2M Strip line

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by K4CKB, Nov 29, 2014.

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  1. K4CKB

    K4CKB Ham Member QRZ Page

    Question: under what conditions does a flat strip over a ground plane begin to act like a strip line? I'm struggling to get a 2M tank circuit to work. It's a 8 inch x 1 inch flat line, 1 inch above and 2 inches below ground planes (chassis). It forms the L of a parallel LC tank circuit for a pair of 4CX350As, with the far end of the strip line grounded. It's link coupled to the output through a link parallel to the line. The problem is that the tank isn't resonant anywhere near the 2M band (although this is a well- documented circuit that's been sucessfully built many times). When I "look" at the line with my grid dipper, it looks resonant at ~85 MHz, exactly what I would expect if I were calculating the inductance of a flat strip over ground, but about 3 x what I'd expect for a strp line from all the calculators I've used. Thoughts? Any help would be greatly appreciated.

    73
    K4CKB
     
  2. W0AAT

    W0AAT XML Subscriber QRZ Page

    Are the tubes connected? Tube capacitance will shorten the line considerably bringing resonance up to 2m...
     
  3. K4CKB

    K4CKB Ham Member QRZ Page

    They are (through the coupling cap). I was calculating using 11pf as the Cout of the 2 4CX350As, an estimate of 7pf for stray capacitance, and I fabricated a variable cap with ~3pf of center-range capacitance - 21pf total. I calculate the Xc to be ~52 ohms at 144Mhz. I then fabricated the strip line for an Xl of 52 ohms at 144. After trying various width strip lines from 1in to 3in with no resonance anywhere near 2M. I settled on 1 in. wide and fab'd a sliding ground so I could vary the line length. To my surprise, I had to get down to less than 4in to reach resonance. I felt that was clearly the line acting as a regular inductor. Calculating this length for a strip over a ground plane (rather than as a strip line) yielded an Xl of the correct amount which matched the resonance. It was not acting like a strip line, or the Xl would have been much lower (by a factor of ~ 3) - hence the reason for my initial question. Btw, I cannot apply B+ to the end of the line opposite the tubes, because the far end of the stripline needs to be grounded for this to be a parallel LC tank (the Xl of the line at resonance cancels the the Xc which is in parallel) I need to calculate the series reactance of the coupling cap, but I'm guessing it's insignificant.

    thanks for the response - any other ideas? This one really has me stumped!

    73
    Chris
    K4CKB
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Check your other post in Q&A.
     
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