Problem, possible solution, question

As I await the arrival of a new tuner rated for 2kW, my current 600-watt tuner is failing miserably at 250 watts on 20 and 40 meters. Bizarre because an old 100W tuner did fine at 100 watts – never a hiccup.

The new tuner will be rated for higher power but I believe the complex (and widely varying) impedences presented to the tuner will stress it as well, especially since I will be operating at 1000 watts.

So far, the antenna series-of-components has been simple: Transceiver-Tuner-15 feet of RG8-4:1 Balun-70-feet of ladder line up to the 80m dipole.

You can read about what this does to the tuner here.

Here’s a diagram of what I’m going to try – in a nutshell, it simply involves adding an additional balun (of a different ratio), selectable via the A/B switch on the tuner and a remote relay on the output of the baluns.

My question is, what type of relay might be suitable for this application at 1000 watts, 10-80 meters?

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  7 comments for “Problem, possible solution, question

  1. Steve
    June 3, 2011 at 7:36 pm

    You have committed the sin of making the feeder a multiple of a wavelength. 70 feet is just exactly the wrong length! If you make it 90 feet I bet the tuner will be happy with either balun but the 1:1 should be better. Your coax between the balun and tuner must be REALLY SHORT. None of this 10 ft. stuff. Do whatever it takes to keep it short…Like ONE foot. The capacitance of that coax can really upset the impedance presented to your tuner. As for the extra feeder and what to do with it; Make yourself a wood or pvc frame about the size of a small washboard and wrap it around it. Try to space the turns at least 1″ apart and twist them once per turn.
    I have my tuner outside under the antenna. It’s an MFJ-998 and no balun. Yes the feedline does radiate a bit but that is fine with me. I have a large relay that flips the polarity of the feeder and I can often see considerable change in signal strength. I used a large power relay from WW Grainger. It’s coil is 110v and it is designed for 240v. So far with 1.2kw, no problems. 73

    • June 3, 2011 at 7:52 pm

      Thanks Steve – Well it certainly would be cheaper to add feedline and gives me the kick I need to raise the antenna a bit more. I’ll give it a try.

  2. Steve
    June 3, 2011 at 9:32 pm

    It’s amazing how forgiving that ladder line is. I’ve wadded it up in a pile, tossed it on the ground etc. and it works just fine. I just like being neat though.

  3. June 4, 2011 at 1:57 am

    Hi John, I would certainly get rid of the RG58 between the rig and amp. At least use RG213 or if that is too thick use Aircell7 instead. I had some miserable faults and feed-back in the past wit RG58 between rig/amp or SWR meter especally on 10m and 12m and was only using 100-150W then. Imagine what happens if you run a KW. Good luck with the new setup. 73, Bas

  4. June 4, 2011 at 10:14 am

    If you actually decide to use relays for this or future projects, ArraySolutions sells replacement relays used in their remote switches. Rated for power. High voltage ratings are probably around 5KV, but not given on their site. Digging through the parts catalogs might turn up several suitable and similar open frame relays. The HV rating is the critical rating in this application.

    Also the American Zettler AZ755-1C-12DE(SPDT) or P&B RTE24012F(DPDT). Both parts are enclosed/sealed and used by a lot of hams in remote switch applications. Less expensive than open frame relays. Also rated at 5KV[although I'm skeptical].

    The P&B DPDT is rated a 8A per side. When used for RF switching, most folks tie both poles together and use it as a SPDT.

    Most antenna switches expect a matched load, where the voltages are less likely to be very high.

    The potential problem with your idea is the possibility of being at a high voltage node at the switch.

    I agree with both previous suggestions. Jiggle the lengths of the ladder line, and use a well shielded cable between components, maybe even add a choke/isolator on the line coming out of the tuner to the amp if RFI is persistent. Locating the tuner away from the amp and radio might help too if they are now very close together.

    73 es tnx fer Q’s – hope for many more!
    de w4kaz

    • June 4, 2011 at 1:03 pm

      QRO definitely wrings out problems that were invisible at 100W levels. You hit the nail on the head with my concerns of very high voltages possibly being present at the relay’s location. This antenna was never a problem for me at 100w on any band, nor was performance lacking.

      My hopes are that a 1:1 balun by itself will solve my 20/40m problems without migrating them to other bands; if not, experimentation continues…

  5. WB6BYU
    June 5, 2011 at 6:23 am

    Assuming 135′ for the dipole, 70′ for the twinlead, a 1 : 1 balun, and 15′ of coax, my EZNEC model shows
    8 – j50 ohms at 4 MHz, and 4 – j90 ohms at 3.5 MHz. That’s in the range where tuner losses can be
    excessive, though they likely will still be able to get a match. At 3.5 MHz the SWR on the 50 ohm coax
    is over 50 : 1.

    The source of your 40m problem is apparent: at 7 MHz the model gives 0.8 – j25 ohms pretty much
    across the band. How many amps would have to flow in a 0.8 ohm load at the output power you
    are considering? That’s way beyond a reasonable impedance to match with a tuner. (Of course, you
    won’t actually see such a low impedance due to losses in the feedline, but it still points out the
    problem with your installation.

    20m comes in around 1.2 + j32 ohms.

    You’ve got about the worst possible combination of lengths imaginable for those bands. Your coils are
    overheating due to high currents at the extremely low impedances – and a 4 : 1 balun may make it worse.
    The biggest source of problem is the coax – at the balun, sample impedances are much more matchable:

    80m : 250 – j600 ohms
    40m : 200 – j900 ohms
    20m : 70 – j450 ohms

    So one solution is to run the twinlead all the way to the tuner in the shack. Then, if you want to
    switch between a 4 : 1 and a 1 : 1 balun, you can use a set of banana plugs and move the leads
    by hand. But the tuner will be a lot happier matching the higher impedances. (I’d use a 1 : 1 balun
    and not worry about switching.)

    (Note: I don’t guarantee the exact impedances, but they are at least representative of the problem.)

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