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Author Topic: Tuning a linear into a dummy load  (Read 5190 times)
W9KDX
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Posts: 770




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« on: March 21, 2013, 09:06:18 AM »

Well, I am stumped.  I had been led to believe that you needed to tune an amp into the antenna due to the fact that the antenna had characteristics quite different from a dummy load. 

And yet, I am contimually reading comments and rants from people who impy that an amp can be tuned into a dummy load, often from someone commenting on a LID who can't afford a dummy load.

So, which is the case?  And, assuming that a tuner was used and the SWR was reduces to 1.1:1, would the answer be any different.
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Sam
W9KDX
K6AER
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Posts: 3516




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« Reply #1 on: March 21, 2013, 09:47:25 AM »

Most amplifiers that are tuned into a resistive 50 ohm load will put out about the same power providing you antenna load is not to much on the capacitive or reactive side of 50 ohms.  Loads of 45-55 ohms will be a close enough match for the amplifier to be happy. It is when the amplifier starts to look at loads from 25-100 ohms that the power output will be reflected back to the tank circuit and depending on phase relationship you can have excessive high voltage (arching) or heat dissipation problems.

I furor over hams tuning into the antenna is from an interference perspective.  A lot of hams peak their amplifiers right on the DX station thinking every last watt of power will enable then to break the pileup.  The guy on the other end will not notice 500 or 1000 watts (3dB) when the noise level of stations calling is +20 over S9.

Take the time to write down your tune and load settings for the bottom, mid and top of each band.

Some times depending on the antenna like a SteppIR the load for a particular frequency my change from day to day. Then just move to a clear frequency to do you amplifier tune. The settings 50 KHz away will change little when you move back to the DX frequency.
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N3QE
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« Reply #2 on: March 21, 2013, 10:28:23 AM »

On the "1.1:1"... how do I know that a SWR meter is correct and there is actually is a match, unless you have a 50 ohm dummy load to check against? Circular logic, my favorite kind :-)

More often here, I see folks saying "I cannot transmit for even a few seconds with 1.5kW amp into my 1kW rated Cantenna", kind of a lame-o excuse if you ask me. I have a little air-cooled dummy load here, thirty 1.5K 2W resistors in parallel, and I put bursts of 600-800W into it all the time. So I'm quite certain the Cantenna will take legal limit for a little while without problem.
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K4RVN
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Posts: 772




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« Reply #3 on: March 21, 2013, 01:09:08 PM »

The antenna can be used by finding an empty spot near the QSO not on top of it to be considerate.  A dummy load in my view is useful to eliminate equipment problems such as a transceiver, amp, or coax and antenna issues. Most antennas will not be exactly 50 ohms impedance while most dummy loads will be close to 50 ohms. If you are asking should you tune on a dummy load, then reconnect to your antenna without retuning, my opinion is no. You need to tune on your antenna but not interfere with someone on a frequency.I do hear operators new and old timers tuning up right on top of qsos because they either don't know better or don't care. It only lasts for less than a minute usually which I can tolerate without saying anything. I find that my views are along the lines of what K6AER posted previously.

Frank
« Last Edit: March 21, 2013, 01:16:10 PM by K4RVN » Logged
W1QJ
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« Reply #4 on: March 21, 2013, 01:38:50 PM »

A cantenna will easily hold the power in the time it takes to load up the amp privided you know how to load the amp.  After you load into the dummy load a QUICK touch up taking about 10 seconds will readjust the amp for antenna that has a low swr.
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G3RZP
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« Reply #5 on: March 21, 2013, 03:19:43 PM »

W1QJ is totally correct. Do as he says in this respect.

73

Peter G3RZP
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W1QJ
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Posts: 1446




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« Reply #6 on: March 21, 2013, 03:43:41 PM »

W1QJ is totally correct. Do as he says in this respect.

73

Peter G3RZP

Thank You Peter.  This 800lb Gorilla knows a couple things. Hi Hi  Bananas anyone?
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W9KDX
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« Reply #7 on: March 21, 2013, 05:22:41 PM »

OK, it sounds as though my conclusion was correct; to accurately tune for a particular antenna, you need to tune using the antenna.  So no one thinks otherwise, I already have a dummy load and the tuner takes care of my SWR and impedance matching.

Given that I already have the the amp settings for all my bands, my tune up takes less than 10 seconds so I'll keep doing what I have done all along, off the transmitting frequency of course.

Thanks
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Sam
W9KDX
AH6RR
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Posts: 803




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« Reply #8 on: March 21, 2013, 07:43:34 PM »

Why not just tune up in the pile up Grin the only one listening there is the DX station and with hundreds of callers it will never be noticed Cheesy otherwise use the dummy load first. I have a home brew Cantenna that I made from a Navy Surplus SWR/ Watt meter/Dummy load that used 8 50W non inductive resistors that are immersed in mineral oil. You can key 1KW for 12 minutes before it starts to get warm. If you cant tune it by then you should sell your amp. I use it to test amps after repair and I have put 1.9 KW into it for 5 minutes before it started to get warm.
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G3RZP
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« Reply #9 on: March 22, 2013, 01:30:25 AM »

One thing you do need a good dummy load for is you have to start troubleshooting.
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W9KDX
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Posts: 770




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« Reply #10 on: March 22, 2013, 06:26:29 AM »

I should have mentioned; I always tune 20M on the official tune up frequency, 14.313.  Wink
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Sam
W9KDX
KG4RUL
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« Reply #11 on: March 22, 2013, 07:27:55 AM »

A cantenna will easily hold the power in the time it takes to load up the amp privided you know how to load the amp.  After you load into the dummy load a QUICK touch up taking about 10 seconds will readjust the amp for antenna that has a low swr.

A good idea as long as that "quick" 10 second touch-up is NOT done on an actively used frequency.  I think that is what most advocates of the dummy load method are against.
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W5CPT
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« Reply #12 on: March 22, 2013, 08:24:30 AM »

In response to the argument that "My amp tunes differently into my antenna than it does into a dummy load." my Elmer, W1ARE (SK) would tell you to fix your antenna!

Clint - W5CPT -
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K8AXW
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Posts: 3839




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« Reply #13 on: March 22, 2013, 09:28:04 AM »

CPT:  Sounds good at first blush but what happens when you tune to a frequency off the resonant point of an antenna? 

Now you're back to the original question.  The only answer is to have your amp pretuned as much as possible and do a very quick touch-up on a clear frequency.
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K4RVN
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« Reply #14 on: March 22, 2013, 10:33:13 AM »

Sam I know what that means about 14.313.  Just recently learned it when I called CQ on 14.314. Believe it or not I had a good qso with someone in Canada who gave me some good info on my Ten Tec transceiver. I stay off that freq. now. I was surprised later when I heard the language used  on 14.313

Frank
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