My Rock-Mite 40M

Discussion in 'Homebrew and Kit Projects' started by KB3YLI, Apr 7, 2012.

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  1. KB3YLI

    KB3YLI Ham Member QRZ Page

    Hey guys,

    After thinking about getting into Ham Radio for a couple years, I finally got around to taking the exam and now I'm licensed! I just find it fascinating that I can solder something together and use it to talk to people hundreds of miles away. I haven't used this transceiver yet (still waiting on my dipole to come in the mail), but I just finished building this Rock-Mite 40M QRP CW rig. It was a relatively cheap option, and it definitely fulfilled my need to apply some of my college physics courses. I'm almost finished learning CW, so hopefully my antenna will come soon.

    Really worked on my soldering skills putting this together! Drilled some holes into an altoids tin for the enclosure.

    Quick question: if an antenna isn't connected to the radio, does the radio transmit any sort of signal if I practice with my key?

    Thanks and 73,

  2. K7MEM

    K7MEM Ham Member QRZ Page

    I'm not trying to be snarky but it is called a "transceiver" not a "code practice oscillator" (CPO). If you use it as a CPO you should connect a 50 Ohm dummy load to the antenna terminal. I believe that Radio Shack has, or use to have, 50 Ohm 1-2 Watt non-inductive resistors. Just wire then across a connector and plug them into the antenna terminal.

    But it's also pretty easy to build a separate CPO for learning CW. There are dozens of CPO drawings available on the internet. For example here is one, at, that uses parts that are available at Radio Shack. You can get a little pre-drilled prototype board at RS. There are many more just like it on the internet.

    When you finally put it on the air, you will need a lot of patience. When you are very low power, it often takes a bit of work to make a contact. I have a HW-8, HW-7, and 40 Meter Vectronics TX/RX for QRP work. They are a lot of fun to use, but they are often a lot of work. Just don't get discouraged when no one seems to be answering. Sooner or later you will be heard and someone will give you a call.
  3. AB9LZ

    AB9LZ Ham Member QRZ Page

    Yes, but make a dummy load first. Hopefully your nearby rat shack has a couple of 200 ohm half watt resistors, tie four of them together in paralell and hook them up to the antenna jack. When you transmit, you should be able to hear the "sidetone" in the headphones.

    Btw, you could fabricate a nice portable antenna with a long piece of lamp cord. Split the cord partway so each leg is approx 33 ft (use the formula) and use the unsplit part as the feedline. This works much better than you might think .

    Have fun with your little rig, i usually listen for those when the nasty psk guys aren't plugging up the bands with thier howls.

    73 m/4
  4. KB3YLI

    KB3YLI Ham Member QRZ Page

    Lampcord? That's pretty cool. I guess the main reason I'm interested in amateur radio is hearing the stories of WWII soldiers building radios out of scrap bits and pieces, just whatever was laying around, and I figured, heck I could do that too. I still don't quite understand how antenna's work, but that's why I'm getting into building my own equipment, I want to be able to understand all this stuff!
  5. AB9LZ

    AB9LZ Ham Member QRZ Page

    A lampcord dipole works out to an approx 75 ohm impendence depending how high it's hung, a close enough match for most rigs, including the rock mite, to work just fine without any need for an external tuner. I've used one several times while backpacking with an sw+, I've since switched to an end fed halfwave, but only because it's easier to hang (lampcord is pretty bulky too, space is at a premium in my pack). All in all I've managed about a dozen blue ridge to europe Qrp qso's with the lampcord.

    73 m/4
  6. WD5GWY

    WD5GWY Ham Member QRZ Page

    Nice first build! As others have stated, build some sort of dummy load before keying the transmitter. If not, even
    at such low power levels, you could damage the output transistor keying it without an antenna or a dummy load.
    Patience is the key with QRP!!
    Have fun!
  7. KO6WB

    KO6WB Premium Subscriber QRZ Page

    I just cringe everytime someone says they are buying a dipole antenna. It's just wire and sometimes you add a balun (it will work without one) and the feed line. The measurement to make a 1/2 wave dipole is 468/F = Feet. F is the frequency in Mhz. From this formula you can get the diminsions of every dipole antenna that is a 1/2 wave. When you get the wire measured fold it in half (or if you want to be picky, measure it) and cut it. The length is usually a bit too long for immediate use. You would trim the length to obtain a low SWR (1.5:1 is good). Don't forget you need to have the center and the ends insulated from contact with anything. Your insulators can be anything non-conductive, pieces of plastic are a good source, PVC pipe works and you can buy them. Just use your imagination. Zip cord can vary in impedance, one QST article measured it 100-110 ohms.
    There is a good source of information you can download at; Everything is in metric so you would need to covert it or use a metric measuring system.
    Hope this helps
  8. AB9LZ

    AB9LZ Ham Member QRZ Page

    It's a good thing I don't bother with QST anymore, else none of the stuff I do would actually work ;)

    My impedance bridge (it's homemade, so may not be perfectly accurate) measured my lampcord antenna under a typical deploy... thrown over a low branch, at 67 ohms.

    If you liked the rockmite experience, you may want to try the sw+, it has a good filter a usable vfo and enough additional power to make it a daily user. I used one from a London hotel room over a three week period (used an end fed wire thrown over the edge of the balcony), was able to work russia, north africa, and lot lot of middle eastern countries that i don't normally see. I made a jig so i could power it from my work provided laptop battery.

    73 m/4
  9. KO6WB

    KO6WB Premium Subscriber QRZ Page

    The QST article was really a long time ago. It was kind of amusing. It started off with the measurements of various zip cord then it announced "Anybody that needs a 110 ohm feedline this is it!!". Then it asked "So who needs a 110 ohm feedline?". There were fewer manufacturers of zip cord back then and it was usually made the same way at each plant. The zip cord today is all over the place. So yes you can get different impedance just by looking for zip cord that has different spacing or insulation. There is one place where a 110 ohm feedline is good and that would be on a full wave loop. Zip cord works well on the lower bands but as you go higher the losses become very high.
    Have fun
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