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Author Topic: Radio to Amp SWR question  (Read 2207 times)
N4NOO
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Posts: 106




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« on: May 11, 2009, 05:05:24 PM »

I have an Icom 7000, SGC SG500 amp, Hi-Q 6/160 RT/MC-3-H and Better RF tune system in my car.  It all works great and I get great reports.  However I do have a question.  I tune the the antenna with the amp off and I get a 1.1 SWR reading from the radio.  I then turn the amp on and I am seeing an SWR of up to 3:1 on the radio when talking.  What gives?
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ZL4IV
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Posts: 44




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« Reply #1 on: May 11, 2009, 05:31:02 PM »

I don't know the amp but it sounds like it does not have a tuned input but just a near enought match. There are input matching devices on the net you can fiT. I am sure you will get a more detailed answer but I am guessing thats what it is.
ZL4IV
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WB2WIK
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« Reply #2 on: May 11, 2009, 07:52:49 PM »

First, if the amp is putting out full power, this doesn't matter in the slightest.

The SWR reading on your IC-7000 is trying to indicate the match looking into your amplifier, which does not have a tuned input, but is passive.  Frankly, unless something is wrong with the amp, I doubt the reading is accurate.

You might try a different (independent, not in the IC-7000) SWR bridge and see what it indicates.

If the SWR looking into the amp is 3:1 but the rig still generates enough power to drive the amp to 500W, this doesn't matter, anyway: Irrelevant data.

However, again, I'm guessing the IC-7000 SWR indication is probably incorrect and should be verified by a separate instrument.

WB2WIK/6
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KM3F
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« Reply #3 on: May 13, 2009, 05:20:50 PM »

If--IF your radio has auto match feature, use it when the amp is in the key up position and see if it settles the issue.
I can do this with my KW 480 and works very well to match to a 1 to 1 at least on the radio's indicator.
In my opinion it it poor practice to design an amp with an untuned input for mulitband use but it's only my opinion.
I have done both and see the difference in drive requiements as well as possible IMD and harmonic output.
Keep in mind that IMD mainly comes about differently than harmonic outputs but both have a little of each in the end result.
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WB2WIK
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« Reply #4 on: May 13, 2009, 06:05:02 PM »

>RE: Radio to Amp SWR question  Reply  
by KM3F on May 13, 2009  Mail this to a friend!  

In my opinion it it poor practice to design an amp with an untuned input for mulitband use but it's only my opinion.<

::It's common practice with solid state amplifiers.  Most are so easily driven you can swamp the input with a noninductive resistor to provide a great match to the exciter all the time and still run full power from the amplifier.  The FET amps I've built have so much gain I need to use a 6-10 dB resistive attenuator on their inputs to prevent severe overdrive from a typical HF transceiver; that gives them a nearly 1:1 input VSWR all the time no matter what the drive or power levels are.

WB2WIK/6
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KM3F
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« Reply #5 on: May 14, 2009, 07:32:02 PM »

That is true.
My transverter uses a 50 ohm 30 watt noninductive termination and an adjustable level off to top end to the mixer.
Min input level from the radio is 5 watts.
The transverter only needs a half watt or less to drive it full out.
My solid state amp is a near perfect match to the transverter.
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