Discussion in 'Amateur Radio Amplifiers' started by WA4SIX, Jul 3, 2017.

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  1. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, here I go buying junk on Ebay again. But, the price was too good to pass up, and they are my LDMOS of choice, BLF278s.
    I have 8 pallets coming. All LDMOS tested & guaranteed. I will use 4 pallets for a 1.2KW 2M amp.
    These pallets are rated 120-200 Mhz (I am sure I might have to tweak a bit) 300W.
    This may be my first water or freon cooled amp.
    Now, for splitters/combiners....
    I am looking at Wilkinson S/C & probably using 75 ohm TL transformers (Just 1/4 wave*VF) & the proper resistors for isolation. I would probably use RG302 or just Quad shield RG6. Both have the same voltage rating. I cannot find the wattage rating for the RG-6. Also, the RG6 quad shield does have a better Db loss rating at 1Ghz.
    Would I be better off using lumped components? Sure would be smaller. If so, I could use a bit of help here.

    THank you,
    Last edited: Jul 3, 2017
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd use the Teflon cable instead of any kind of RG-6 for this.

    For TL combiners why not follow what Jim W6PQL has already done?

  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well. I nèd a 4 port combiner set
    I could do strip line but would rather use tl or lumped.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    For four ports I've used three 2-port combiners, works fine.
    AI3V likes this.
  5. WA4SIX

    WA4SIX Ham Member QRZ Page

    That is my intention. If I try lumped, should I just use the inductance & capacitance per foot, or a transformation calculator from 50 ohm to 100 ohm?
    Example: http://www.mantaro.com/resources/impedance-calculator.htm

    Thank you,
  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    OK, using an impedance transformation for 100 ohm to 50 ohm at 146 Mhz, I get 54.505 nh series inductance & 10.901 shunt capacitance.
    For the inductor, #16 AWG gets me .2"D .203"L & 3.99 turns.
    Do you think I can fudge it to a 10pf cap? 11pf is an odd number.
    If I try this, can I put an MFJ 259C on the single input & 2 matching 50 ohm dummy loads at the output & sweep it?
    I don't have access to a network analyzer.

    Thank you,
  7. WA4SIX

    WA4SIX Ham Member QRZ Page

    OK, now thinking about it, I may swap the capacitor to the series & the inductor to the shunt. If so, I can make a test unit in a PL259 with a 100 ohm terminating resistor, to see if it works properly. Then, wind the inductors to test/tune them on the Boonton 250. I am looking at 1.5KV capacitors.

  8. K7JEM

    K7JEM Ham Member QRZ Page

    According to my calculations, changing to a 10pf cap will change the output impedance to 53.9 ohms or so.

    You could build a HV coaxial capacitor out of a few inches of Teflon coax, trim it to anything you want.
  9. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, I did find some 11pf 1KV SMT caps. I will start experimenting soon.

    THank you,
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, that's what W6PQL does (caps made from open-ended sections of PTFE coax).
    WA4SIX likes this.

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