1296 lobuck station build

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KK4YWN, Nov 13, 2017 at 6:46 PM.

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  1. KK4YWN

    KK4YWN Ham Member QRZ Page

    I was thinking about a 4-way stack of quagis since they're easy and cheap to build. Thoughts on these?

    I have an SDR play for reception. Also have a LNA. Haven't found any filters yet. Recommendations welcome, otherwise I'll get around to googling.

    What to do about feedline? 1/2 hardline losses are quite high. I could possibly get the feedline down to 15 ft if the antennas can fit in the attic (i know, not ideal).

    I'm not-yet concerned with TX. I want to verify that there is some activity around before going full-bore into this band.

    Thanks for any guidance.
     
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Many on 23cm and the higher bands (especially the higher bands) put the "whole station" up at the antenna and just feed DC power up to it and baseband signal at some lower frequency down from it (and also up to it, for transmitting -- using a transverter) and that takes care of the cable loss problem.

    Here's a cool example: The "box" mounted on the mast above this 70' tower actually contains several transverters, SS power amplifiers and preamps for several UHF-SHF bands. One big DC power cable leads up to it (about 120' long or so) and one ordinary coaxial cable like RG-213/U leads up to it also; the signals up/down the coax are at 144 MHz so there isn't much loss in the 120' or so run and the RF end going to the antennas which rotate along with the "box" are 432/902/1296/2304/3.4G/5.7G/10.3G.

    The antennas are small, but they're up pretty high (around 75-85' above ground) and feedlines to the antennas are very short: The longest one is maybe 10' for 70cm; the other lines are shorter, in the 3' range. Since the "box" with all the electronics in it rotates along with the antennas, no "rotator loops" or anything are required.
    upload_2017-11-13_15-40-0.jpeg

    This setup is at N6NB's QTH in Orange County, CA pretty near me.
     
  3. KK4YWN

    KK4YWN Ham Member QRZ Page

    Cool stuff. Certainly not cheap though :)

    i might get away with putting the sdrplay in the attic. USB 2 specifies operability to 5m. A powered hub makes a cheap repeater. Or i could bite the bullet and purchase an amplified cable.
     
  4. W0AAT

    W0AAT XML Subscriber QRZ Page

    Loop antennas are easy and cheap on 1296. You do have activity down there but it will be during tropo conditions and contests. Rest of the time dead air...
     
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    In some places (maybe many places) there are VHF-UHF-SHF "activity nets" one night a week, which lights up those bands for an hour or two at least weekly.

    Here in So. Cal. we have those, and the net control changes by band but is always someone well equipped; when I lived in NJ, we always had them there, also -- the "Packrats" (Mt. Airy VHF Club) members used to run them and even on 23cm there would usually be 20-30 check-ins from all over the place (1296.100 SSB). Net control was usually well equipped and well sited so he could hear everybody, but not everybody could hear each other -- the challenge for those of us participating would be to tweak stations 1 dB at a time (1 dB at 1296 is easy at the beginning and way harder once you're doing well) in an effort to hear "the other guy" next time.

    UHF-SHF is unlike anything on HF, although for "challenge," 160m probably comes closest. 23cm VUCC is like 160m WAS, and 23cm "100 grids" award is like 160m DXCC.:p
     
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    If you know what you are doing, a 1296 station can be assembled for a very "low buck".

    The station that I put together as a penniless EE student in about 1980 consisted of this:

    • Receiver: Homebrew copy of the Microwave Modules MMc 1296, 2 MRF901 RF stages in front,
      3 resonator interdigital image rejection filter. Existing 144 MHz converter+surplus receiver
    • Transmitter: Two varactor triplers in series (a "no-no" if I had known how difficult they were to align),
      from an existing 25 W 144 MHz VXO controlled transmitter.
    • Antenna: 2 stacked G3JVL loop yagis, description from DUBUS. Amphenol BNC coax relay salvaged from scrapped maritime VHF. Feedline 3 m of 1/2"
    Total investment wasthe equivalent of about $40 in 1980's money.
    The varactors were the base-collector junctions of BLY93 and 2N3375 power transistors, which turned up as surplus a year before.

    A similar station using modern components and building blocks would be perfectly possible to build today, but it would be nice if there was some activity...

    73/
    Karl-Arne
    SM0AOM
     
  7. WB2WIK

    WB2WIK Premium Subscriber QRZ Page


    Might work for CW, but for any other mode probably won't.

    x9 the modulation bandwidth, doubt it would work on SSB and for sure a 5 kHz deviation signal on FM would now be 45 kHz deviation that nobody could discriminate.
     
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    It would not work on either AM, SSB or NbFM.

    But the mode of choice for weak-signal work then and now is Morse or CW. Also, in my case the NBFM deviation was continously adjustable down to zero, so if I have had needed to work a station on "phone", I would have turned down the control to +/- 0.5 kHz deviation or so.

    Actually DJ4ZC and PA0EZ came up with the concept of "constant amplitude SSB" where the SSB IF signal was hard-limited to remove the envelope fluctuations. Then the IF was divided with the multiplication factor (in this case 9) and mixed back to the original IF for further mixing to 144 MHz.

    This modulation scheme was in principle very narrow FM, but sounded remarkably like SSB when tuned in on a not too selective receiver.

    A variant, where the amplitude envelope was remodulated on the signal after class C multiplication was used in the still operational OSCAR 7 transponder that was the subject of DJ4ZC's doctoral thesis.

    73/
    Karl-Arne
    SM0AOM
     
  9. K6CLS

    K6CLS Ham Member QRZ Page

    (I forget if SDRPlay app will run on a raspberry pi) I'd put the Pi with the SDRPlay in the attic (or in a box on the mast!), and use the Pi Wifi to stream the SDRPlay data to the shack computer, run the decoder/waterfall app there. that's still in the $$$cheap == lobuck range as you mentioned.


    I'm surprised no one has commented on your quagi antennas. I think they'd be fine. I haven't built many quagis but I am a fan. would you build the combiner too?
     
  10. KK4YWN

    KK4YWN Ham Member QRZ Page

    :D

    I say yes. I probably mean no. I've built 144mhz combiners and they worked, but were a tad short. Perfectly acceptable but given the margin for error at 1296 I'm fairly certain I'll build 2 or 3 before giving up and purchasing one :D
     

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