atu performance

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  • Updated 1 year ago

I'm not seeing the matching performance of the ATU in my 6700 that I would expect.  I'm trying to use a G5RV on all bands, which is possible with the ATU in my K3. I realize that the K3 ATU is spec'ed at 10:1 whereas the Flex ATU is spec'ed at 6:1, but I'm seeing cases where the Flex is unable to match an SWR under 5:1.

For example, the ATU is unable to match the antenna at 3.690 MHz. According to my MFJ 259B analyzer, the SWR is 4.7:1. The reactance is 65 ohms resistive and 59 ohms capacitive, which is about 88 ohms, well within the published range of 8.3 - 300 ohms.

Similarly, the ATU will not match the antenna at 28.0 MHz which has a SWR of 4.8:1 and a reactance of 35 resistive and 50 capacitive or 61.

I think I also found a glitch in the SSDR display:

 I have my tune power set to 10 watts. On 10m and below, the tune power slider stays at the 10w level during tuning, but on 6 meters, the slider which is still set at 10w, jumps to zero during the tuning and then back to 10 after the tuning is complete.

When the tuner is unable to find a match, none of the indicators (Success Byp or Memory) light up, but the ATU button is still blue. Clicking the ATU button causes the ATU button to dim and the Byp indicator to light up. What is the status of the ATU if it is unable to find a match? Is it engaged and at the last setting tried?

Before running my tests, I cleared the ATU memories and I ran the tests with the tuner memory disabled.

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Mark Erbaugh

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Posted 3 years ago

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Mark Erbaugh

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I've done some more testing and I'm starting to think there is a problem with the tuner in my 6700. It has trouble tuning out an SWR of about 4.7:1 (according to my antenna analyzer. I disconnected the coax from the 6700 and plugged in the analyzer at the same connection, just to rule out changes in the coax length). In places where the SWR is under 2:1, after considerabke tuning, the tuner sometimes only gets the SWR down to 1.6:1.

Also, I've seen some glitches in the SSDR display during tuning. First, a question. Does the Tune power slider set the auto tune power? I've tried changing that slider and the slider seems to reset to 10w on HF/MF, and 0w on 6m. It appears that the tuner uses a couple of algorithms, at least according to the sound of the relays. Initially, the rhythm is irregular for a few seconds, during which the tuner usually finds a match if it is going to find a match. Up to that point the power and SWR displays seem to be what I would expect. If it hasn't found a match, the relay clicking rhythm becomes a regular pattern, which I assume is the tuner trying every combination sequentially. Usually, but not always, if the relays go to this second rhythm, the tuner fails to find a match. Also at the point the rhythm changes, the power and SWR displays on SSDR go blank.

Would a master reset be in order? I have reset the tuner memories and disabled ATU memory. This is with SSDR 1.5.1.


FWIW, the ATU in my K3 has no problem finding a 1:1 match with this antenna
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Bill -VA3WTB

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I think you should open a help ticket for this....
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Mark, Not all ATUs work well with all combinations of resistance and reactance.  Now I am NOT an electrical engineer and others should be able to chime in on this.   The ATU in the flex probably can not match such a complex combination.  I suspect nothing is broken.. just a limitation of the ATU.

PS - Try adding or removing feed line - 6 feet can make the difference.  Also make sure your not using any "ugly" baluns - this adds inductance thus will changes the rectance component of your impedance.

PSS - ATUs that operate off of processors (ie any modern ATU) are very temperamental to common mode (however rare it is I have had this issue in the past).  Try putting a 1:1 Balun (ideally) or ground the shield where the feed line enters your shack... I have found this can cure ATUs that are not tuning to their specs.  
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Robert Fraley

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This is an old post but I may be offering a practicle suggestion. If you use a G5RV and try to tune beyond its size capability then you will surely not be effectively gaining anything even if it does tune the frequency. The lower bands need longer wire. It is simply not going to be an efficient performing antenna because it lacks the inductance or the length. If the tuners have an inductor at the tuner brings the inductance higher then you will be losing almost all of the power transmitted from the radio.Try getting a bigger antenna. Pound for pound I would see gains from a Carolina Windom over a G5RV antenna.