Should I use a balun for a G5RV?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KE0CPH, Oct 14, 2018.

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  1. 2E0VSS

    2E0VSS Ham Member QRZ Page

    Go to IZ2UUF You tube chanel re the use or no use of a Balun and this thread will no longer be so agitated and ill informed.


    2e0vss
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    Mr Varney did not have NEC; if he had he would have done things quite differently.

    Here is the impedance looking into 34ft of 600 Ohm, Velocity Factor=0.91 air dielectric ladder line center feeding 102ft of #14 wire, 50ft in the air: (As I understand G5RV's description):

    5rvZ.png 5rv50.png

    Swr is shown both for a reference of 75 or 50 Ohms. Pretty Awful, isn't it? Notice it comes close to resonance (and low SWR50) only on one frequency, namely 14.0MHz. A tuner is required everywhere else, and it will be a very lossy system. A ZS6BKW is much better.

    If the goal is to run 50 Ohm coax to the station, I think you are screwed. There is no balun impedance ratio that will improve the situation other than just using a 1:1 Common-Mode-Choke.
     
    WB5YUZ likes this.
  3. N5WVR

    N5WVR Ham Member QRZ Page

    That really supports my decision to always bring the window line from multi-band doublets back to the shack!
     
    WB5YUZ and AG5CK like this.
  4. N5WVR

    N5WVR Ham Member QRZ Page

    Unfortunately, I think the current housing situation has made things much worse.

    Back in the day, practically everyone started with one or more half-way dipoles. They were (and are) cheap and easy to build, and easy to install. Beginning hams were not likely to be battling HOAs, CCRs, or living on microscopic lots. (Some were and did, of course, but it wasn't the norm in the 1950s and 60s.) Everyone had outdoor clotheslines, and they saw a house as a place to live, not an investment that was supposed to appreciate.

    That changed beginning in the 70s, and has accelerated since.

    Today, they often have to go straight to understanding stealth or indoor antennas and the tradeoffs needed, or more-complex but small approaches like small tuned loops. They can't start simple because they can't start with the basic outdoor dipole. They did not select their homes with radio in mind.
     
  5. KA2RRK

    KA2RRK Platinum Subscriber Platinum Subscriber QRZ Page

    XLR8

    KA2RRK
     
  6. WA7ARK

    WA7ARK Ham Member QRZ Page

    Yeah, but if you extend the ladder line all the way to the shack, have you ever asked yourself what that transforms the impedance that the tuner has to deal with to? Also, the SWR on the twin-lead can get high enough to where its loses exceed what would be lost in coax in a well-designed multiband antenna like a ZS6BKW.

    Depending on the feedline and antenna lengths and band, the tuner with it's built-in balun might have to contend with some really bizzare, tough to match, very-lossy-to-match combinations. Only if you determine the impedance looking into the tuner end of the feedline as a function of the antenna and feedline lengths, including the velocity factor of the feedline, on all bands of interest, and you are willing to change some lengths to avoid those tough-to-match combinations, can you be sure to avoid those potentially high losses.

    The "throw up a random length doublet, feed it with a random length of ladder line or twin lead and connect it to the balanced terminals on my commercial tuner, and I will have a low-loss, tune anywhere system" is a pipe dream.
     
    Last edited: Oct 19, 2018
  7. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Yes.

    Another aspect of modern American life that makes it harder for the propertyless to put up antennas: back in the late '70s, '80s, and early '90s, every rental apartment complex I lived in was very small by today's standards and either owned by one individual, or managed by one individual. One person. After a few months of keeping the place clean and paying my rent on time, and yakking about their families and their interests when I dropped the rent check off, I was able to approach all these people and get permission to string up a fan dipole in nearby trees. It wasn't hard.

    I can't imagine what that would be like in a typical very large, modern apartment complex that is owned by a faceless corporation, and managed by a staff of a half-dozen or more who can only stay out of trouble with their supervisors by saying "no" to just about every request by every tenant, regardless of how harmless or even beneficial the requested action is, and how reliable and pleasant the tenant is.
     
    Last edited: Oct 27, 2018
  8. N5WVR

    N5WVR Ham Member QRZ Page

    It's usually easy to solve problems like that by adding a few more feet of ladder line, assuming that the antenna is at least 3/8 wavelengths long on the lowest band, and is not a multiple of a 1/2 WL on any band for which it is used.
     
  9. N5WVR

    N5WVR Ham Member QRZ Page

    The friendly or easy-to-understand responses heard in person at clubs and hamfests are not always correct, either!
     
  10. W5DXP

    W5DXP Ham Member QRZ Page

    A Double Zepp is a multiple of (2x) a 1/2WL on its design frequency and is a time-honored good performer. Let's say we have two antennas, a 1/2WL dipole with a feedpoint impedance of 50 ohms, and a 2 x 1/2WL dipole with a feedpoint impedance of 4050 ohms. We feed both antennas with 450 ohm ladder line. The SWR on the ladder line feeding the 1/2WL dipole is 9:1. The SWR on the ladder line feeding the one wavelength dipole is also 9:1. The impedance at the current maximum point on both ladder lines is the same, i.e. 50 ohms.

    If I tell you that the SWR50 is 1:1 in the shack, can you tell me which antenna is being used?
     

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