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Author Topic: switching antennas with amplifier without retuning?  (Read 6801 times)
KX2P
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« on: July 11, 2012, 08:59:48 AM »

I'd like to know if I should be able to (without hurting anything) switch between two different dipoles that are both approximately SWR 1:1, on the same frequency, without retuning my AL 572 amp? My guess is the amp should see them both the same?
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WX7G
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« Reply #1 on: July 11, 2012, 09:10:41 AM »

There will be no problem doing so.
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AA4PB
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« Reply #2 on: July 11, 2012, 12:01:07 PM »

Just don't switch antennas while you are transmitting  Roll Eyes
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KX2P
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« Reply #3 on: July 11, 2012, 02:49:35 PM »

Thanks guys.

Jeff
KX2P
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RUSS324
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« Reply #4 on: July 25, 2012, 06:09:05 PM »

Why not put the amp in standby and check each antenna for SWR then check amp settings into a dummy load. Tha is the safe way. When in doubt don't. Good luck.
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KM3F
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« Reply #5 on: July 25, 2012, 10:51:02 PM »

Here is a tip about changing frequency and antennas.
Take some time and uses a noise bridge to find the best match settings.
Look for the Cin and Cout settings that will only need a change in L when F is moved.
What that means is if you plot F vs Tuner setting for L, only L has to be changed.
You may find out that L setting is a linear function vs F such that all you need to do is dial in the L and be right on and never have to touch Cin and Cout per band once you do the testing and plot the data to see it.
Works for me everytime.
Without extended tuning time to go to a new frequency.
Good luck.
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W1QJ
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« Reply #6 on: July 26, 2012, 04:31:34 AM »

Here is a tip about changing frequency and antennas.
Take some time and uses a noise bridge to find the best match settings.
Look for the Cin and Cout settings that will only need a change in L when F is moved.
What that means is if you plot F vs Tuner setting for L, only L has to be changed.
You may find out that L setting is a linear function vs F such that all you need to do is dial in the L and be right on and never have to touch Cin and Cout per band once you do the testing and plot the data to see it.
Works for me everytime.
Without extended tuning time to go to a new frequency.
Good luck.

Great idea, but it does not answer his question. He mentioned nothing about antenna tuners.  He says he has 2   1 to 1 swr diploes  which he does not need a tuner for at all.  Maybe someone else could use that advice.
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K8AXW
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« Reply #7 on: July 26, 2012, 07:20:20 AM »

I've always had a problem with the concept of first tuning into a dummy load and then switching to an antenna.  I've never seen an antenna that didn't require retuning.  So the point of tuning into a dummy load first seems lost to me.

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AA4PB
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« Reply #8 on: July 26, 2012, 07:45:14 AM »

The antenna shouldn't be very far from 50 Ohms (especially if you use a tuner) and only require some minor tweaking of the amp. Doing the major tuning with a dummy load keeps your tune signal off the air for the most part.
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KM3F
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« Reply #9 on: July 26, 2012, 11:31:23 AM »

Maybe this isn't addressing the op first posted question but extends to others who maybe reading and have different setups.
A dummy load may or may not be a pure 50 ohm termination as they are still frequency sensitive to some small degree on the upper bands.
Unless you test the termination with your meter  from 160 to 6 meters you don't really know what you have.
At a reflected reading of less than 1.1 to 1 and depending on the quality of the meter and how it is designed, you may still not read accurate.
Trying to split hairs on accuracy makes little difference to normal operating at reflected values less then 1.2.
For an easy read and example, suppose you have 2 antennas both reading 2 to 1 in match and feel you can change over from one to the other without retuning an amplifier.
Fact is you 'may' not and still be dialed in.
Why: for easy understanding, suppose one antenna is 25 ohms, the other 100 ohms in match.
They will both show very close  to 2 to 1 on the same SWR meter. Any difference in reading is due to the meter circuit reaction to the reactance sign (X sub C or X sub L) as the case may be.
An tuned amplifier tank sees  these reactances differently.
Only way you would have saw it is with a antenna analyzer or recognized the amplifier tank tuning is different on each antenna.
As far as a dummy load goes it pays to have one that is 'reasonable' for testing and trouble shooting as long an you know what you should be seeing with each use and application.
Using a tuner in line changes the setup and the results to get to a match but again you have to pay attention to what is happening when you make the adjustments.
Present day plug and play without this attention causes questions to asked and then even the answers may not be understood in the absence of technical understanding of  AC reactance.
I have been around this 'horn' many times and it is even more critical at 2m and above having custom built a near flat dummy load for as high as 450 mhz just for reference, equipment, transmission line, and antenna load comparison  testing alone in attempts to put together the best weak signal station at those frequencies.
Key to it all is understanding all the possibilities that influence what you see so you recognize when a parameter needs to be changed or improved or just understand what you have.
Good luck.
« Last Edit: July 26, 2012, 11:48:41 AM by KM3F » Logged
W4VR
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« Reply #10 on: July 26, 2012, 12:25:58 PM »

I do that all the time with my two 40 meter beams.  One has a slight SWR and the other does not.  Just make sure the amp is not putting out power while you're doing the switching.
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KA6MLE
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« Reply #11 on: July 26, 2012, 03:08:17 PM »

I have wondered about this very thing... incidentally I have the same amp. I guess the only way to see if there much of a difference is to fully tune into your dummy load and make note of the settings.. Then completely retune into the antenna and see what the difference is in settings. You would pretty much know for future reference after that
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K7KBN
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« Reply #12 on: July 27, 2012, 02:55:16 PM »

I do that all the time with my two 40 meter beams.  One has a slight SWR and the other does not.  Just make sure the amp is not putting out power while you're doing the switching.

You have an antenna that has no SWR?  How's that work?
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73
Pat K7KBN
CWO4 USNR Ret.
W1QJ
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Posts: 1438




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« Reply #13 on: July 27, 2012, 03:00:40 PM »

I do that all the time with my two 40 meter beams.  One has a slight SWR and the other does not.  Just make sure the amp is not putting out power while you're doing the switching.

You have an antenna that has no SWR?  How's that work?

I'll bet it works pretty good Don't you think?
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AA4PB
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« Reply #14 on: July 27, 2012, 03:17:43 PM »

Every antenna feed line has an SWR (Standing Wave Ratio). It may not have any standing waves, which means it has an SWR of 1:1  Cheesy
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