135' Dipole Problems

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC9QQ, Oct 30, 2009.

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    Steve,

    I failed to find my 80m half-wave dipole data, but I found data for a similar exercise I did with a 100ft doublet. I've posted a screen-shot here:

    http://www.karinya.net/g3txq/temp/balun_choice.png

    I used EZNEC+ to predict the impedances, I used the W9CF tuner simulator with its standard settings to predict the tuner loss, and I assumed the Baluns would be loss-free and would transform the impedances exactly 1:1 or 4:1.

    Here's the "Exec Summary":

    1. Averaged over the 32 combinations of Band and line length, the 1:1 balun was better by 0.08dB
    2. The only losses exceeding 2dB were with the 4:1 balun
    3. Of the 32 combinations, the 1:1 was better on 10 occasions, the 4:1 was better on 11, and they were the same on 11.

    Those numbers would seem to make a small case in favour of the 1:1. If we then remember that core flux in the 1:1 depends only on the common-mode voltage, whereas on a 4:1 voltage or current balun the core flux depends on the differential-mode voltage, the case becomes more persuasive.

    Of course it's fairly easy to find combinations of antenna/ladderline length which would tip the tuner losses either way; but I find it odd that folk continue to recommend 4:1 as the preferred choice for the "general case".

    73,
    Steve G3TXQ
     
  2. W5DXP

    W5DXP Ham Member QRZ Page

    Someone said to switch between two baluns. That is good advice if one knows which one is better for each particular set of conditions.
     
  3. NL7W

    NL7W Ham Member QRZ Page

    Agreed!

    Each person's antenna and environment will be different. If one has the wherewithal, trying both the 4:1 and the 1:1 baluns, to find out which "performs" better, would be ideal methinks. :)

    We must all keep in-mind that there's no multi-band "magic bullet" antenna -- they have their limitations.

    73.



     
    Last edited: Dec 15, 2009
  4. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    One of the problems I have with EZNEC and the whole host of "modeling" software is that they are only as good as what is programmed into them and can't possibly account for all real-world situations.

    4:1 baluns have been used for years to match 300 ohm lead to 50 ohm coax and have worked quite well, *LONG BEFORE* there were computers around.

    So I say forget the computers, put the darn thing up and see what happens!

    :):)
     
  5. W5DXP

    W5DXP Ham Member QRZ Page

    Or could it be that ignorant hams mistakenly thought they were working well? :) The two characteristic impedances existing at an interface have nothing to do with the transformation ratio required unless both lines are flat, i.e. SWR = 1 (e.g. the case of single-band folded dipoles). It's certainly NOT the case for most multi-band antennas.

    Consider the following case:

    XMTR---50 ohm coax---balun---1/2WL 300 ohm feedline---50 ohm antenna

    Does anyone really think a 4:1 balun would be the balun of choice at the Z0=300 ohm to Z0=50 ohm interface? Why would anyone want to transform the perfect 50 ohm match to a 12.5/50 ohm mismatch???
     
  6. W4PG

    W4PG Super Moderator Lifetime Member 279 Volunteer Moderator Platinum Subscriber Life Member QRZ Page

    :D:D

    Funny how the older I get, the more a problem math becomes!! :eek:

    Fix the numbers to read what you like. The point is that we have used those "baluns" for years *BEFORE* rigs all had tuners and folks actually had to know something about how to make an antenna resonant, without using a tuner to match the antenna/feedline. So I guess maybe they didn't work.

    Time to turn in all those QSL cards from the DX stations I never should have worked . . . :D
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    I guess you'll never know how many more you might have worked with an optimised system ;)

    Steve G3TXQ
     
  8. KL7AJ

    KL7AJ Ham Member QRZ Page

    Actually, I ran coaxial-fed dipoles for YEARS with NO baluns whatsoever, and as far as I knew at the time, it never made any difference. However, this just proves that QRP works!


    eric
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    I did the same thing while a Novice in the early '50s. But when I upgraded, bought a metal microphone, and got my lips close to it, RF-in-the-shack made a believer out of me. :)
     
  10. KL7AJ

    KL7AJ Ham Member QRZ Page


    Hi Steve:

    This is precisely the reason the 1:1 balun is almost always the best choice, since the impedance it encounters has about a 50/50 chance of being either HIGHER or LOWER than 50 ohms. The only exception would be an antenna that happens to have a feedpoint impedance of 450 ohms, in which case a 9:1 balun would be best. How many antennas have you seen with a 450 ohm feedpoint impedance?

    eric
     
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