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Matchbox On 160 Help




 
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Author Topic: Matchbox On 160 Help  (Read 470 times)
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wa2fxm
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« on: September 21, 2022, 01:39:57 PM »

I'm having problems with drifting SWR with my Kilowatt Matchbox modified for 160 meters. It has been suggested that the added doorknob capacitors are heating up and changing value. This is with a Viking II, 70 watts output on CW. The drifting seems instantaneous on key down. I've replaced the originals with another set of 5KV doorknobs and then a set of 10KV "mushroom" (similar shape as in the Yaesu 2100 amp) type capacitors. The drift still happens. I have used the Matchbox successfully on 160 with QRP rigs so the heating idea sounds plausible. Other than trying to get some air variable capacitors shoehorned inside the Matchbox are there any other specific types of capacitor that would work better? Or any other possible causes of the drifting?

Mark - WA2FXM
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W2JBL
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« Reply #1 on: September 21, 2022, 08:09:24 PM »

Don't know what your antenna is but it sounds like the RF current at the tuner is very high, like feeding an 80 meter dipole with open wire line on 160. Your problem is not the voltage rating of the caps, but current. Doorknobs are designed to handle high RF and DC voltages at relatively low current. You need caps that can handle high RF current. Try some mica transmitting caps in place of your doorknobs. They are all over Epay and dirt cheap. 200 watts carrier was 7.5 amps! 
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wa2fxm
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« Reply #2 on: October 25, 2022, 12:07:33 PM »

Picked up a couple of 300pF micas at NearFest. So far they seem to be stable and doing the job. Thanks for the info.

Mark - WA2FXM


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W3SLK
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Just another member member.


« Reply #3 on: October 25, 2022, 06:22:28 PM »

Also remember by adding more capacitance to the coil you will be raising the Q of the circuit significantly!
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Mike(y)/W3SLK
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