Balun or No Balun on 80 meter loop?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC8PJS, Nov 14, 2009.

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  1. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I first installed by 80 M loop with the traditional 1/4 wave section of RG59 and no balun. I noticed that on some bands I could see SWR variations by handling the coax in the shack. The 1:1 choke balun killed the common mode current nicely. In some installations you might get away without the balun as it depends on what impedance is presented to the common mode path. Apparently in my installation the common mode path had a low enough impedance to allow significant current to flow.
    RK3BU, My loop was only 3 or 4 meters above the ground and the impedance at the feed point was still in the range area of 100 ohms. I used this antenna for alot of impedance measuring exercises in the mid 80's. I built a number of bridges at that time and then built individual temporary LC circuits for each band as part of re-learning the Smith Chart. 73, Pete
     
  2. R3BU

    R3BU Ham Member QRZ Page

    To WB2UAQ
    Pete ! When the loop is so close to the ground, the antenna is strongly captured by soil condactivity. The rezistance of your loop at 3-5meter hight is
    direct formula of your real soil, that you have in your garden.For example, under the salt water,at 3meter hight, the rezistance may be very low with almost no loses.The 80m full size dipole as for experements purposes on my concret(beton)roof at 2meters had 150ohms instead of 73 ohms.. Half the power for heating the roof.. :)
    On this picture it is seen how rezistance of the horizontal dipol is effected by the hight of antenna at different hights at AVARIGE soil an IDEAL SOIL. At ideal soil we have No ground loses.Only low impedance---higher currents in the basement...and probable loses in the wires at high current's antenna's places.
     
    Last edited: Nov 16, 2009
  3. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    Hi Alex,
    So what you are saying is that with a perfect ground at low heights above ground the signal is reflected back and coupled to the antenna in a phase that increases the current at the feed point (lowers the Z). No different than signals on a transmission line.
    I appreciate your reply and the graph!
    73,
    Pete
     
  4. 4D1N

    4D1N Ham Member QRZ Page

    Why?

    Why loops don't need baluns?
     
  5. R3BU

    R3BU Ham Member QRZ Page

    Even loop fed with lader line may have some assimmetry due to phisical suraunding,sach as trees,houses,ground objects.Then the ladder line will begin
    to radiate a little.And known self-simmetry of loops is valid only on ONE freq.The RF choke on cable wuold be a good addition.
     
  6. KC8PJS

    KC8PJS Ham Member QRZ Page

    Thanks for all the responses! It has been very interesting reading all the posts. Thanks again for sharing your knowledge with me. 73!
     
  7. NL7W

    NL7W Ham Member QRZ Page

    It's interesting you should mention this over and over.

    I'll mention my kick-arse "secret weapon" antenna now... :cool:

    I built and ran a full-wave, 75 M band, practically equilateral triangle loop sloping from a 90-foot "monster" telephone pole to two others at 50 feet high on Keesler AFB in the early 1990's. The feedline consisted of paralleled RG-62/U -- using it as coaxial balanced feeders with a characteristic impedance of 186 ohms, or twice that of RG-62's 93-ohm Z (ref stolen figure 24A for coaxial balanced feeders). Terminating the paralleled RG-62 on the rig end were a :eek: decent 4:1 balun and 10-feet of RG-213 coax brought up through the floor of K5TYP's WWII barracks turned military station recreation facility or club house.

    [​IMG]

    This loop's height averaged well over 1/4 wavelength above "ground," so the feed-point impedance was nearly 165 ohms on it's design freq of 3.875 MHz -- presenting an EXCELLENT match for the 186-ohm paralleled feeders, 4:1 balun, and 50-ohm RG-213. On the higher bands, the impedance of the loop may have dropped to nominal 120-ohms or so, yet the over all match was excellent. This antenna easily handled legal-limit... without any noticeable heating effects to the balun -- even on RTTY. All in all, this design and actual result was a "match made in heaven." So was it's heavenly performance.

    This antenna was the MAIN REASON K5TYP won Sweepstakes two years in a row (1991, 1992?), working a Clean Sweep, and setting a Delta Division record for this contest at the time. The setup included my TS-830S with matched 2.1 KHz International Radio filters, a Henry 2K Classic-X, the LOOP, and a TA-33 at 40 feet (the tri-bander was hardly used -- required rotating from west to north to northeast during contests). Must admit... the Henry with it's stiff 4000 Vdc military power supply (3800 volts under TS-830 full-drive) didn't hurt either. :cool:

    This loop, high up and in-the-clear, was efficient, and kicked some serious arse on all HF bands, except 30 and 12 meters, and 160 meters -- of course. It did NOT need a tuner for 75, 40, 20, 17, 15, or 10M. :D

    I plan to build another when my 80 foot tower goes up... and for good reasons. :)

    Besides sharp operating skills, this is my now not-so-secret weapon. :cool:



     
    Last edited: Dec 18, 2009
  8. W5DXP

    W5DXP Ham Member QRZ Page

    The engineering rule of thumb is that a balun needs to be installed at each BALanced to UNbalanced junction in the antenna system. If a balanced loop is fed with parallel line, no balun is needed if a balanced tuner is in use.
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Not sure what this posting had to do with my quoted posting warning against the use of a 4:1 balun with a G5RV ??

    Steve G3TXQ
     
  10. AD5ND

    AD5ND Ham Member QRZ Page

    A 1/4 wave matching section works on the fundamental frequency but may cause problems when the antenna is used as a multiband.
     
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