Weird problem tapping 10 meter tank coil

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Feb 8, 2020.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    Need some advice from some of the more experienced amplifier/homebrew guys. I’m building a homebrew gs-35b amp. I’ve had good luck tapping the low bands....160 80 and 40. I’m using my antenna analyzer into the output s0239 with the relay keyed and resistors equal to my calculated plate impedance from anode to ground. The low bands were easy to tap so I thought I’d try 10 meters......haven’t attempted 15 or 20 yet. A very weird thing is happening with the SWR on my analyzer trying to do 10. The SWR is continually up and down really jerky like. It’s almost like a bad connection or such. I cannot find a point at which the SWR stays low indicating resonance, it’s just all over the place jumping up and down mostly when making adjustments to the load cap. Has anyone run into this before? It kind of acts like a bad connection, but I know for a fact it isn’t at the bandswitch itself. The high band coil is wound using 1/4” copper tubing and I’m using 1/4” copper strap from the band switch to the tank coil. Any suggestions. This kind of has me baffled.
     
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    On
    One more thing I’ll add is that I’m just wrapping the strap around the copper coil in spots in my attempt to find where it needs to be. The copper tank coil isn’t very tarnished or anything and neither is the strap. I do have the strap actually soldered at the bandswitch. Could this weird SWR thing be because I’m not actually soldering the strap to the tank coil? All other connections are done and known good except the connection to the tank coil itself
     
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sure, it could be that. Copper, like aluminum, oxidizes instantly when exposed to oxygen (air).

    Of course, it could be something else, also.

    I've built a lot of amplifiers, several of which are published articles, but never used an analyzer...just formulas.
     
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    Anything else that you can think of that could be a possibility? I’m gonna check and try a few more things like lightly sanding/polishing that copper and maybe actually soldering it
     
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sure, I can think of "lots of things," but none of them may be the problem.

    One is, analyzers aren't perfect.:) When you change the part of the spectrum involved, they can change how they work, including being interfered with by local signals having field strength that exceeds their +10 dBm maximum output, so the detectors create errors.
     
    WQ4G likes this.
  6. KN4CTD

    KN4CTD Ham Member QRZ Page

    I have an LCR meter that will read capacitance and inductance. I don’t have a dip meter. Should I just measure the inductance of the tank coil/coils and just tap them as per the pi network software I’m using?
     
  7. N8CBX

    N8CBX Ham Member QRZ Page

    What is the Pi's circuit "Q" for 10M? You may have found a tank resonance with a very high Q? Maybe add a little more inductance and try again.
    Jan N8CBX
     
  8. W1QJ

    W1QJ Ham Member QRZ Page

    I asked a question and never got an answer. What is the minimum capacitance of that tune cap? That could be your problem. The minimum may be more than you need throwing everything off.
     
    KK4RSV and AG5CK like this.
  9. AG5CK

    AG5CK Ham Member QRZ Page

    From the pictures I remember the case was pretty small. The anode capacitance of the tube may be too high for 10m if it's close to the sides.

    You shouldn't need more than 75 to 100 pf on the tune cap for 10m. Just thinking out loud.
     
    KN4CTD likes this.
  10. KN4CTD

    KN4CTD Ham Member QRZ Page

    Lou, the minimum on the tune cap is 23pf. Based solely on this software with a plate voltage of 3900vdc @ 1 amp and using 1.7 as a constant, I’m getting 2295 ohms to use from anode to ground. At 28.500 MHz, the value of the tune cap is supposed to be 26pf. The value of the load cap should be 139pf and the inductor tap point should be 1.32uh.....which equates to about 5-6 turns on this coil I wound out of 1/4” tubing
     

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