Homebrew Antenna tuner for QRP

Discussion in 'Homebrew and Kit Projects' started by W1JAV, Sep 16, 2011.

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  1. W1JAV

    W1JAV Ham Member QRZ Page

    I was surfing around the internet for ideas on my QRP antenna tunner. I settled on a basic design withe parts I have around the house.I read in the article that for QRP a small poly variable capacitor used as a tuner in older AM/FM radios should do the trick.I have such a old radio and de-sodered the cap. It is a small square with a adaptor for a shaft and knob,but there are three leads 2 solder out of each side,six total.Its just a basic lc circuit and I can not figure out what the leads are for or how I need them all.Any Help or ideas would be great.
  2. AB9LZ

    AB9LZ Ham Member QRZ Page

    While you'll see them used in lots of designs, (and some kits as well) for reasons of being small and cheap... those poly caps are pretty lossy, not a good thing at qrp levels. This is the one place where a metal air variable is worth it's weight.

    Btw, what are you trying to tune? Knowing matters as it will determine whats right for you, i.e. and end fed would want an L match, open feeder a balanced (tuner) etc. In any case the designs remain pretty much the same, parts selection is typically based on power level.

    If it's a portable antenna, a better approach is to try and make the thing resonant so you can eliminate the tuner from the transmit chain. If it's a fixed station antenna, your regular shack tuner may be a better bet as it won't be using the lossy caps mentioned above.

    73 m/4
  3. W1JAV

    W1JAV Ham Member QRZ Page

    Thanks for the reply

    I want the tuner for feeding random wires,I go backpacking and camping alot.I thought that a small tuner would be good so I can just throw a wire in a tree and be able to tune it.I will be using 40 or 20 meter mono band qrp rigs.I really want to build it myself and any suggestions would help,
    Thank You
  4. AB9LZ

    AB9LZ Ham Member QRZ Page

    I'll sound like a broken record here to the regulars, but I too do a lot of backpacking and trailside operating. Remove "random length" from your qrp vocabulary, and go with an end fed halfwave. Look at AA5TB's website for info, Steve has several good articles on how end feds work, how to make a l-match tuner and a resistive swr bridge, and how to get the whole thing to perform.

    73 m/4
  5. WA9SVD

    WA9SVD Ham Member QRZ Page

    Unfortunately, "QRP" operation and miniaturized antenna tuners are sort of mutually exclusive. When running QRP, you would want the MAXIMUM efficiency from a tuner, but miniaturized components (small coils made of small diameter [high AWG #] wire and miniature variable caps) increase the loss in the system, making your QRP signal even weaker. If small size and light weight IS your requirement, realize that a small size tuner will further reduce the efficiency of your station, and hence, produce a weaker signal.
  6. WA4OTD

    WA4OTD XML Subscriber QRZ Page

    IF you can get a VOM that also has capacitance then you can figure out the part.

    As mentioned, you really should get a resonant antenna. Random length wires have....well random performance. Most people would not put up random length antenna at home and with QRP levels you need as much efficiency as possible and you might not even need the tuner.
  7. AB9LZ

    AB9LZ Ham Member QRZ Page

    This is quite true, with my current backpacking rig, the one place i don't skimp is with the tuner. I use a full size receiving type air variable in my l-match over the lossy poly types. You literally can hear the difference with a much sharper peak when tuning for maximun noise while listening... So much so, I could almost do away with the swr bridge.

    73 m/4
  8. W4HAY

    W4HAY Ham Member QRZ Page

  9. W9GB

    W9GB Ham Member QRZ Page

    Hendricks QRP Kits -- Balanced Line Tuner (BLT Plus)

    The BLT is a simple Z-Match tuner that works from 10-40 meters, designed by Charlie Lofgren for NorCal. Charlie is probably the number one man in the world when it comes to Z-Match Tuners.
    The tuner tunes balanced feedlines, thus the name "BLT" for Balanced Line Tuner for the mnemonically-challenged among us.

    Charlie originally designed the tuner to go with the SMK-1 and the NorCal Doublet for 40 meters, but when he delivered his design we found it tuned other bands also.
    With the NorCal Doublet, it tunes 10 - 40 meters. We rate the tuner at 5 Watts, but it will probably be safe at 10 W.

    The BLTPlus comes with several modifications including the coax/balanced inputs,
    and the Dan Taylor, N7VE LED SWR indicator are built in, and is an absorptive bridge, so you won't fry the finals as you tune.

    Great design, by a great designer. All parts for the tuner are included, including the case. It uses polyvaricon caps
    We have upgraded the tuner to now include all of the connectors and switches needed to tune balanced lines, coax and long wires.
    There is no need to do the mod as on the older cases. This kit has had the mods incorporated into the kit.
  10. AB9LZ

    AB9LZ Ham Member QRZ Page

    Not only do polyvaricon caps suck, deploying a doublet after a full day on the trail sucks as well. The end fed simply requires that you shoot the end of the wire over a branch... And yer done. Maybe its just me, but it's about all i have energy for at the end of a long day.

    73 m/4
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