# 2M Strip line

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

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1. ### K4CKBHam MemberQRZ 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. ### W0AATXML SubscriberQRZ Page

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

3. ### K4CKBHam MemberQRZ 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. ### WB2WIKPlatinum SubscriberPlatinum SubscriberQRZ Page

Check your other post in Q&A.