Re: Coax to connect between exciter and SB-200
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Hi Bob ¨C I would like to read that too, if you would put it on here or send it to me directly. ? Thanks ! ? John VK6JX ?
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From: [email protected] <[email protected]> On Behalf Of Bob
Sent: Monday, October 30, 2023 8:44 PM
To: [email protected]
Subject: Re: [ham-amplifiers] Coax to connect between exciter and SB-200? Jim, I sent you an email directly. The address copied kinda strange so not sure if you'll get it or not. Let me know. You'll be interested in reading. Not that long. ? Jim, If a 50 ohm coax is NOT terminated into a 50R - j0 load, the impedance will change with length. Somewhat like using a 1/4 wave 75 ohm coax to match/change impedance of certain antennas. Now some will say that the impedance will change but the SWR does/should not. I have a little write up from a feed line expert explaining why the SWR does indeed change, just like your 3 foot to 6 foot example changed the SWR. Personally I've noticed this change more so on ten meters than lower bands. Makes sense because for example a 3 foot piece of coax is a larger percentage of a wabvelength on ten meters than on the lower bands. ? On Sun, Oct 29, 2023 at 2:43?AM Jim VE7RF <jim.thom@...> wrote: Watch out with length of coax between xcvr + input of the amp.? ? On all 4 of my Drake L4B amps, if coax is 3' long, input swr is flat on all bands.? ?But when I use a 6' jumper, input swr increases to aprx? 1.4:1? or? 1.5:1? ? and no amount of slug tuning will reduce it.? And the Q of the tuned input PI networks on the drake amps is a helluva lot higher? vs a SB-220.? (cap values are higher) ? On my big hb amps, they all use a manually tuned, PI tuned input.? Same deal, but at least I can tune it dead flat.? ? I still have not figured out why the interconnecting coax length affects? input swr.? ? Jim? ?VE7RF
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