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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. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Here's the real rub: Many end-feds are fed with effectively zero transmission line loss because there isn't one. You can bring an end-fed wire directly to the tuner, and then if the tuner adjusts the match to SWR = 1.0, where's the line loss? There isn't any.

    In the old days, this is exactly what many did (and not just hams).

    As Cecil pointed out, the objective of the tuner isn't to make your transmitter happy (I don't think they have feelings) but to transfer maximum power from source to load. With a big enough tuner (very low-loss components, primarily the inductor and interconnection), tuner loss can often be so small it's nearly impossible to measure. "Small" tuners generally have more loss.

    So, the "difficulty" becomes only that return current, which is required or no current would flow at all, will now be carried by whatever happens to be connected to the "other" pole of the source (in our case, usually a coaxial connector which is common to the equipment chassis), so now you have the same RF current flowing through everything common to that. With a current-fed end-fed antenna (1/4-WL, 3/4-WL, 5/4-WL etc) this current would be substantial; but with a voltage-fed antenna (1/2-WL, 1-WL, 1-1/2-WL, etc) it can be very small .

    To help match the voltage-fed designs, which are the most popular because they will work with almost no counterpoise, a high-transformation unun is usually employed and even 9:1 may not be an effective ratio; 49:1 might be, and is still something reasonably easy to wind and compensate.

    A field strength meter can be your friend in determining the actual radiation efficiency of various antennas installed at the same location.

    I think a lot of amateur antennas, whether end-fed, center-fed or off center-fed, lack optimum deployment and therein lies their handicaps. I'd rather have an end-fed with one end at my hamshack window and the other end up 120' in a very tall tree than a center-fed horizontal dipole at 25 feet. I've done both, and the much higher end-fed performs better.
     
    WB5YUZ, K0OKS, WG8Z and 1 other person like this.
  2. W5DXP

    W5DXP Ham Member QRZ Page

    I fail to see the real world distinction except for semantics. The tuner resonates the system. The antenna is a component in that resonant system. In a conjugately matched system, every point in the entire system is tuned to resonance as defined by the IEEE dictionary. That includes the feedpoint at the antenna. One can just as easily consider the combination of tuner impedance transformation and transmission line impedance transformation to be the two parts of the matching system (which they are).

    Put an RF voltmeter at the feedpoint of a mismatched non-resonant-length antenna. Adjust the tuner in the shack for maximum voltage at the antenna feedpoint. By IEEE definition that antenna feedpoint is resonant. Maximum voltage at the antenna feedpoint seems to me to be an effect at the antenna caused by the tuner in the shack. How can one argue otherwise?

    Are you saying that some of the source power doesn't reach the load? That is true for both matched and mismatched systems. No antenna system is 100% efficient. So that we can get on the same page, assume the following lossless system:

    100w XMTR---50 ohm coax---+===1/2WL 300 ohm feedline===50 ohm load

    What percentage of the source power is dissipated by the mismatched load?
     
    Last edited: Feb 28, 2018
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Seems to me what's dissipated is very, very little save for Ohmic loss (skin resistance) of the conductors.

    I try to envision an "antenna tuner" as part of the transmitter. With tube-type transmitters, the "tuner" is built into the transmitter in the form of an adjustable network that both resonates the plate tank and also transfers maximum power to the load.

    Why does it suddenly become "different" if you move it outside the transmitter?
     
    WB5YUZ, AG5DB and KD6RF like this.
  4. W5DXP

    W5DXP Ham Member QRZ Page

    Sorry, trying to keep it simple, when I said 50 ohm load, I meant a resistor, not an antenna.

    The difference is that with an external tuner, a Z0-match to 50 ohms is achieved at the input to the tuner where it is easy to make measurements. Knowing Z0=50 ohms is an advantage.

    With the tube finals with an adjustable network, a Zg-match is achieved somewhere inside the transmitter where it is difficult to make measurements. Not knowing the value of Zg is a disadvantage.
     
  5. W9XMT

    W9XMT Ham Member QRZ Page

    Below is an example of this, for the conditions stated there.

    It is true that the power delivered to a mismatched antenna load can be maximized by using a matching device at the input end of the transmission line. However it won't be equal to that available if the load impedance at the antenna feedpoint terminals inherently matched the Zo of the transmission line connected there.

    The difference in radiated power in this example is nearly 30%.

    Another consideration is that the power dissipated in the transmission line might exceed its rating, and the line might fail.
    [​IMG]
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    This is primarily for Brian, @KI7QVR since I suggested that he get familiar with TLDetails.

    Using the data in post #15, TLDetails shows that 40.8W out of 100W is lost in the cable:
    (Click to enlarge): tld.png
    Note that if you were using a tuner at the transmitter end, it would have to match 11.4 - j16.5 Ohms, radically different than the feedpoint impedance of 200 + j200 at the antenna.
     
    Last edited: Feb 28, 2018
    KI7QVR likes this.
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    But one of the conditions stated was "assume the following lossless system".

    Steve G3TXQ
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    Even the 1/2 wave of 300Ohm feedline in Cecil's post #14 is not lossless.
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    I don't see any mention of 300Ohm feedline in Cecil's post#14, only in his post#12 where he describes it as a lossless system.

    Steve G3TXQ
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    "100w XMTR---50 ohm coax---+===1/2WL 300 ohm feedline===50 ohm load"

    Didn't have a model for 300 Ohm, but here it is with 400 Ohm:
    tld1.png
    To do this right, you would need to include the losses in the balun to get from 50 unbal to 300/400 bal twin-lead, assuming that the antenna is intrinsically bal and doesn't need a balun at that end.
     
    Last edited: Feb 28, 2018

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