth creates loss... It is even worse if you "ground" that end using direct connection to earth or radials...

    I figure that 90% of EFHW antennas that are bought by hams are deployed that way, mostly due to lack of proper instructions by the antenna maker. After all, the maker is trying to sell antennas to naive folk that hope-beyond-hope that they can put it at 6ft agl in their hoa backyard and it will "work". If deployed like a mobile whip would be, it will work about as well as a mobile whip does...

    A EFHW antenna naturally resonates on all harmonics of the band that it is cut for. Add a small compensation coil, and that aligns the Swr minimums on the harmonic bands. A 80m EFHW is usable on 40m, 20m, 15m, and 10m without a tuner. With only a bit of help from an in-station tuner, you can operate most of 80m, and 17m.

    2. A non-resonant Marconi vertical antenna, which normally expects to be fed against the earth or a field of radial wires These are sold with various wire lengths and a 1:4 or 1:9 balun at the feedpoint, but the kits sold to hams come with very vague instructions as to how to make the required connection to earth... Some makers perpetuate the "myth" that if you just make the coax long enough, and/or lay the coax on the dirt, it will "work". This is bogus advice offered to those who hope against hope not to have to do the work that it actually takes to make it work.

    The proper way to deploy a non-resonant Marconi vertical antenna is the way that DX Engineering tells you to do it in with its non-resonant 43ft vertical. Download and read what it takes to install it, particularly the radials requirements (PDF here), and you will quickly appreciate why the magic "wire with balun and coax" doesn't have a hope...

    Being a non-resonant system, the coax feedline on this type of antenna is always operated at an Swr of 3 to 6, leading to substantial loss in the coax. Someone spending the money for a proper non-resonant Marconi installation like the DXE system I linked would be well served by getting an outdoor remote tuner and putting it right at the Marconi's feedpoint.
     
    Last edited: Aug 10, 2020
  7. KD6RF

    KD6RF Premium Subscriber QRZ Page

    You keep posting this Mike :cool: but it is not really accurate - at least if you are ok with an additional dB or so of coax loss.

    Keeping SWR reasonably low leads typically to coax losses less than 1.5 dB or so - hence the reason to feed a radialed multi-band non-resonant antenna with a 9:1 or 4:1 UnUn for the higher bands, and ideally a loading coil for the lowest band of operation. (I guess it depends upon your definition of "substantial" - in Dave World 1.5 dB is acceptable for 100 ft coax runs !!)

    Very true that the remote tuner you suggest is a fine system, but it's not really needed when the above UnUn / Loading Coil prescription is followed. If you are OK with 1.5 dB, or even 2 dB of coax loss (instead of the 0.5 or 1 dB loss for a resonant system), then it's perfectly ok to put the tuner in the shack.

    This idea is what makes a 43 ft Vert or Inverted-L (WITH a decent radial system for 80 M and 60 M) a nice idea for keeping total radiator length down in the 3/16 wavelength range for the 80 M band and all bands above. Ditto for a 90 ft Inverted-L for 160 M and all bands above.

    The trade-off for using such a comparatively short all-band, DX targeted, antenna is perhaps a dB extra coax loss over a resonant system, and the need to have a decent counterpoise / radial system.

    More info: "Why we use a Base Matching Unit (BMU) with End/Base-Fed Verticals and Inverted-Ls" ---> http://vtenn.com/Blog/?p=1158


    For example, coax loss for 42 ft Inv-L, 100 ft LMR400 (slightly better than RG213):

    [​IMG]


    Data partially measure (antenna Z) and partially calculated (TLDetails).
     
    Last edited: Aug 10, 2020
  8. KX4O

    KX4O Ham Member QRZ Page

    Coax can play a role, but it doesn't have to.
    Likely the feed profile with high impedance and low current negate whatever "need" there might otherwise be for something conductive to work against... even something as small as the unconnected lug on the other side of the power source placed at the feedpoint. Simulations and various experiments with measurements back up the notion the EFHW works surprisingly well with no inherent need for providing extra conductors on the other side of the power source to the antenna... not that such conductors aren't available aplenty with mounting masts, coax, etc.

    I use to be an EFHW doubter, but the ever growing empirical evidence shows it works... sometimes with efficiency costs, but certainly not inoperable. This coupled with decades of use in many products, marine especially, along with ample chamber testing at my employ have changed my mind. Several LMR commercial radio vendors show competence at EFHW design and I suspect the Silver Rod designers do as well. I do find it odd they seem to also offer a 5/8ths wave antenna for CB that probably works just as well (with radials), but some like choices in their product lines.
     
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  9. KM1H

    KM1H Ham Member QRZ Page

    I compare the EFHW with a properly designed T2FD having the lowest frequency used being a bit longer than a half wave and increasing the resistor to ~ 800 Ohms based upon others models.

    It also works as well on all harmonic and non harmonic bands and has no RX suckouts on any frequency in its range making it great for SWLing also and even down in the BCB. Matching is straight forward without the ridiculous power eating balun needed for the EFHW.

    Carl
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    ... and you really believe that a 1:49 EFHW transformer has higher loss than a 1:16 ferrite transformer plus the loss in the 800Ohm resistor used in the T2FD?
     

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