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Author Topic: How can I tell if my MFJ-269 works properly?  (Read 1039 times)
K0BG
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« on: August 20, 2003, 09:04:31 AM »

Tom Rauch, W8JI (a frequent contributor to this forum), designed the electronics for this unit. His web site (http://www.w8ji.com) has and interesting treatise on calibrating and/or checking the unit.

There is also an article in the ARRL's Antenna Compendium number 7 on checking the accuracy, and maybe easier to understand.

Alan, KØBG
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K5DVW
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« Reply #1 on: August 20, 2003, 08:54:31 AM »

Why dont you measure some resistors at 1 Mhz and see what you get. You can simulate a 1.5:1, 2:1, 3:1 with resistance and then you'd know which meter is more accurate for VSWR.
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KK2QQ
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« Reply #2 on: August 20, 2003, 02:55:50 AM »

I recently purchased the MFJ-269 Analyzer.  I read the reviews of both the Autek and MFJ-269 and while neither is perfect, I decided to buy the MFJ unit.  I also purchased the Deluxe Accessory Pack.

I want to say up front that although I've thoroughly read the manual cover to cover, I've never had a piece of test equipment like this before and I am very concerned as to whether it is working properly.  I am trying to get some "benchmark" readings that show me the device is working.  I've used only what I have available to me around my shack to help me try and bench test this instrument.  I try my best to describe what I've done in order to solicit the help of this group of Elmers.

When I first tried out the unit I did a few easy SWR measurements into a dummy load.  I compared the readings to my rig's meter and they seem to concur.  However, the digital readout on the 269 did not  agree with the needles on the analog meters. For example, digital might say 1.2, Meter reads 1.4, Digital says 2.1 meter says 3+.  I had read some of the reviews of this unit on E-Ham and some of the owners said they had to send back their units for various minor fixes.  So I called MFJ Tech Support and they suggested I send the unit in for calibration.  So I had maybe an hour of use total on the unit before I repacked it and shipped it away.

Today I got the unit back.  I tried it again with the dummy load and the digital readings seem accurate, however the analog meters are still off (they are closer than where they were before sending the unit in, but they still don't nessarily match the digital readout).  

So just on a hunch, I decide to try a few more features of the unit to see if it works correctly.   I decide first to try the frequency counter.   I attach a small duck antenna to the TNC connector and then take a HT tuned to 144 MHZ and key it up.  The MFJ reads 143.999 Mhz.   I figure that works.

Next I decide to test some Brand New 9913-type Coax I just got from Cable Experts.  I see that the MFJ Unit can display two a few interesting things: Coax Loss and Distance to Fault measurements.

Coax Loss, I figure, should be right on the money with the specs of Cable Expert's coax.  I try the measurements on two lengths of coiled coax, a 75 ft length and a 6 ft length.    The Coax loss on the 6ft length at the 150 mhz and 450 mhz settings are close to the 100 ft spec.   On the 75 ft lengths, the Coax Loss reads about .6db higher than the specs for 100ft.  
QUESTION:  Does this sound like correct behavior or is there something wrong with the unit?

The second type of measurement I try (on the same two lengths of brand new, unconnected coax) is the DTF measurement.  I figure this measurement should tell me the physical lengths of each coil of coax.  I carefully follow the instructions in the MFJ manual and both coils show ludicrous measurements:  Like 5261 Feet or 1285 feet -- Crazy numbers.

QUESTION:  Does this sound like the unit is working correctly?

So I'm very frustrated and don't know if the unit is working or not.  The repair slip that MFJ sent back with the unit say that the tech spent 96 minutes calibrating this thing.  So I would assume that it should be working.

However, here's, in summary, what I know:
1) Digital Impedance R&X readings don't match up exactly with Analog Meters (perhaps that's a design flaw and the digital reading is the gospel).  
2) Frequency counter seems to be accurate
3) Coax Loss readings don't seem to be accurate on the new 75ft coil of coax, they seem close to the cable spec on the new 6ft coil of coax.

4) Distance to Fault readings on either the 6ft coil or the 75ft coil of coax were ludicrous.

I'm going to call MFJ Tech Support back tomorrow and explain the situation, but I was hoping one of you Elmers might have some suggestions on where I am and what I should do.  I've invested over $400 on this equipment and have waited an additional two weeks to try and get a properly working unit.

I feel very helpless in this situation, so I appreciate any feedback I can get.

Thanks in advance

Bradley
KK7QI
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DC0ED
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« Reply #3 on: August 20, 2003, 04:47:57 AM »

Hi Bradley,

here are some good links that may help you:

http://www.seed-solutions.com/gregordy/Amateur%20Radio/Experimentation/EvalAnalyzers.htm

http://www.mfjenterprises.com/man/pdf/MFJ-269_Calibration.pdf

73 Attila
DC0ED
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KC7YRN
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« Reply #4 on: August 20, 2003, 11:38:59 AM »

Bradley,

Precision at RF is a difficult and expensive thing to achieve. For example, the respected professional-grade Bird power meters are only rated for accuracy of 5% of full scale. Not 5% of the reading, 5% of the maximum the meter's able to measure.

If SWR readings are within about 25% of other cheap meters you're doing fine.

Please keep us posted on your problems with the distance-to-fault measurements.
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KC7YRN
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« Reply #5 on: August 20, 2003, 11:50:09 AM »

Alan pointed out W8JI's site, which contains calibration information from MFJ. Worth checking out, especially the part which says "Loads most sensitive to harmonic-induced errors include, but are not limited to, antenna tuners, tank circuits, very short resonant antennas, and distance to fault and stub length measurements. If you notice something "funny" going on with a stub measurement, it may be a fault of incorrect bias. ". There's a resistor you can tweak to correct problems, which may not have been part of the calibration that MFJ did for you.
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KK2QQ
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« Reply #6 on: August 20, 2003, 01:57:00 PM »

Although a lot of you have replied to use various resistors to check the unit -- I simply don't have them. So it really doesn't help me out.

I've read the MFJ Calibration instructions -- which obviously require some electrical engineering equipment and expertise, likewise, I've read the W8JI stuff (which again requires all sorts of test equipment and is really about the 259B and not the 269) and the W8WVV stuff (which, of the lot, is easiest to read and do).

I pretty much don't have any doubts that the "basic" SWR functions of the unit are pretty accurate. As I mentioned, I just received the  re-calibrated unit back directly from MFJ.

What I was interested in finding out from you Elmers is why the Coax Loss DTF functions seem out of whack. All the references that you have provide don't even touch the subjects of DTF and Coax Loss, so I am still without any help in those areas.

Does anyone in the group have a 269 and can say, "Hey I have that unit -- here's how you can try to test Coax Loss and DTF"?

Thanks again in advance for your continued feedback and help.

*Bradley
KK7QI

 
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DC0ED
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« Reply #7 on: August 20, 2003, 04:10:19 PM »

Hi Bradley,

i have own my MFJ-269 about a year and i sold it this month at ebay, cause i want more accuracy. I think your problem is the coiled coax, cause you are measuring a coil! Uncoil your coax coils, put the coax on a straight line and measure the coax again... and see what happens...

73 Attila
DC0ED
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NH2GX
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« Reply #8 on: June 03, 2009, 04:07:55 PM »

I know this is an old thread, but a definitive answer on the coax DTF measurements were not answered.

I agree with two of the earlier posts. As DC0ED mentions, don't measure wire (even coax) when it is coiled. Next, KC7YRN gives the answer all the way down to a quote. Take that, calibrate it accordingly, and this meter seems to get very, very accurate in my case. In fact, if you have the precise length of coax, that single calibration step fixed a lot in my case.

73
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AD7WN
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« Reply #9 on: June 03, 2009, 07:40:38 PM »

I once borrowed an MFJ-269 to make some SWR measurements on an inverted vee.

Before attempting any measurements, I connected it to a 50 ohm dummy load, and it was right on.  Then I connected it to a 100 ohm resistor and it correctly read 2:1.

When I connected it to the antenna, through the coax feedline, that's where I ran into trouble.  I was getting impossible (maybe improbable) SWR readings, no matter what I tried.  Turned out a local AM broadcast station was upsetting the measurements.  I cobbled together a high-pass filter and the situation improved, although the very simple filter introduced enough of an impedance bump that the results were not acceptable.

Once I set the 269 aside and built a cheap and dirty impedance bridge on a piece of perf board, my troubles were over.  By running a test signal at the two-watt level from the rig, I started getting useful results.

It turns out that MFJ lists AM broadcast signal ingress as a possible source of error in their manual.  I think that the 269 is a useful instrument, but it must be used with a great deal of care.

And, in my own case, when all else fails, read the manual :-)

Hope this info is useful, 73 de John/AD7WN
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KI4SDY
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« Reply #10 on: June 04, 2009, 08:02:58 AM »

I have used various meters over a 40+ year period and I can tell you that the meters in the radios are always suspect. They are only good for an initial guide and should not be considered as accurate. If you happen to have a radio that has accurate on board meters, count yourself as lucky, you have just won the radio lottery!

The best and easiest way to test your meter for accuracy is with another known accurate test meter.  When the meters are not even close, use a third test meter. Even then, there will be some variance in the readings. All kinds of things, internal and external, account for this. However, this is where belonging to a ham club comes in handy! Get with some meter Elmers and get that thing checked out before you send it off to get calibrated or sell it. It could be that you have an accurate instrument and don't even know it.  

As far as coax reading, I agree. You must uncoil the coax. You are getting the reading on a giant balun coil!
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