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1 Coil 2 Coax ?

Discussion in 'Home Brew' started by W9WD, May 24, 2009.

  1. W9WD

    W9WD Guest

    Anybody ever tried this on a 5/8 wave ?



    You've got a tuning coil going from the ground plane to the vertical element.
    Normally you would run the jumper from the center of the coax to somewhere along the coil to get a 1:1 match on the frequency you want to tune the antenna on.

    What if you ran two pieces of coax up to the antenna and took 2 taps off the coil?

    In my case I would tune one of the taps at 28.4 for 10 meters and one at 27.205 for 11 meters. That would allow a 1:1 match on two bands.
    You could use a switch at the radio to switch between the two pieces of coax.

    Would that work or would the two taps interfere with each other?
     

  2. Robb

    Robb Yup

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    If I wanted to use other frequncies on my antenna, I would use an antenna matcher. But I am using an IMAX antenna; they are fairly resonant between 25-30mhz if I were to re-tune it for whatever band.

    But an antenna tuner would do the same thing with a small loss to output w/o having to mess with the antenna tune each time I wanted to change band usage. Changing the antenna tune on a antenna that is on a 40 ft push-up pole on a 10 ft garage is a waste of time and a pain in the cool-o...
     
  3. W5LZ

    W5LZ Crotchety Old Bastard

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    No, it won't work that way. They would interfere with each other. If you really want to have a remotely tunable antenna I only know of two ways of doing it. One's mechanical, and a nightmare, the other is electrical and expensive. Or, do it like Rob KI6USW suggested, use a tuner. It'll make your radio happy, not do a huge amount of good things to a coax feed line, but it'll certainly work.
    A resonant antenna is only going to be resonant in one spot (that spot isn't microscope sized). The further you get away from that spot, the less resonant it will be until it hits a harmonic. That does make some difference but it isn't a huge-never-good thingy at all.
    - 'Doc
     
  4. W9WD

    W9WD Guest

    Drat

    Looks like I'll look for an antenna tuner.
    I wound up with 1.4:1 at 28.5 and 1.9:1 at 27.3

    A little higher than I hoped for on 11 meters
     
  5. Robb

    Robb Yup

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    So long as you aren't running any power - then any cheap tuner will do just fine. Around ~$50 or so. But if you are going to run some power, then you will need to get a good tuner. More $$$.

    A manual tuner with a higher power rating than you intend to use will be best and more efficient. A cheap tuner that is rated at 100 watts won't cost that much.
    Just two cents more...

    Astatic PDC5 Power SWR Meter & Antenna Tuner
     
  6. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    As already said the two cables will interact. The unused cable becomes a coaxial stub and depending on exact length and operating frequency it may even appear as a dead short to the other cable. Tt's one of those crap shoot things whether it happens or not. At the very least the unused cable will present a large value of capacitance (coax cable has a certain amount of capacitance for every foot) from the feedpoint to ground and will grossly detune the antenna system.
     
  7. 1342

    1342 Active Member

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    adjust your radiator for the middle of those two frequencys and then adjust your tap point left or right you should be able to get that way down to flat line in the middle with under a 1.5 top and bottom.i have a maco and it took a while to get it down but when i figured it out the antenna was workable from high 26 mhz up to bottom of 28 mhz the antenna will do it some fine tuning will help.
     
  8. Wire Weasel

    Wire Weasel Senior Moment

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    Don't be afraid of antenna tuners. The loss is unnoticeably minimal and will take take your resonant antenna anywhere you want to go on both sides on the band edges.

    You'll be happy with a MFJ 949 Series tuner. The current model is a 949E. I lucked into a excellent used one on ebay a couple of months ago for $65 There's a C and D model going back to the recent past that are essentially the same. These handle up to 300 watts and have a nice large cross-needle movement that reads power and SWR at the same time.

    Prices are down right now on used gear. Don't pay more than $90 bucks for a used one.

    good luck!
     
  9. W9WD

    W9WD Guest

    I've pretty much abandoned 11 meters. Too unruly and helter skelter.

    But I did get an antenna tuner from a local guy. It's an AEA ET-1 made locally (or at least it was). Next time I get up to the antenna or move it I'll re-tune for the center of 10m and pull the antenna tuner and save it for other bands and antennas
     
  10. linearone

    linearone King of NY

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    or you can make one. all you need is a coily an alligator clip some scrap wire and a variable cap and you can tune in a fence
     

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