ATU power loss

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  • Updated 3 years ago

My antennas have SWR's of less than 1.5:1 and when I click on ATU it tunes says "success" and the indicated SWR on the bar graph is a little better but the power output measured on my Alpha 4501 s always about 18-20 percent less than without the tuner... is this normal for a 6700???

Tnx

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AH0U

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Posted 3 years ago

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Chris DL5NAM

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Only to understand right: for what reason do you need a tuner in TRX and a tuner in Amp at same time?
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Paul

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I believe the Alpha 4501 is a lab grade power meter, not an amplifier
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Ross - K9COX

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All tuners have some degree of power loss. 18-20% is something less than 1db (26%). You probably don't really need a tuner to drive the Alpha however if you do just increase the drive level slightly to compensate for the slight loss.
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Jim Gilliam

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Remember that the forward power as read on a meter is the power generated by the transmitter plus the value of the reflected power. So the forward power will always read higher when there is an SWR.


Jim, K6QE

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Paul

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Jim, true but since the reflected power is travelling in the opposite direction to that of the forward power I believe it should be *subtracted* from the power produced in the transmitter (not added). So the external meter reads lower than the internal meter by an amount which depends on the VSWR. If the VSWR is unity the two should differ only by an amount equal to through loss of the ATU +/- the accuracy of the external instrument and the metering in the transmitter.
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Jay / NO5J

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Keep in mind that the external meter and the radios internal metering are sampling at two different points along the feedline, so they aren't seeing the same power and SWR values.
And both might be correctly displaying power and SWR accurately, and still display different results. 

73, Jay - NO5J
(Edited)
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Jay / NO5J

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Ok maybe I can skip the full presentation for a bit. but I'll redirect my efforts toward finding/blaming the losses in my shack. 

I remembered another issue that hasn't been mentioned ...
there are mismatch losses due to the SWR, and also cable losses thru the cable with that SWR in existence on the  cable. along with ATU losses inside the ATU all to be figured in.

In my shack I've got @30 feet of mixed RG8/RG8X coax and a 1.14:1 mismatch looking at my dummy load. Oh yeah, and 10 "perfectly hand soldered" PL259s, coax switches, and a bypassed Amp, wattmeter, and Tuner between the radio and the dummy load.  

Using online cable loss calculators, I can blame a 6 watt loss on @30' of copper. I know that each termination will produce it's own reflection. The  switch in cable types from RG8X to RG8 will also produce one.
After letting the 6500/ATU find a match.

Using Flexmeter.exe to take the readings inside the rig.

I  see 87.07 watts of forward power
I see .87 watts of reflected power 
I see a 1.14:1 SWR mismatch

With the ATU bypassed.
I see 93.39 watts of forward power
I see 2.94 watts of reflected power
I see a 1.3:1 SWR mismatch 

So at first glance it looks like I get better power output with a mismatch.
or maybe ... somethings been miscalculated in the software. But which piece of the software. Flexmeter or the radio firmware. or is it just measurement error.

I'd expect to see higher forward power into a mismatch, it's the matched
readings that make me go HUH?

So somethings not quite right somewhere.

Not all that important to me because, my amp requires me to turn down to @70 watts output from the 6500, otherwise I trigger it's protection circuits.

I still think I'm overlooking something simple though!

The mystery remain mysterious!

Definitely worth figuring out, and applying the effort that the calculations might require.

73, Jay - NO5J
(Edited)
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Jay / NO5J

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More Google research, reminds me that Power and SWR measurements are not accurate when there's an impedance mismatch between the source, load, and transmission line, So SWR measurements of anything other than 1.1:1 when feeding a 50 ohm load, from a 50 ohm source, thru a 50 ohm transmission line, mean that whatever forward and reflected power readings might need to be taken with a few grains of salt. and that with my  1.14:1 SWR mismatch that seemingly small mismatch, is causing the losses in Forward power and also the increase in reflected power. Ok, I kinda knew that too. The point I was, well, ignoring is that with a mismatch, your SWR meter is likely inaccurate, and also the point, that a dummy load spec of "lower than 1.5:1" might mean that it's impedance is anywhere between 50 and 75 ohms. And that it might be the complete cause of the inaccurate readings, the only thing it's really good at is converting power to heat, while adding uncalibrated losses.

So! Last months project was to place an order for 50' of RG8X, and 20 new PL259 and UG176 connectors. Which arrived this week. I plan to re-patch and re-shuffle all the equipment on the desk to create a spot for a Maestro. Hopefully I'll do as good, or better job as I did last time. and maybe remove some losses, by removing some coax length. 

Once I'm done with this project, I plan on Swearin off, SWRs fer good!.

Maybe, not knowing, is the best approach.

73, Jay - NO5J
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Paul

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Mismatches are the *cause* of reflections without which there would be no standing wave and no VSWR to measure.

There are some excellent papers on the Bird web site which discuss VSWR measurement and errors in great detail.

Priciples discussed at;
http://www.birdrf.com/~/media/Bird/Fi...

Directivity error discussed at;
http://www.birdrf.com/~/media/Bird/Fi...

My conclusion is that in practice, as long as the transmitter protection isn't backing off the output, the match is as good as necessary and the actual figures are superfluous. We're sure we've maximised the power into the antenna as best we can for our particular system.
(Edited)
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Jay / NO5J

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Paul

I'd still like to figure out which of my wattmeters is closest to the truth.
I suppose it's probably going to be the PowerMaster II, I paid the most for that one. It comes pretty close to Flexmeter.exe, most of the time.

Thanks for those links. The whole point of this exercise is better understanding. The truth is out there, might as well use that!

73, Jay - NO5J  
(Edited)
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Jay / NO5J

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Paul

I'm beginning to suspect that, the reality is that you can get a reasonable power measurement from any of my meters, if you allow for the stated accuracy tolerance, and perform the test at the same point in the circuit. then all I need to do is decide which measurement I like the most.  and then sell the rest of them.

73, Jay - NO5J