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Author Topic: SWR Readings  (Read 257 times)
N3EON
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Posts: 21




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« on: April 17, 2008, 07:09:29 AM »

I just removed my external antenna tuner from my line as I don't think I need it with a Kenwood TS-570 and an Ameritron AL-572 (I used to have it on bypass, so I was really not using the tuner at all. I also use an M2KT-34, four element tribander. My SWR on 20 and 15 is very good and consistent with or without the amp on.  On 10 meters, the SWR reading with 100 watts is about 1.2 to 1.  However, when I apply amp power, the SWR goes to 1.5 to 1.  Is this common or is there an issue I need to address?  I'm not particularly concerned about that reading because the AL-572 can more than handle the higher SWR. My power is slightly lower on 10, but still in good shape.  My question is why isn't my SWR consistent on 10 meters, with or without the amp, than on the other bands.  Any comments would be appreciated.  Thanks
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WA3SKN
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« Reply #1 on: April 17, 2008, 07:23:44 AM »

It sounds like the input circuitry of the amplifier is not a perfect 50 ohms, probably because of a little "stray capacitance".
Unless the coaxial cable is terminated in it's characteristic impedance, the impedance will vary with line length.
73s.

-Mike.
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W2AEW
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« Reply #2 on: April 17, 2008, 07:41:46 AM »

As Mike said - when you have the amplifier off, it sends the RF signal around the amp circuitry, so your rig is "seeing" (is loaded by) the feedline and antenna.  When the amplifier is on, the rig "sees" (is loaded by) the input circuitry of the amp.  The amplifier output "sees" the feedline and antenna as a load.

So, the reason that you see a difference with and without the amp, is that the rig has a different load in each case.

Of course, all of this assumes that you are making your SWR measurements with the rig's SWR meter...
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N3EON
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Posts: 21




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« Reply #3 on: April 17, 2008, 07:53:19 AM »

Thanks guys for the feedback. Your comments make sense. I'll leave well enuf alone as the SWR loss in the amp's output power is minimal and will not make any difference to the one receiving my signal.  
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N6AJR
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« Reply #4 on: April 17, 2008, 01:01:01 PM »

if you hit the tune button, the radio will tune to the amp input, and then you will be matched into the tuner.

it is not unusual for an amp to fsll off on power when you get to 10 and even 15 m.
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N6AJR
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« Reply #5 on: April 17, 2008, 01:01:31 PM »

make that matched into the amp....
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N3EON
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« Reply #6 on: April 17, 2008, 01:42:21 PM »

N6AJR - Do you mean using the TS-570's internal tuner as a means to reduce the amp's SWR?  
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AA4PB
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Posts: 12844




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« Reply #7 on: April 17, 2008, 05:32:50 PM »

The only SWR that really means anything is the SWR on the feed line between the amp and the antenna. That's where you should have your SWR meter located - between the amp output and the antenna.
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