High power L-network tuner

Discussion in 'Homebrew and Kit Projects' started by KU0DM, Sep 6, 2008.

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  1. KU0DM

    KU0DM Guest

    Hello, have a question for anyone who wants to answer.

    I am looking into getting a AL-811 amp, but the problem is I don't have an antenna tuner that can take 600 watts. So I was thinking about building an L-network tuner that I could mount on the side of the house and just lean outside to adjust.

    The antenna I am using (at the grandparents farm) is a 320' dipole, fed with about 40' of ladder line to a 4:1 balun, then a run of coax to the shack.

    The only thing is, I am very much an appliance op. However I have a strong urge to learn how to build matching networks and tuners like this, because I enjoy experimenting with antennas very much. There is a ham in the area who has elmered me for a long time who is willing to show me build it, if I can get a schematic (he likes to help you help yourself).

    If you have any plans laying around, or schematics to an L-network that can handle 600 watts that you think would work perfectly for this situation, please post them here, send me a PM, or send me an email! Your choice. :p


    --Duncan, KU0DM
  2. K7FE

    K7FE QRZ Lifetime Member #1 Life Member Platinum Subscriber QRZ Page

    Last edited: Sep 6, 2008
  3. W3JN

    W3JN Ham Member QRZ Page

    Stop messing with baluns. Since you're already running balanced line to the ant (a good strategy by the way!), build a balanced tuna from the git-go. It's much simpler than a T network but probably a bit more difficult to tune than an L network as you need to find the right coil taps but much more efficient than a T or L net and a balun. One home-wound coil and one HV capacitor.

    Here's a drawing of one (although the article focuses on short 160M antennas, the tuner schematic is pertinent to what you want). C2 can be eliminated, in my experience. http://www.amwindow.org/tech/htm/160smallants.htm

    I built one using an edgewound tank coil out of an old BC xmitter and a vacuum variable. It handles anything I can throw at it, full carrier 120% modulated AM. Total cost was $20 - YMMV depending upon ability to score applicable hamfest goodies :D

    Alternatively you could use a Johnson matchbox. The 275 watt version was designed for full carrier AM so it'll handle a KW SSB no problem. I've found ratty Matchboxes at hamfests for well under $50.
  4. AC0FP

    AC0FP Ham Member QRZ Page

    I'm thinking you should "lose the balun" also. Get the TLW program from the ARRL, it will tell you the value's you will need. The tuner evaluator is in the lower right hand corner. ;)

    Last edited: Sep 8, 2008
  5. WA7KKP

    WA7KKP Ham Member QRZ Page

    Easy to build . . .

    I'd start with some #12 or #10 bare wire, and space wind a coil, with taps every other turn or so. You'll need a lot of coil, if you want to run 160 meters. Then find a variable capacitor, 200 pf or bigger, with fairly good spacing on the plates; I'd say at least 1/8" or better. Use a clip lead to select taps, and tune both for minimum SWR. If your antenna impedance is greater than 50 ohms, the variable cap goes on the antenna end of the coil; if it is less than 50 ohms, it goes on the rig end.

    That should be the basics. On 15 and 10 meters, you might be better off using small diameter copper tubing -- 1/4 or 3/8" OD or so.

    Gary WA7KKP

    howdy neighbor . . . St. Joe here!!!
  6. KL7AJ

    KL7AJ Premium Subscriber QRZ Page

    First, I would like to say that anyone who likes experimenting and LEARNING bout things like you do is not an appliance operator. The definition of an appliance operator is someone who DOESN'T CARE, regardless of his license class. (This is the vast majority of the hams I encounter). So, my hat's off to ya!

    The L network is an incredibly instructive project. Network theory tells us that ANY impedance can be matched to ANY OTHER impedance with an L network. Now, sometimes certain impedance combinations might require IMPRACTICAL component values...but the fact is, a match IS possible, assuming you have a large enough assortment of values.

    One thing that I consider an absolute requirement (almost!) in a simple L network is a ROLLER inductor. Though you CAN get by with a tapped inductor, you will seldom get a perfect match at any particular tap...except at a few specific frequencies. (This is the one main advantage of a T or Pi network...you can get a perfect match with tapped inductors.)

    So...beg borrow or steal a good roller inductor. And get the best capacitor you can find.

    You're probably going to want a means of reversing your in and out on your L-network, unless you're always going to be using an impedance that is lower (or higher) than your transmitter impedance. As a general rule, if your antenna is short, you want to have a shunt input and a series output....if your antenna is long, you want a shunt input and a series output.

    This should give you a good start! Keep at it!

  7. M0DSZ

    M0DSZ Ham Member QRZ Page

    This is pretty much what I would do. An L-match won't lose you that extra 3dB or so that some T-match networks would. If you use a permanent coil former make sure it can stand getting a little warm.
  8. KU0DM

    KU0DM Guest

    Thank you all very much!!

    Gonna get crackin' and hopefully get it done in time to play around during the 160 contest! I think I have some time...

    I'll post pics of the finished product if I'm not too busy enjoying it. :D

    Duncan, KU0DM
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