Posted: 7/3/2015 10:58:14 AM EDT
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Going to be building a 3 or 4 element 2m yagi for low power portable use. Size and weight are major factors. Do I need a choke? it's only going to be 5 watts, and only about 10ft of coax so I'm not really concerned about "RF in the shack", but does it make the antenna any more efficient if I were to put a few slip on ferrites near the feed point to keep the RF where it's supposed to be? Considering the size and weight factor I'd rather not to do an air choke, or a toroid winding. Thoughts/suggestions? |
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Well, it's pretty much true that you don't "need" one....
But not having one causes feedline radiation, pattern skew, gain reduction etc. IOW, if small and light is your goal, then a 2 el yagi WITH a choke will be smaller and lighter than a 3 or 4 el yagi WITHOUT a choke, but will probably have roughly the same gain. From an eHam smart-guy: "A choke with the highest impedance will be a choke that has its self resonant point close to the operating frequency. Self resonant chokes work very good over a small frequency range. If you wind a choke with RG58 size coax on a 1 inch diameter PVC form with about 5 turns it should resonate close to 141 MHz. That should give you about 6000 ohms at 146 MHz. If you really want to optimize it, you could spread the turns some to get the resonant frequency closer to the operating frequency." http://www.eham.net/ehamforum/smf/index.php?topic=39351.0 |