Which Tuner to build and why?

Discussion in 'Amplitude Modulation - AM Fans' started by N6YW, Jul 17, 2017.

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  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    I am looking for ideas to help make a decision which tuner design to build, and more importantly, why.
    The reason I am building one is simple. I need to be able to flatten the line when I QSY to a frequency
    where my antenna is no longer resonant. For example, the new antenna I am building and erecting soon
    will be coax fed without the use of a balun. It's a 3 band fan dipole for 75/40/20 meters and the 75 meter
    antenna will be cut for resonance at 3850 kHz. I move between the Extra Class portion to the General Class
    portion daily. The 40 & 20 meter bands will not need likely the tuner at all, so it will be switched out of circuit
    when using those bands.
    So, considering my requirement for band usage, the real reason for doing this is to allow my Bauer 707 to
    be happy at full power. The Johnson KW Matchbox isn't capable of handling a folded dipole and ladder line
    at full power... it arcs. I don't want arcs, so I am building the new antenna and ditching the open wire feed.
    The components I have chosen for this antenna can withstand 10KW with ease but that's because they are
    on hand and of the highest quality. I just want to build a tuner once and forget about it and never worry about
    voltage breakdown etc. I will not have to be matching complex loads way off frequency either like attempting
    to run 160 meters on a 40' ft whip, crazy stuff like that. And, I will not be using ladder line, just coax.
    Which design would you suggest? Pi-L, T network etc, and why?
    I am very curious as to the suggestions and the discussion ahead.
    Thank you in advance!
  2. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    The fewest parts are the best.
    How about an L-Network.
    Since you are using coax, no need for balanced line and a link coupled tuner.
    The L-Network should be able to match the range you are talking about without excessive loss.
    You only have one part that has to be isolated from the chassis (or ground).
    I don't see using a PI or T as having any advantage.

    N6YW likes this.
  3. N2EY

    N2EY Ham Member QRZ Page

    I suggest something different: Broadband the 75 meter antenna.

    This can be done several ways:

    1) Use multiple conductors of the same length (a "cage") for the 75 meter part.

    2) Use multiple conductors of different lengths (a "fan") for the 75 meter part.

    3) Use parallel wires not connected to the main wire

    4) Other ways.

    But, if it's a Transmatch you want, I suggest the classic Ultimate Transmatch. See QST for July 1970, page 24.

    73 de Jim, N2EY
    N1BCG likes this.
  4. K4KYV

    K4KYV Subscriber QRZ Page

    A simple L-network worked for me when I fed the Gates into a coax-fed antenna. I had a switch to transfer the variable capacitor to either end of the coil depending on what part of the band I was operating.

    One caveat: make sure you NEVER feed the transmitter into the tuner without the full antenna load on the tuner. The circulating RF current to the unloaded L-network (and probably any other type of tuner) will likely burn out any thermocouple RF ammeter you have as a line current meter. I found that out the hard way twice. I burnt one up when the antenna changeover relay failed due to a broken connection. The second time, I forgot to re-engage the antenna feedline after I had disconnected it before a thunderstorm.

    I added an in/out switch for the meter, and now I never take a line current reading until I know for sure the transmitter is close to loaded properly.

    I got rid of coax feed altogether and went 100% to open-wire feeders. Now my antenna system is coax-free, not a sprig of coax anywhere between the transmitter tank coil and the radiating element of the antenna.
    VK2WP, AF6LJ and N6YW like this.
  5. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    That's good advice from all of you, and Don, yes... I would hate to burn up anything especially an RF Ammeter.
    So, if the antenna is a bit long I would switch the cap to the input? Also, does it matter which part of the cap goes
    to ground, stator or rotor? I really need to hit the books on this subject so I can understand the subject better.
    AF6LJ likes this.
  6. K5UJ

    K5UJ Ham Member QRZ Page

    It's pretty simple: Balanced load => balanced line => balanced matching network.
    Unbalanced load => unbalanced line => unbalanced network.

    Keep things simple, no baluns etc. You were on the right track. The KW MB arced because it didn't have the spacing needed between the cap rotor and stator plates. the balanced line concept is fine. ditch the fan dipole, go with one dipole 1/2 w. at lowest frequency, make a high current high v. balanced feed and build a link coupled tuner with 10 KV bread slicers and copper tubing. You just need what you had only beefed up. Everything bigger spaced farther apart. You need to start rounding up big ceramic spacers and insulators, copper strap and punch tools, brass hardware copper tubing etc. Mount the tuner on a big shelacked wood board. big bowl feed throughs. you get the idea. Don't over think it.

    If the dipole has a weird pattern at 2 wavelengths put up a second 1/2 w. dipole for the high part of HF and switch them with one of those mil. surplus giant DPDT knife switches.
    N6YW likes this.
  7. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    My problem is that I don't have enough room here to get the antenna high enough and the balanced feeders long enough. The idea is to have a self
    resonant antenna on 3 bands and switch in the tuna when needed on 75 meters. I suppose it would be better to have it a bit longer but still, the idea
    of using coax makes sense for my installation. The 450 ohm window line sucks when it rains and having to chase the tuner settings is a real drag
    when that happens. My last antenna was the classic 130' Doublet fed with 4" wide spaced #14 AWG stranded THHN. I had nothing but problems
    because of the length dilemma. Then again, all of this is trial and error so I'll never know unless I try.
  8. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Tell me about it, I have the Window Line Blues (there is a song in there someplace) every time it rains.
    KA9JLM likes this.
  9. KM1H

    KM1H Subscriber QRZ Page

    Why not just run two parallel fed dipoles, one for each band segment? When trimmed a classic W VSWR curve will result with ~ 150 kHz 2:1 points for each if you take it to the edges. Separation can be about 20 to 90 degrees. Ive used 90 for decades for low end CW and the middle of SSB DX area, and the VSWR is still a manageable 2.5 at 3885 without any stinkin tuner or added coax loss. There is no intelligent life above 3900 anyway.:rolleyes:
    A side benefit is both sets of wires take power so there in no noticeable directivity/nulls. Another dipole for 160 runs down the middle since I do have the room:D and the combo also kicks ass on 30 and 17M without a tuner.

    AF6LJ likes this.
  10. N2DTS

    N2DTS Ham Member QRZ Page

    Ignore moderate swr with a tube rig.
    At those low frequencies the loss will be low with good coax.

    Use thick wire, like #10, it gets very broad banded if the antenna is full size.

    If cut for the center of the phone sections, you should be fine.

    My 40 meter standalone dipole got up to 3 to 1 swr and I had no problem loading the 4X150 rig into it.
    People at the far end said there was no difference from the low swr fan dipole on 40.
    The 40 meter dipole also had a longer run of coax on it.

    TRY it without a tuna first...

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