uBITX Transceiver semi-kit build

Discussion in 'Homebrew and Kit Projects' started by KD8UYQ, Aug 14, 2018.

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  1. W0VRA

    W0VRA Ham Member QRZ Page

    I don't wanna get too far off @KD8UYQ's exquisite build, but my sense is that a lot of legitimate experts there are overreacting a bit, and it's brought a real downer on that group.

    Farhan's argument about the issue was that the harmonics would still be way less than a conforming 100W rig, which is true, and some operators are thinking "spirit of the law" v "letter of the law".

    And I don't think anybody's gonna be knocking on anybody's door about the harmonics.

    Mine is dead in the water because of shoddy cheap components (a relay that was sticking) and my lack of desoldering skills replacing it but I'll be working on it more this weekend. The exploding TDA- audio chips was another example of shoddy components.

    That said, I have been thrilled to get on HF with it while it lasted and I have learned A LOT about radio from it.

    I have been a big recommender of it here, but now I'm a lot more hesitant, and am looking forward to QRP Labs' QSX.
    N6ALT likes this.
  2. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Ripley again,

    Took some time today to do two tone testing on the uBITX transceiver. The exercise was to measure the SSB verses CW harmonics across the bands that I frequent and what I see largely jives with what others have reported.
    80 meters; SSB 2nd and 3rd order harmonics are at or below -43dBs. The CCW 3rd order harmonic is about 5dBs higher then spec.
    40 meters; SSB the 2nd order harmonic is 3dBs above spec and CCW is way up there. The third order harmonic is -33dB.
    20 meters; CCW is good but SSB 3rd order harmonic is about 3 dBs over spec (-40dBs).

    So here is the thing. I am not going to worry about it. I am a relative new comer to the hobby and had no interest in CW operations. Now that I have been a ham for a while that is starting to change. I am trying to learn code but I have a long way to go before I can even think about using a code key. Thus removing half the objections for using the uBITX on the air. Also, you could consider me as one of those guys that hang out on one frequency on one band 90 percent of the time. I am not a DXer or Contester. I haven't tried my the digital modes yet either. The attraction to the hobby for me is the technology. Specifically building and/or fixing things .

    I am going to proceed with the build and operate most of the time on 75 meters, occasionally on 40 and 20 meters. The power that I will be putting out operating QRP on SSB mode will be low with the harmonics, even above FCC specs, a wispier of a wispier. Please don't get me wrong. When I was build my 600 watts SS linear amplifier last year I made sure that it met or exceeded the FCC regs.

    Amp with wood sides.JPG

    40m Spectrum 28w drive.jpg

    However, I am curious to see if strategically placed shielding will have any effect or as Ashhar had suggested, tweaking the idle current for the IRF150s to null out the harmonics on 20 and 40 meters. One thing that I am not going to do is add any microphone gain. Two tone testing with a speaker and my home brewed uBITX hand mic showed how easy it was to overdrive the radio.

    Two tone test.JPG

    Running up the speaker volume from the two tone generator sitting on top caused the bead pattern waveform on the scope to flat top big time above ten watts. Seven watts looked to be optimum for my set up.

  3. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    The question remains, what about the 140 watt PA kit that you ordered from CCI. I am still going to build it an use it with the uBITX docking station. All my efforts to filter out the harmonics will be done there.

    KC8SBV likes this.
  4. N7EKU

    N7EKU Ham Member QRZ Page


    Good job on the testing. I wish I had a nice spec. analyzer like that! But 2-tone testing is very important too and can reveal a lot so I use that for testing my stuff. It's easy as all one needs is an oscilloscope (which doesn't need to be a fast one either) and a tone source which is easy to find on the web.

    Of concern on the kit for the higher bands are transmit spurs that are both above and below the operating frequencies (which can't be seen on an oscilloscope so needs a look at with an SA). It looks like 17m and above will need not LPF's but BPF's. This seems to depend on the kit and the gain of the individual transistors supplied. The main problem is too much gain in the transmit IF chain and it needs to be lowered as both mixers are overdriven. There is lots of information about fixing these things on the group list.

    I don't think the people on the list are being too hard on it. According to statements from the designer, essentially the kit was not designed as a final refined product, but as something for people to learn and experiment with. Some users are already designing outboard filter modules that can be made to work with the kit's controller. As it stands maybe the kit is not too bad to be used on its own unless people try to get max power out. And if one wants to use it with an external amplifier, then things like the spurs become even more important.


  5. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hi Mark,

    Thank you sir. I don’t operate on the higher bands (17 meters and on up) mainly because those bands are dead most of the time at my QTH. However, I did look at the 15 and 10 meter bands with my spectrum analyzer in CW mode, maybe I just lucky but they were the cleanest of them all. My issues are all on the lower bands as described in my previous post.

    I agree with your assement of the uBITX transceiver. It was designed to be a good development and experimenters platform. A community grew from it and to me, that is what is so great. Individuals adding their own twist to the design, finding problems and then sharing ideas as to how to address them. What could be better?

    If you are looking for a $140.00 semi-kit that is equivalent to a commercially available HF transceiver out of the box the uBITX ain’t it. I can tell you this, I am having a grand time experimenting with this thing.

  6. W0VRA

    W0VRA Ham Member QRZ Page

    SDRplay has beta analyzer software for there boxes. I have not tried it yet.
  7. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello again,

    This is the 80 meter snapshot of my uBITX prototype fixture for SSB and CW.

    Harmonics 80 meters.jpg

    The yellow trace is the output into a dummy load at approximately 10 watts SSB mode. It looks OK. On the other hand the purple trace is the same physical setup, slightly higher output power (14 watts) in CCW mode. Lots of harmonics and the odd ones exceed the -43dB spec for FCC compliance.

    Here is a look at the 40 meter band.

    Harmonics 40 meters.jpg
    Once again the yellow trace is SSB, Purple is CW. This time single side band 3rd order harmonic is 41dB down from the fundamental. The 5th harmonic -40dB. Close but no cigar. Look at the 3rd order harmonic for CW. Thus all the discussion on the uBITX support groups.

    I read somewhere that RF shield might help. I experimented with aluminum foil covers with a connection to ground. The first one that I made covered just yhe LPF section on the main board but as I expected it made now difference. Next I made one to cover the PA section with a ground connection.

    Foil hat on PA.JPG

    Bad idea! I though I had blown something up with all of the unpleasant noises coming out of the speaker while transmitting in CW mode. Everything was fine when I removed the foil hat and please note that there is plastic tape placed on the edges and inside surfaces so nothing was grounding out. Even with the ground connection removed and the foil cover just placed as shown above. Dark magic happens.

    RF Shield on and off.jpg

    40 meter band; CW mode. The yellow trace is without the foil hat on the PA section and the purple trace is what you get when it is just sitting there with no connection to a chassis ground. Now I am wondering what a happens when the uBITX board is put in a tight fitting all metal case.

  8. N6ALT

    N6ALT Ham Member QRZ Page

    KK5R likes this.
  9. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello Mark,

    I ordered four of the 45Mhz crystal filters myself today. I am also going to do the second relay mod to the LPF section of the uBITX main board. In the mean time I am working on a A/B switch box to compare different microphones to the mic element that comes in th uBITX kit. I want to see what different microphones do to the spurs on 15 and 10 meters.

    Oh, and the PA board kit from CCI arrived yesterday.
  10. K5MIL

    K5MIL Ham Member QRZ Page

    Who are you ordering these 45 mHz filters from?

    Bill - K5MIL

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