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Author Topic: MFJ-998 Intellituner Question  (Read 1844 times)
ND9B
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Posts: 49




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« on: June 13, 2012, 10:18:58 AM »

I have a MFJ-998 antenna tuner that's not doing a very good job of automatic tuning on the 20M band. At my favorite 20M frequency the tuner will tune and read out 1:1, but my external SWR meter (which I trust) reads 2:1. I can go to manual mode and get the SWR down to 1:1. I can then press [TUNE], [C-DN] and [L-DN] to store this setting in memory. BUT, how do you recall the stored setting? I can find no way restore the setting in memory, the tuner always initiates a new tune cycle and lands up back at 2:1 ! What is the point of the memory banks, if the tuner is going to retune anyway?

Bobby Dipole ND9B
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K4HXC2
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Posts: 51




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« Reply #1 on: June 13, 2012, 10:57:41 AM »

Have you tried setting the default target SWR from 1.5 to 1.1 in the setup?  My understanding with this tuner is that the default is set at 1.5 with a .5 tolerance allowing it to settle on 2.0 or thereabouts. With the default set at 1.1 the worst it can be is 1.6 making it work harder but isn't why you got it? So you wouldn't have to?
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Dennis, WK1A
Formally K4DAZ
In honor of Dad, K4HXC(sk)
AD5X
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Posts: 1426




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« Reply #2 on: June 13, 2012, 11:02:55 AM »

The reason it retunes is that it must think that the SWR is 2:1 (that's the default retune setting).  Have you put a dummy load at the output and verified that the SWR really is off on the MFJ-998?  If it is, then the MFJ-998 needs to be re-cal'd.  Probably you just need to null the SWR null capacitor on the SWR board.  Put a dummy load on the output of the tuner.  Then transmit a signal through the tuner with it bypassed and adjust the null capacitor for minimum SWR.  You should be able to get very close to 1:1.  I recommend using a 20 meter signal for setting the null capacitor.  I think the manual says 7MHz, but I found that 14MHz gave a better HF-band result.

Phil - AD5X
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G4AON
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Posts: 520




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« Reply #3 on: June 13, 2012, 01:31:21 PM »

If there is RF on the outer of the coax feeder you can see a different SWR at different points on the feeder. Do you have a balun or choke on the antenna end of the feeder?

This could explain why your two reading differ.

73 Dave
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ND9B
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Posts: 49




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« Reply #4 on: June 13, 2012, 06:52:08 PM »

The MFJ-998 is in a chain: Rig-Amp-SWR meter-MFJ-998-Antenna. The linear amplifier is an old Heathkit SB-1000. I just discovered that with a dummy load directly on the output of the amp (with the amp in Standby or Off), the SWR measures 1.5:1 on 20M on the rigs meter. Evidently, the amp is not providing a good 50 ohm feedthrough. (The SWR gets better at lower frequencies.) So maybe the problem is that the input to the tuner is not a good 50 ohm source, at least not on 20M. Does this sound plausible?

BTW, the antenna relay in the Heathkit amp looks like it came out of a washing machine. It definitely wasn't designed for RF!

Bobby Dipole ND9B
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AD5X
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Posts: 1426




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« Reply #5 on: June 14, 2012, 08:23:05 AM »

The MFJ-998 doesn't hare what the source impedance is.  What does the SWR read on both the external SWR meter and the MFJ-998 when terminated in a dummy load?  That'll tell you which meter is good, and whether you need to re-cal one or the other.

Phil - AD5X
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NO9E
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Posts: 383




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« Reply #6 on: June 14, 2012, 01:20:47 PM »

I have MFJ-998 from Phil. Lots of lessons learned.

SWR measured by radio, LP100, MFJ998, and SWR analyzer can be all different, up to .6. For many reasons:

1. One jumper bearing RG8X on the sleeve was actually changing SWR by 0.4. Probably cheap CB type.
2. Amp and switch are uncompensated and change SWR.
3. Not sure LP100 is always accurate.
4. Something else I don't understand.

My antenna is 80  doublet. 998 tuned 160m-15m easy and took all the power of TT Centurion without a  complain. It had trouble tuning 12m with one balun and 10m with another balun. Strange cycling between SWR of 1.0 and 2.3 on 10m with one balun. Also blank screen at high power on 12m until the display and the power supply were bypassed, as in one of recent reviews of MFJ-998 on eham.

My balun is from MFJ-989 and is 1:1 current. It does not fully balance the voltages. I will try a 1:4 voltage balun. Perhaps MFJ-998 may be better able to match low than very high impedances.

 
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WA8UEG
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Posts: 339




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« Reply #7 on: June 16, 2012, 05:03:15 AM »

I have the MFJ giant peak reading meter. When placed in line with the 998 it never reads the same SWR as the 998, always higher when connected to my antenna. The power reading is the same on both and when measuring with my dummy load both read 1:1 swr but when any of my antennas are connected the giant swr meter reads higher regardless if the amp is in or out. The amount higher is consistant regardless of antenna and regardless if the amp is in or out.

Not really a problem, I just use the Giant meter for power readings and tuning the amp and the 998 for swr but it does bug me!
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