Illegal high power HF amplifiers

Discussion in 'Amateur Radio Amplifiers' started by N8FVJ, May 20, 2019.

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  1. LU6PSG

    LU6PSG Ham Member QRZ Page

    Ed,
    Those power figures are for saturated, tuned, and sometimes pulsed operation (FM Broadcast, ISM), not linear AB untuned boradband suitable for AM and SSB, which will leave you with only 700W for 50V supply, and and around 1250W for 65V supply, both using a single device using fixed 9:1 output circuit. For dual devices, using 16:1 transformation, linear output limits are 1290W and 2200W respectively. Anything above, and compression starts to show up, and IMD start to climb to unhealthy levels.
    Regarding the FCC, there is no IMD specification for ham use. In this regard, a splatterboxes might be legal, but violates good amateur practices. Many FCC approved rigs, like the FT857 have terrible IMD emmisions (splatter)
     
  2. WA4SIX

    WA4SIX Ham Member QRZ Page

    Well, I surely will keep from any compression. I just want a healthy & clean amp that can take some abuse. I will keep this in mind during tuning.

    Ed
     
  3. KA2BLG

    KA2BLG Ham Member QRZ Page

    Years ago when I was REALLY active, I wanted to step up to an amplifier. I saved up a bunch of money... and spent it on antennas. Never owned or operated with more than my 100W barefoot rigs.
     
    KU4X likes this.
  4. K4YNZ

    K4YNZ Ham Member QRZ Page

    Antenna 4 to 8 db gain both ways. Mostly 10-20 meters. Very hard to get antenna gain on 30-160 meters. Stacked mono band antenna cost more than my amp. Amp gives me 13 db gain.
    So to overcome path loss you use gain. Antenna gain, power gain and pre-amp gain. Reduce feed line loss.
    Now some just do not like amps!
    This is a hobby. to each his own.
     
  5. W2VW

    W2VW XML Subscriber QRZ Page

    Your transceiver contained a 100 watt amplifier. You should have removed that and spent even more money on antennas.
     
    AF5XF, K5PO, AG5CK and 1 other person like this.
  6. LU6PSG

    LU6PSG Ham Member QRZ Page

    Even without compression, all commercial LDMOS amps are only partially linear up to 7MHz, maybe 14MHz if carefully hand crafted on critical areas like bifilar feed choke.
    Above this frequency, all LDMOS amps start to behave like class D current mode and class E, generating lots of IMD.

    I have been designing (not copying) LDMOS amps during the last 6 years, but just for fun. If I wanted to sell you an amp, I would tell you that LDMOS is the very best and cleanest technology ever... But sadly, this is not true for a broad band topology on such a low drain impedance.

    You want a clean amp, go vacuum tubes. LDMOS were just ment to work tuned, not broadband. Unless your rig has the option to use predistortion, which solves all IMD problems.
     
    Last edited: Dec 26, 2020
  7. K6LPM

    K6LPM Ham Member QRZ Page

    It really is beneficial to setup with decent antenna first before installing an amp. Otherwise it is like placing the cart in front of the horse.
    Too many inexperienced hams are installing amplifiers yet are using a G5RV as an inverted V fed with RG59 or RG8x coax or an ended dropped out of the upstairs apartment window.....
     
  8. K6LPM

    K6LPM Ham Member QRZ Page

    I think that it just goes by way of the nature of each device, in the fact that a vacuum tube is a voltage amplifying device and a solid state device is a current amplification device.
    Also are not most all amateur amplifiers are not operated willy nilly broadband such as the CB mosfet amplifiers but are band switched...
    At least as the output filter networks require some sort of matching .....
     
  9. LU6PSG

    LU6PSG Ham Member QRZ Page

    Nope.

    Both vacuum tubes and mosfets in general are basically the same: current sources driven by voltage (hence transconductance is the "gain")... and both are square law devices (non linear). Both can also be linearized using negative feedback. Bipolar transistors (not mosfets) are current amplifiers and are more linear, but these are no longer used for power RF.

    Regarding output filtering/band switching, this will eliminate harmonics, but will DO NOTHING for IMD/Splatter as these can not be filtered once generated. In fact, CB amps are single band tuned, not broadband. Even push pull CB amps are tuned by using tuning capacitors on both the output circuit and the output transformer. The real problem is overdriving. This can create tons of IMD, no matter how well tuned is your amp. And this becomes a problem for HF amps also, as they are usually overdriven and operated class D or E to reach full power above 7-14 MHz, and this also creates IMD.
     
    Last edited: Dec 28, 2020
    K6LPM likes this.
  10. K6LPM

    K6LPM Ham Member QRZ Page

    I stand corrected , thank you for clearing up my understanding. So what is needed for pure hf multiband linearity would be some sort of adjustable or switchable output transformer?
     
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