Loop switched to end-fed?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by AB3LD, Nov 7, 2016.

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  1. AB3LD

    AB3LD Premium Subscriber QRZ Page

    OK, this might be a crazy idea. What happens if I put a switch on one side of the feed to my HF loop antenna, so it can be switched to ground, leaving the other end of the feed open, so the antenna is now kind of a loop-shaped end-fed wire?

    My smallish property and sparse trees make it hard to construct an effective antenna for 80M. I put up a horizontal loop that is approximately a full-wave at 40M. The feed from the antenna to the shack is 20 feet of 450 ohm ladder line. Where the ladder line enters the shack, it connects to a home brewed 9:1 current (Guanella) balun. Between the balun and the operating position is 25 feet of 50 ohm coax, LMR 240. I have an antenna tuner in the line near the transceiver. The antenna can be tuned to low SWR on all HF bands and works pretty well. On 40M and higher the SWR on the antenna side of the tuner is reasonably low, so losses in the coax are not too bad. But on 80M, the SWR is 25:1. So I am probably losing well over 1db in the run of coax.

    On a hunch I tried an experiment. On the antenna side of the balun, I disconnected one leg of the ladder line from the balun terminal nearest the coax shield in the balun schematic. I connected that terminal to ground. I think this makes the balun act as an unun? The other leg of the ladder line remained connected to the balun terminal nearest the coax center in the schematic. So the loop antenna is now open at the disconnected end of the ladder feed line. Now the SWR on the antenna side of the tuner on 80M reads just 3:1 or so! It seems like a great result, but also too good to be true? I am considering making the setup more permanent by installing a SPDT switch to easily change between closed loop and open loop configurations. Not sure what find of switch I should use, considering a heavy duty toggle or a knife switch. But before I do that, I wanted to get some feedback from smarter hams.

    In case it matters, here are a few facts to note. The loop is approximately a triangle, about 35 feet high. The shack is on the second floor, and the run from rig to ground is long, more than 30 feet. I run 100 watts from a Yaesu FT950, and use an LDG YT450 automatic tuner. I have an antenna analyzer, MFJ 226, and it approximately confirms the readings my rig gives me. With the loop open, the FT950 can get a match on 80M easily using only it's internal tuner, and can even run without a tuner on some 80M frequencies with SWR < 2.5:1.

    This all raises lots of questions. Here are a two to start:

    Would losses in the switch be just as bad as the losses in the coax run were when SWR was 25:1? If so I suppose this would mean large losses in all bands now? How could I measure this?

    Even though the Loop is open, because the beginning and end of the loop are at the shack end of the 20 feet of ladder line, I suspect the signals in the 2 conductors are interacting. What effect would this have?

    Thanks!
     
  2. KD6RF

    KD6RF XML Subscriber QRZ Page

    Lotsa words there! Diagrams would help.

    One observation I can make - when grounding things willy-nilly, seeing a lower SWR is almost certainly a BAD thing - it likely means that you are end feeding something on the one side, and dumping that same current into lossy ground on the other.

    Low SWR only means that the measured impedance is closer to 50 ohms. It doesn't say that the system is efficient.
     
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  3. K8JD

    K8JD Ham Member QRZ Page

    Can you manage to install two 66 ft sections of wire around the perimiter of your property and connect coax to them where they meet ? Probably work MUCH better on 80M than a hodgepodge of coax, ladder line and baluns !
    (if you read many of my posts, you see I prefer making single band antennas)
     
    Last edited: Nov 7, 2016
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  4. WA4SIX

    WA4SIX Ham Member QRZ Page

    Many have used a loop with the feed shorted & fed against ground. Basicly a short vertical with a huge capacitance hat.

    Ed
     
  5. AI3V

    AI3V Ham Member QRZ Page

    Since your original antenna is just a random collection of parts, with a random radiation pattern and a random impedance,

    Adding a random relay will change the random pattern and random impedance.

    Since you are making random HF contacts, anything that can be switched during a QSO may or may not make a difference.

    Should the new random combination cause vswr/tuner/rfi issues, the proper course of attack would be to add and subtract antenna system components at random and hope for the best.

    Rege
     
  6. AB3LD

    AB3LD Premium Subscriber QRZ Page


    Thank you all for the good perspectives and suggestions! Since one of you suggested I provide a diagram, I am uploading one now, to see if it sparks any additional thoughts.
     

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