600 watt HF Linear Amplifier Project

Discussion in 'Homebrew and Kit Projects' started by KD8UYQ, Feb 9, 2017.

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  1. N1OOQ

    N1OOQ Ham Member QRZ Page

    Nice clean layout, and plenty of room to work in there. Love the (external) look of it so far.
    50A shunt? Yikes. High power ain't for sissies.
  2. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Looking good, my only concern is your lack of internal shielding.
    The one thing you have going for you is the control boards are mounted at right angles to PA.
    Potentially the fun is about to begin, this where the performance of the breadboard meets the complications of the compact system level design.

    Great project, I love it.
  3. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Thank you for the compliment, N100Q.

    Hi Sue,

    Yes mam, I had been thinking about shielding too. I have a piece of mu metal that will be mounted on the bottom panel at a 90 degree angle to that metal plate and parallel to the front panel to shield the displays from RF.

    The control lines to the Low Pas Filter Diplexer board just provide ground connections to the relays for each bands filter. On the mock up board the PA board and LPFD were close to each other and in parallel lines to each other. I had no RF interference issues on any band but we will see if that holds true with everything inside of a 11" x 12" box.

    I have a few more connections to complete before I move to the shielding. Keep an eye on me and please point out anything that might be questionable.

    Remember, I am making this all up as I go along.

    AF6LJ likes this.
  4. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Good to hear...
  5. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hello again,

    The RF shield for the displays in the front panel is done. Before I tackled that challenge I need to complete a few more connections between boards on the bottom panel and assemblies inside of the cabinet.

    Wiring bottom boards.JPG

    These cables are dressed such that they fold neatly out of the way when the bottom is closed.

    The RF shield was first cut from cardboard using the trial and fit then try it again technique. Once the template was finalized I traced the outline on a piece of mu metal then cut, drilled, and folded it into shape.

    RF shield.JPG

    This shield is secured to the "L" bracket with two 6-32 screws and the bottom panel with three more flat head 6-32 screws.

    Front RF shield mounted.JPG

    RF Shield bottom view.JPG

    Another task checked off of the list.

    AA7QQ likes this.
  6. WW1WW

    WW1WW Ham Member QRZ Page

    Nice work. On the subject of shielding, have you seen any birdies from the switch mode supply? I was forced to move mine to a separate enclosure to get proper isolation from the receive path.
  7. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Hi Woody,

    With everything setup on the mockup board I didn't notice any birdies in either the Kenwood TS-120 or the Icom 7300 that I was using for test radios. I can't say yet if there are issues with the boards inside of a metal enclosure.

  8. WW1WW

    WW1WW Ham Member QRZ Page

    Sounds like you are in good shape. I was using the 3000 watt version from Mean Well so yours is probably quieter, also my amplifier draws about 6 amps of idle current since I don't gate the bias. If you do see any interference try loading power supply input and output with type 31 clamp-on beads.
  9. KD8UYQ

    KD8UYQ Ham Member QRZ Page

    Thank you for the tip Woody,

    6 amps of idle current......wow.

  10. KD8UYQ

    KD8UYQ Ham Member QRZ Page


    I was putting off this next part of the build because I knew it was going to be tricky. The "it" is the amplifier bypass relay. With the amplifier off the transmitter's output is feed directly to the antenna via this relay. Turning on the linear amplifier's AC power switch energizes the relay such that the output of the transmitter is feed to the amp's input protection board. What made it tricky was getting the relay with all of the coax cables attached placed in a very narrow space between the copper heat spreader and LPF board. The first step was prepping the relay.


    I needed the coax cables to be very flexible to fit in the narrow space between the PA assembly and the LPF board plus freely move when I opened up the bottom panel. I decided that RG 58/U would best be suited for this application. I would have liked to use RG 8X but it couldn't make the tight bends required in this situation. Plus RG 58 works well with the SO-239 RF shield cups that I used on the RF input and output connectors.


    It is hard to see but the RF output connector is at the bottom corner of the chassis and the RF input connector is located above it. There are four coax packed into that corner of the chassis. Here is another view of the two RF connectors.


    The relay stuck in the corner.


    Finally the back of the cabinet. I tested the relay and it makes a very satisfying click when I power on the amplifier.


    Until next time.


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