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Author Topic: stripline swr meter  (Read 4242 times)
K8ZG
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Posts: 5




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« on: May 01, 2013, 09:50:00 AM »

I picked up a cheap Dentron SWR-1A swr meter at a hamfest last weekend. The schematic of a Dentron tuner that uses the identical circuit and PC board can be found here:

http://www.hayseed.net/~jpk5lad/Assorted%20Ham%20Stuff/Dentron%20AT-3K%20Tuner/AT-3K_Schematic_Final_Rev1.JPG

Why did they put the 3.9pF capacitors across the input and output of the stripline? All of my design experience with directional couplers tells me this makes no sense.

Steve K8ZG
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G8HQP
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Posts: 120




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« Reply #1 on: May 01, 2013, 12:49:19 PM »

Probably to adjust the characteristic impedance of the tails which connect the stripline to the sockets. With no screen immediately adjacent, these tails would otherwise have a higher impedance. The small caps are a lumped approximation to whatever distributed cpacitance is missing. This will improve the match of the whole meter at the upper end of its frequency range.
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WA3SKN
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Posts: 5434




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« Reply #2 on: May 02, 2013, 05:43:03 AM »

"Stray capacitance"
They are being used to minimize the affects of stray capacitance, and increase the useable range of the meter... and yes, it increases the high end frequency limit in the meter.
73s.

-Mike.
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WB2WIK
Member

Posts: 20542




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« Reply #3 on: May 02, 2013, 02:25:22 PM »

I picked up a cheap Dentron SWR-1A swr meter at a hamfest last weekend. The schematic of a Dentron tuner that uses the identical circuit and PC board can be found here:

http://www.hayseed.net/~jpk5lad/Assorted%20Ham%20Stuff/Dentron%20AT-3K%20Tuner/AT-3K_Schematic_Final_Rev1.JPG

Why did they put the 3.9pF capacitors across the input and output of the stripline? All of my design experience with directional couplers tells me this makes no sense.

Steve K8ZG


It makes sense only at higher frequencies.  Obviously 3.9 pF across a 50 Ohm line at 3.5 MHz is doing absolutely nothing.  But as the stripline and its connective wiring becomes less perfect (at higher frequencies) it can require some reactive compensation to try to pull it back to 50 Ohms Zo.  I'd leave them there.
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