Most Efficient Antenna for QRP

Discussion in 'QRP Corner' started by W7CJD, Dec 15, 2015.

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  1. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Just Google folded dipole. Making it out of twinlead is just really convenient and works well at lower power but there's nothing magical about it vs making a folded dipole out of other materials and feeding it with balanced line of homebrew or commercial origins.

    Basically you just cut enough twinlead for your feedline and then cut another section half a wavelength long (the 468/f MHz gets you close but best to cut a little long and then trim it during fine tuning) for the actual antenna. You strip the wires at each end of the antenna piece and twist them together a bit of solder can make it last longer but until you've trimmed to length just twist them together. Then you find the middle and cut just one but not both of the individual twin lead wires at the middle and connect your feedline.

    Yes, it's technically a balanced feedline and ideally should have a characteristic impedance near 300 ohms but that isn't critical. You can tune it with a balanced feedline tuner or any tuner through a balun. You can use a 1:1 balun but a 4:1 is a good option as well since ideally you'll see something close to 300 ohms looking into that feedline assuming the twinlead characteristic impedance is close to 300 ohms and the antenna is tuned to resonance. But realistically most any outboard tuner can handle the mismatch you're likely to see as long as you don't try to run the antenna multi-band on even order harmonics of the fundamental frequency for which the antenna was tuned.

    If you get really lucky with twinlead impedance (near 300 ohms) and antenna tuning or if your rig has tube finals and a decent pi network for loading the tubes you could just run the rig into a 4:1 balun into the twinlead and you should tune up just fine. Most rigs and especially tube rigs with pi network tuning should load just fine into a 75 ohm load. But since this was posted in the QRP section I suspect you're not running tube finals. Still with a bit of twinlead impedance luck and a 4:1 balun you should only be looking into a 75 ohm load on the band for which the antenna was cut.

    -Dave
     
    Last edited: Feb 8, 2016

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