MFJ-929 ATU – Power & SWR Reading Accuracy

[M1BWT]

Does anybody have the MFJ-929 ATU (or similar) with digital readings of forward/reflected and SWR? I’ve got a new unit, and the accuracy of the reflected power (and hence SWR) reading is a bit off when compared to an inline power/SWR meter (in chain TX, SWR/PWR, ATU, Antenna).

For example, in cases of low reflected power (say 20w forward, 0.2w reverse) the inline SWR/PWR meter may show an SWR of 1.5 say, and the ATU shows 0 reflected and SWR 1.0. On other frequencies at the same power the ATU might show 1.8 SWR but the meter reads 2.2. Oddly, however, in some cases it shows an accurate reading for power and SWR, even at low power (so doesn’t appear to be a consistent accuracy issue).

I’ve had the unit recalibrated by the shop for power, although that only calibrates based on a 100W value (so quite possibly didn’t check 0-2w reflected readings).

Just wondering if anybody has seen the same behaviour? It works fine apart from this, and quite happy automatically detects and tunes at high SWR, just it often thinks it’s tuned to a target SWR of 1.0 but it’s still actually 1.5 (can manually tune lower). I'm sure my reference meter isn't the most accurate bit of equipment, but if it shows a bit of reflected power would have thought ATU would register something at least.

Perhaps I'm expecting too much from the ATU!

[G4LNA]

Simon, we are going to have to stop meeting like this, people will start talking

Do you have a dummy load you can try? That will hopefully indicate which meter is reading accurately.

One way to test if your in line SWR meter is working properly is to feed it onto a dummy load and make a note of the readings, it should be 1:1 or very close to it, then reverse the connections, the meter readings will obviously be reversed, but the values should be the same, if they are not then there is something amiss with the meter.

[M1BWT]

Indeed, you’re obviously following my train of thought!

Yes, I’ve tested into a dummy load with the standard power/SWR meter and no reason to believe that is incorrect (various power levels match those both set and reported by the radio, plus SWR reading based on forward/reflected power is in line with approximate manual calculation).

I’ve not tried in reverse but will try that, maybe with a handheld to test some lower power readings for ATU reflected. I think it may just be a case that it’s not designed to be that accurate - perhaps the technical specs have more details, I'll check.

[K7KBN]

I see that MFJ doesn't specify the accuracy of the power readings, according to the manual. Digital bargraphs are very nice eye candy but they're no substitute for an actual meter reading. I wouldn't worry about things - the cumulative tolerance of the two meters may well show that each by itself is fine.

"He who has a watch knows what time it is. He who has two watches is never really sure." - Some old guy. Same for meters.

[M1BWT]

Hello Pat,

The unit has both digital bar graph and a digital decimal reading – it does mention in the manual that the power display resolution is ‘0.1 for < 10 watts and 1 for >= 10 watts’, although that’s ambiguous enough not to specify the resolution of the actual measurement, just what it displays!

Quite possible it’s just not that sensitive as you mention, although it leads you to believe that it’s able to accurately measure SWR down to 1.1 by having that as an option for target SWR. Not a big problem though, as you say.