What's the story with end-fed antenna's?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KI7QVR, Feb 27, 2018.

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  1. KI7QVR

    KI7QVR XML Subscriber QRZ Page

    Hello. I'm a new ham. I got my Tech. in October and my Extra early Feb. I'm not completely new to RF, radios or electronics in general as I studied electronics across the gamut many moons ago at a trade school...that was in 1981.

    So, on to my question. As I understand, an antenna tuner doesn't actually "tune" the antenna at all; it simply presents a load to the radio that won't cause it to fry. The losses between the tuner and the antenna still exist. The SWR along the feed line/antenna don't change; just the SWR that the radio "sees". I get all that, but here's the rub...

    I was chatting with some folks on a net one day and they were talking about end-fed long/random wires. I pointed out the losses incurred, even with a tuner and somebody said, "you need to read up on end-fed antennas; those rules don't apply to end-fed". I didn't ask him to clarify, so I'm asking now, is there a difference? For instances, are the same losses incurred when matching a 4:1 SWR into a vertical vs. matching a 4:1 SWR into an end-fed? Please explain.

    Brian, KI7QVR
     
  2. KL7AJ

    KL7AJ Ham Member QRZ Page

    You got bad information here. Read this article. "My Feedline Tunes My Antenna" I've posted this a dozen times on the Zed, but folks don't seem to get the message.....so I'll keep reposting it periodically until I croak, I guess
     

    Attached Files:

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  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Hi Brian,

    With a transmission line between a transmitter and an antenna, if the feed-point impedance of the antenna doesn't closely match the characteristic impedance of the transmission line, then you have to do some math to figure out what the transmitter sees looking into the driven end of the transmission line. There are several ways of visualizing what happens, but one way you can study it is to download and play with this program: TLDetails.exe.

    If most modern transmitters are connected to a transmission line that presents an SWR higher than about 2:1, the protection circuit built-in to the radio prevents it from harming itself, but in the process will throttle back its output to much less than its rated power output. In most cases, if the SWR is good-enough to satisfy the protection circuit in the transmitter, you are good to go with out a tuner.

    Putting a tuner (internal or external) between the transmitter and the feedline satisfies the protection circuit so that the radio delivers its full rated power, but does nothing to mitigate the losses in the transmission line due to high SWR in the line, plus the tuner adds its own losses to the mix.


    The end-fed dipole antenna is a case where a transformer is placed between the transmission line and the antenna's feedpoint. The goal of the transformer is to step down the very high (~1300 Ohm) impedance at the antenna feedpoint to 50 Ohms for coax, so the SWR on the coax could be quite low, and therefore the losses in the coax are low. However, there are losses in the transformer, so the overall efficiency still might be less than if the antenna were center-fed.
     
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  4. K8AC

    K8AC XML Subscriber QRZ Page

    Eric - that of course is a classic article, not well understood by me when I first read it in 1991, but seems rather clear to me now. But I think even a good understanding of the principles as described may not answer Brian's question. Brian - Being an EE, I'm sure Eric could go into great detail explaining why there are still losses in the tuner regardless of the nature of the load, but in my opinion you're concerned about the wrong thing when talking about an end-fed antenna. When I was a novice operator in 1959, there were a number of us high-school age guys in the local radio club and everyone of us used the same type of antenna - a random length end-fed wire with the far end tied to a tree with an insulator, coming into the house under a window and terminating at an L network tuner sitting on top of the rig. There were several companies making this type of tuner and they all operated about the same - there was an NE-2 neon bulb on the front panel and you tuned the capacitor and switched inductor for maximum brilliance of the neon bulb. None of us understood SWR and there were few if any SWR measuring devices available at the time. And - the pi network outputs of the transmitters could handle quite a wide range of load impedances without complaint. And, certainly, none of us could afford to buy coax.

    These stations worked remarkably well and most of us managed to complete WAS on 40 and 15 meters, some within the one year Novice license term. And, most of our transmitters were running 50 watts INPUT or less. But, of course, there were a few common problems that every one experienced but didn't know how to solve. The most serious problem was that on one or more bands, the tuner cabinet and everything else in the shack made of metal was hot with RF. You could get a real bite from just touching the set screw on a knob. Today, we understand that these random length wires are just half of the antenna and the other half was the metal in the rest of the station. There's no way to calculate the losses involved in that situation as every rig and house are different. Today, many "solutions" are offered for the problem and the one that makes the most sense to me is to supply the other half of the antenna by adding a wire counterpoise. Applied to my original station, this would have meant sitting the little tuner on the window sill and dangling a wire connected to the tuner case out the window. Today, I guess I'd make sure that the wire was as long as possible and insulated from ground. All of the other solutions including baluns, un-uns, matching transformers, chokes, etc. simply mask the problem and waste money. If you don't supply the other side of the antenna, the RF current will find a path to use.

    So, if you're going to use a tuner to match a coax-fed vertical on a frequency where the SWR is 4:1, I'd say that your major losses will be in the coax and there are a number of calculators online that will tell you what that loss will be. And, there have been a couple of articles over the past 10-15 years in QST where they measured internal tuner loss on all bands with various mismatched loads, comparing the various tuners tested. Measuring the same thing using a tuner with a random end-fed wire is a different problem entirely, and the loss incurred in the tuner will be very small and insignificant part of the antenna system loss. My approach these days for all-band antennas is to always use a balanced antenna, long enough for the lowest band of operation, and fed with a low-loss open wire or ladder line.

    73, Floyd - K8AC
     
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  5. AF7XT

    AF7XT Ham Member QRZ Page

    ...short story.
    An end fed is a poor , very poor, last resort due to other circumstances. If at all possible use something, anything else.
    If the physical constraints are that an end fed is the best, only option then you have been given some great information.
     
    Last edited: Feb 27, 2018
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  6. AG1W

    AG1W Ham Member QRZ Page

    Every antenna is a comprise in some way. I currently is a short (35ft) end fed that some say isn’t supposed to work. Have it mounted at 6ft to 28ft and have worked 27 States in total 6 Countries including locations in California and British Columbia from a valley in Vermont in 100W at Radio with 100 ft coax.
     
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  7. KI7QVR

    KI7QVR XML Subscriber QRZ Page

    That's a really good document...I'll have to read a couple times for it to set in but it's really good. Thanks. Come to think of it, you'll probably keep posting it until there are no new hams cause, well we all had to learn it somewhere.
     
    Last edited: Feb 27, 2018
  8. W5DXP

    W5DXP Ham Member QRZ Page

    Firstly: The Laws of Physics apply equally to center-fed wires and end-fed wires. Anyone who believes magic antennas exist that somehow avoid obeying the Laws of Physics is out of touch with reality.

    By "tuning an antenna system" we mean doing something to maximize the available RF current flowing through the radiation resistance of the antenna so it will radiate the maximum available RF power. Anyone who believes an antenna tuner doesn't do that should run some experiments using a current pickup device at the antenna. In a low-loss antenna system, when the antenna tuner is properly adjusted for an SWR of 1:1 at its input, the current into the antenna feedpoint will be very close to the maximum available current. It is an old-XYL's-tale that an antenna has no effect at the antenna feedpoint. I just happened to have written an article on that very subject.

    http://www.w5dxp.com/OWT1.htm

    "The main purpose of an antenna tuner is to cause the maximum amount of current to flow through the radiation resistance at the antenna resulting in Prad=I2(Rrad) watts of power being radiated at the antenna."

    i.e. the cause of maximum available power being radiated by the antenna in a low-loss non-resonant system, is the antenna tuner (or some other matching method) that brings the entire antenna system into resonance (satisfies the IEEE definition of "resonance".}
     
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  9. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Cecil, you are mixing up terms and confusing the issue. KI7QVR was correct in his statement.

    -An antenna tuner in the shack does NOT tune the antenna. It tunes the antenna SYSTEM composed of the feedline and antenna. Feedline losses still exist and can be significant.

    -Output from the transmitter will be maximized
    -Input to the antenna will NOT be maxim if there is an impedance mismatch between the feedline and antenna
    -The tuner cannot change the pattern of the antenna, i.e. you cannot make an 80m dipole used on 2om have the classic Figure 8 pattern of a half wave 20m dipole. This is where a trapped or parallel/fan dipole design can be a better choice.
     
    Last edited: Feb 27, 2018
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  10. NH7RO

    NH7RO Ham Member QRZ Page

    Long story short in a rather miniscule nutshell and partially filled with a side order of my opinion:

    As a new ham, whether Tech, General or Extra---will benefit by starting out with a basic antenna such as a garden variety dipole or vertical monopole with a good radial field. Make some contacts, find out which bands work at the time or times of day one is able to play radio and get one's feet wet with experience. Building antennas from scratch is fun, too.

    Once the new ham is acquainted with the basics and familiar with rudimentary antenna theory then the time may follow where experimentation with trickier and more finicky "magic all-band antennas" is better undertaken. This way the disillusionment might not be so overwhelming once the new ham finds out that there is no free lunch here and the benefits of tried-and-true basic resonant antennas that are easily matched will be all the more apparent.

    Btw, the term "antenna tuner" is a blatant misnomer. As they are generally called in the U.K. and elsewhere, "antenna matching unit" is much more fitting---all they do is match loads (virtually necessary with modern solid state transmitters).

    73,

    off my soapbox and outside to paint my latest wire beam antenna (all of which are painted for weather protection and stealth)

    Jeff
     
    Last edited: Feb 27, 2018
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