DIY a highly efficient QRP DX wire antenna

Discussion in 'QRP Corner' started by W7CJD, Dec 22, 2015.

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  1. W7CJD

    W7CJD Ham Member QRZ Page

    I like wire antennas for QRP.

    I am considering purchasing an antenna analyzer:

    1. What antenna analyzer features do I need to determine optimum feedline length, so I do not need a balun or a tuner?

    2. Is the MFJ 801 Field Strength Meter sufficient to measure actual low takeoff angle "lobe" antenna gain, if I walk out the appropriate distance? use a long pole more nearby to get the reading?

    3. Is there a practical method I might use to find ERP effective radiated performance?

    I am also interested in antenna modeling results.
    Last edited: Dec 22, 2015
  2. KK4NSF

    KK4NSF Ham Member QRZ Page

    Since I've not used an analyser, I can't answer question 1.

    But as for question 2, yes the MFJ Field Strength Meter is very handy. I bought one a few years ago to compare the field strength put out by Magnetic Loop antennas to that put out by tradional wires. While it only gives you relative readings, it will indeed show you where the lobes are.

    On a side note, i noticed one of the MFJ FSMs being used to sweep for ghosts on a paranormal investigation TV show not long .... SO if you a bothered by pesky poltergiests, your meter could do double duty!

    Qestion 3- I have no clue.

  3. W7CJD

    W7CJD Ham Member QRZ Page

    I have a good idea if lobes are straight up or broadside a dipole/doublet antenna, for example, typical of a dipole/doublet antenna.

    I am interested in antenna modeling because it gives differences by height and details how the lobes change.

    I am especially interested in low angle takeoff lobes for DX.

    I am interested in ERP effective radiated power: power OUT for power IN because I am considering QRPp.

    If I had a reasonably calibrated Field Strength Meter, I can walk around the considerable acreage or reasonably nearby convenient mountain ridges of the national forest, if necessary.

    I have read attenuating the output of the transmitter may be used to determine ERP.

    I did find an attenuator for signal output.

    However, I have no details how that is accomplished.

    ERP is power OUT for power IN.

    My actual experience is really only resonant dipole antennas.

    I have had the impression highest efficiency in an antenna means having a high current point at the feedpoint.

    This has given me the impression, the feedline length participates in having the high current at the feedpoint, also the feedpoint should be located at optimum height to get the low takeoff angle for DX.

    Yes? No?

    My experience, however, is limited to coax-fed resonant dipole antennas.

    I was thinking, perhaps, a hand-built ladderline fed resonant doublet antenna would have better ERP.

    Yes? No?

    Does an antenna analyzer, with more features, do more for me than my YouKits FG-01?

    Would I be able to determine optimum feedline length?

    Does the YouKits FG-01 give me sufficient information?

    Do I only need to know "how to" use the information already available with the YouKits FG-01 I have?
  4. W7CJD

    W7CJD Ham Member QRZ Page

    My YouKits FG-01 1-72 MHz has SWR and Z.

    I have looked at antenna analyzers.

    I like the looks of the Mini60 with SARK100-for-PC and ZPlots software, and, Mini VNA Tiny with Android software, so far.

    I have made no purchase, because I do not know what features I would need for the wire antennas that interest me for QRP and QRPp.
  5. AG6QR

    AG6QR Premium Subscriber QRZ Page

    Not quite. It is the equivalent transmitter power that would be required to be put into a standard reference half-wave dipole such that the dipole would produce the same field strength at the measured system's main lobe.

    A highly directional antenna might have an ERP higher than its input power. For example, a multi element beam fed with a 100W transmitter might have an ERP of a thousand watts, because ERP is measured in the direction of the main lobe. But because of the law of conservation of energy, and the fact that antennas are passive devices, they can't possibly produce more power OUT than power IN. Any antenna fed with a 100W transmitter will radiate less than 100W in total, even if its ERP is 1000W.

    How can you have an ERP that's greater than the total output power? Because, by the definition of ERP, the ERP is only measured in the direction of the strongest lobe. An antenna with an ERP higher than its input power will radiate less in other directions.

    High ERP from low output power may be a good thing or a bad thing. High ERP from low input power is good if and only if your main lobe is pointed in the direction of the receiving station(s).

    There are a few misconceptions there. High efficiency doesn't mean high current at the feedpoint. It means having low losses, and having impedance well-matched at each transition point in the antenna system.

    If you have a 50 ohm coax transmission line (one of the popular ways of feeding amateur antennas, but certainly not the only way), then the end of the coax should see a 50 ohm non-reactive impedance, for maximum efficiency. If this is the case, the length of the coax doesn't enter into the system's efficiency, except that the coax has a certain loss per unit length, so the shorter the better.

    If your optimal feedline length of 50 ohm coax is anything other than "as short as possible to reach the radio", then you must be using the feedline as a transformer and/or radiator. This would indicate a mismatch at the feedpoint, which isn't something that will give good efficiency. The current on the coax will be higher than would be needed to deliver the power in a well-matched system, and the coax resistive loss goes up with the square of the current, so you'd be wasting power in your coax loss.

    In a well-matched system, optimum feedline length is always zero. But in a well matched system, a nonzero length of quality coax doesn't cost very much in the way of losses, so you usually make the feedline long enough to reach from the feedpoint down to a convenient operating position on the ground.

    The YouKits FG-01 (or pretty much any other reasonable antenna analyzer) will let you know what the SWR of your antenna is as a function of frequency. This gives you the information needed to tune your antenna for a good match, which is what you want in order to deliver the maximum power to the antenna with minimum losses.
    W7CJD likes this.
  6. W7CJD

    W7CJD Ham Member QRZ Page

    It doesn't get better than that?

    At HF, then, feedline length isn't as influential as for VHF/UHF?

    The YouKits FG-01 1-72 MHz has SWR and Z.

    I do not need R and X and other features of an antenna analyzer, then, for other than the resonant dipole antennas I have to try to have other "low loss" wire antennas I would like to have?

    I did find out about the ladderline feedline "doublets" or "center fed Zepp". It's usually just a simple wire, at least 1/3 wavelength long at the lowest band of interest, (1/2 is better) or longer than a half-wave dipole are made to have larger directional lobes.

    Of course, that means nulls, so have more than one.
    Last edited: Dec 23, 2015

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