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Author Topic: Still Looking for Assistance: MFJ 269 Working or  (Read 3350 times)
WB6BYU
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« Reply #15 on: August 22, 2003, 03:34:43 PM »

Measuring capacitance does have its uses:  I remember
my father using an old Heathkit RCL bridge to measure
the capacitance across each end of a 50' power cord
and using the results to calculate the location of a
break in one wire - he was within an inch.

If you are trying to measure the length of a spool of
cable you may be limited by the accuracy of the readings.
For example, 50 ohm coax is about 30 pf/ft (100pf/m).
Including the effect of strays you would be lucky to
measure the capacitance of one foot within 1pf (3%),
though 10% may be more common with the MFJ.  Extending
this to a 300' (100m) length, the error would be about
30' (10m).  (The longer the measured sample length, the
smaller the resulting error.)

A quick way to determine the impedance of a spool of
unmarked coax is to measure the capacitance of a short
piece and calculate the capacitance per unit lenght.
50 ohm cable is about 30pf/ft (100pf/m) and 75 ohm cable
is closer to 20pf/ft ( 65pf/m ).  If it is much lower
than that you might have found some of the 93 ohm coax.

But a DVM that also measurees capacitance costs much
less than the MFJ 269, and, I suspect, is much more
accurate.
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W8JI
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« Reply #16 on: August 22, 2003, 10:36:19 PM »

I helped design the 259 and 269, and was actually the person who came up with the DTF and stub length idea (among other things).

One problem at MFJ, and with some calibration instuctions, is the HARMONIC content of the analyzer is or was not set correctly. This can "hose" the readings, even when the unit tests perfect on dummy loads.

I had a bias adjustment put in the 259/269 that is supposed to be set for minimum harmonic content of RF output under lowest expected battery voltage and lowest expected load impedance conditons.

It turned out that someone somewhere decided I just had that adjustable bias installed for "grins", and it could be set by watching a test point with a meter!! So the people at MFJ were not setting the bias correctly. That's a far cry from adjusting for minimum harmonics in the unit under a specific battery and load condition.

Another person wrote a test proceedure that did not use the low impedance load, and ran the operating voltage high. This was an aftermarket "helpful" paper based on MFJ's incorrect proceedure.

If you do NOT have the diode bridge calibrated correctly, and/or if you have not set the bias correctly, the DTF and stub lengths will be off.

If you have everything set correctly, the DTF works great in the 269. I have some feedlines almost 1/2 mile long, and I can find an open cable within a few feet out of 2200 feet of line length!

Exactly what is the problem you are having?

73 Tom  
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W8JI
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« Reply #17 on: August 22, 2003, 11:20:28 PM »

Let me make just a few more comments :-).

Some of the advice I've read here is **very** good, but some is not quite so good.

The 259 can NOT be calibrated correctly with any proceedure shorter than on my web page, unless you only want to measure resistors!! There is no short cut to the RF calibration. You have to go through ALL the steps with the correct loads. If you are a couple bits off, it can mean a big error with certain loads!!  

The tech at MFJ was not exactly correct. There is no step "missing". You simply find an X=0 low impedance point, and tune upwards (or downwards) until you find the NEXT X=0 point that is also low impedance. If you are using the unit correctly, they will always be in the same band range.

(If you have not calibrated by going into a test menu and setting the A-D bits with known good precision loads, and if you have not calibrated harmonic null with a low Z termination at lower than normal battery voltage, DTF readings will almost certainly be messed up. This is ESPECIALLY true if you have tweaked the pots just to get a correct R reading for various loads, rather than setting for bits.)
 
You do NOT have to set for some frequency where the line is 1/2 wl long or any other length. As a matter of fact, that is the LEAST accurate test setup. As a general rule higher frequencies will give much more accurate readings.

For example, I use several MHz as the test frequency on a 2500 foot line and that's a 190kHz 1/2 wL !!! (There was actually advice published in QST Tech Correspondence that was incorrect. It said something about using 1/4 wl or 1/2 wl frequency!)

I can read ~2500 foot lines within a couple feet with my 269's by doing that.

The 269 at HF is the same as the 259, except instead of using a 8 bit A-D convertor (255 steps or .4% voltage changes)it uses a higher resolution A-D convertor. Calibration is almost the same. The display is in hex (from 000 to FFF), and some components have moved. I just haven't had spare time to write a "consumer test" for the 269 (even the 259 was done on my own time).  

This gets rid of some error in the readings, and allows much higher resolution *IF* the unit is calibrated correctly.

These units are sensitive to external voltages of ALL types, not just from BC stations. You have to be sure what you measure is NOT applying any voltage back to the 269.

The unit also will not be accurate if the line is not totally shorted or open at the fault. It depends on an open or short at the far end of the line for the most accurate distance or degree readings.

73 Tom
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W8JI
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« Reply #18 on: August 22, 2003, 11:34:21 PM »

<If you are trying to measure the length of a spool of
cable you may be limited by the accuracy of the readings.
For example, 50 ohm coax is about 30 pf/ft (100pf/m).
Including the effect of strays you would be lucky to
measure the capacitance of one foot within 1pf (3%),
though 10% may be more common with the MFJ. Extending
this to a 300' (100m) length, the error would be about
30' (10m). (The longer the measured sample length, the
smaller the resulting error.)>

That's a good idea, but the MFJ is MUCH more accurate than a DVM in measuring low values of capacitance because it measures them at or near the operating frequency.

The problem is we can NOT use capacitance to tell how long a transmission line is if we measure the capacitance with a test instrument that uses a high frequency oscillator. That's because a transmission line is a very poor capacitor. It has considerable series inductance, as well as shunt capacitance. Capacitance only holds true when the line is a tiny fraction of a wavelength long at the test instruments operating frequency!

This is also why a 10-foot open stub of 30pF/FT coax is NOT 300pF, except at very low frequencies. As you move up in frequency, capaciatnce increases. Just above 20 meters that 10-ft cable (if .66 vP) would be inductive!    

It is much better to use another method when measuring line length. That's why the MFJ does not use capacitance to calculate length.

73 Tom
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KK2QQ
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« Reply #19 on: August 23, 2003, 12:41:11 AM »

Tom (W8JI):

Thanks for your generous comments.  I would greatly appreciate your help.

Here's a brief background on what's been going on so far and why I started this thread:

I purchased my 269 a few weeks ago, directly from MFJ.  I intended on using the unit for troubleshooting antenna/feedline mismatches (where they shouldn't be) as well as for building/tuning some AMSAT-types of antennas.  I have never had a standalone test unit like this before.  I had always used my rig's SWR capability in the past.  I bought this unit in hope of taking my experimenting and troubleshooting skills to a higher level.

When the unit first arrived.  I used the Impedence X&R functions (probably the most commonly used mode) to profile some "known" characteristics in my shack.  I used two different dummy loads and looked at changes in the Impedence/SWR as frequency ranges changed.

PROBLEM #1:  I noticed that the digital readout looked pretty accurate to what my Rig's meter shows, but the Analog SWR and Impedence Meters' needles often showed quite different measures.   I was confused.   I couldn't understand which reading to trust -- the analog gauge or the digital one.  I thought, shouldn't they be the same?   So I called up MFJ Tech Service and they suggested I send the unit in for calibration.  They thought perhaps the unit was slightly off calibration from the factory.

No big deal.  I figured these sorts of things can happen, so I sent the unit back.  In the meantime, I read the manual again and again and started to notice that they never seem to mention the analog gauges.   Hmmm, why design a unit with such gauges if they're not going to be correct?   I thought could they be there as a "rough approximation" of the readings -- to mostly be used as a easily seen "visualization" of SWR/Impedence trending up and down or staying relatively fixed?   It seems in the manual that the digital display is the "gospel."

Two weeks later I received the unit back from MFJ.  The Tech Notes said that unit was re-calibrated and that the tech spent 96 minutes on the unit.

WAS IT FIXED?
I performed the same profiling of the dummy loads in the Impedence X&R Mode, and did notice that the analog meters still weren't an exact mirror of the digital readout, but the needles were closer to the real thing- definitely an improvement but obviously couldn't be used for the absolute reading -- I'd need to use the digital readout for that.   I figured that's an adjustment that I could make to my use of the unit.

I tried out some of the "Calculator" functions and they worked fine.   I also tried the Frequency Counter functions and they worked fine.

So now I figured I'd try some of the more advance features of the unit, as I had been through the manual so many times as I waited for the unit to come back from calibration.

PROBLEM #2:  I tried the Coax Loss function.  The procedure seemed very straight forward and I had various lengths of brand new, high quality coax I had purchased from Cable Experts for my pending antenna projects.   What a bonus I thought to be able to use this $400 test unit to verify that any of my coax is in good health.   Why not try it on this new coax -- after all, I knew all the specs for it (from Cable Experts) -- velocity factor, loss characterics, etc.

So tested a few different lengths of Coax for the Loss Characteristic.  The Coax was 75feet in length, a 9913 type of high quality, low loss coax with a VF of 84 and spec'd loss characterics of 1.6db/100ft at 150Mhz and 2.9db/100ft at 450 Mhz.   The 269 unit reported 2.2db at 150Mhz and 3.5db at 450 Mhz (overall off about .6db).

I wasn't sure if this was the best accuracy I could get or whether the unit was properly calibrated in this area?

PROBLEM #3:  Within the Advanced 2 area of the unit, I tried to perform the DTF functions.   I used the same cable described above.  I set the velocity factor at 84, and I carefully followed the directions in the manual to try and achieve an Xs=0 on the first reading and an Xs=1 on the second reading.  The DTF result would come out to Thousands of Feet.  

Now I was confused?   Was the unit still out of calibration (other than the simple Impedence modes), or was the issue "operator error" -- e.g., my fault?

So I posted my first thread on the Elmers forum, but honestly, no one seemed to have any information other than using the unit for simple Impedence X&R measurement.  

Because, at the moment,  I didn't have a satisfactory from the Elmer's route, I decided to once again talk to the MFJ Techs to see if maybe I was doing something wrong.   The first tech I spoke to was upfront and said he really didn't know how to use the advance modes of the unit.  No problem, I said, and asked if he could connect me to another tech with better knowledge.   The second tech was very accomodating and we spent almost an hour on the phone trying over and over to do a proper DTF measurement.  

He told me that the manual didn't go into great detail on this function and that the trick to doing the measurement, was to start at the highest frequency range and attempt to get an Xs=0 for the 1st reading.  Then he said that you must switch the frequency range dial either up to the next higher range or down to the next lower range and then try to get an Xs=0 or Xs=1 reading for the second reading.   I was able to do what he said, but the final DTF readings I got were between 3 and 4 feet (still on the 75ft cable).

The Tech was perplexed and said that there probably is something still wrong with the unit.  He asked me to send it back in.   So on Monday, it will arrive back at MFJ Tech Service for repair or replacement.

CONCLUSION:
Even when I get the unit back, I won't be sure if they
truly fixed my unit (if they send the same one back) or if they replaced it, that they sent another unit that is properly calibrated (for ALL modes of operation).

So what I am really in need of is some clear step by step instructions for a few different, solid tests to show me that:

(1) the Impedence X&R Mode works to spec (both in the digital readout and the analog readout
(2) The Coax Loss Function works to spec
(3) The DTF Function work to spec

I'd be eternally grateful if you, Tom, or anyone else could help me out with this, so I can be a happy MFJ 269 user -- and others out there could also use this information as a reference for learning about and trying out their units.

What do you think?

*Bradley
KK7QI
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W8JI
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« Reply #20 on: August 23, 2003, 11:57:27 AM »

Hi Bradly,

<PROBLEM #1: I noticed that the digital readout looked pretty accurate to what my Rig's meter shows, but the Analog SWR and Impedence Meters' needles often showed quite different measures. I was confused. I couldn't understand which reading to trust -- the analog gauge or the digital one.>

The reading should be similar, but not perfect. The meters MFJ buys are what are often called "wigglers". They are cheap and very small, not highly accurate. No one would buy an antenna analyzer with a pair of Simpson 2% meters!

Use the meters as a visual indicator while tuning, but read actual values from the digital display.
 
<<No big deal. I figured these sorts of things can happen, so I sent the unit back. In the meantime, I read the manual again and again and started to notice that they never seem to mention the analog gauges.  Hmmm, why design a unit with such gauges if they're not going to be correct? I thought could they be there as a "rough approximation" of the readings -- to mostly be used as a easily seen "visualization" of SWR/Impedence trending up and down or staying relatively fixed? It seems in the manual that the digital display is the "gospel." >>

That is correct. The meters are a rough indicator easy for eyes to follow. The digital display is for the actual reading.  

<<PROBLEM #2: So tested a few different lengths of Coax for the Loss Characteristic. The Coax was 75feet in length, a 9913 type of high quality, low loss coax with a VF of 84 and spec'd loss characterics of 1.6db/100ft at 150Mhz and 2.9db/100ft at 450 Mhz. The 269 unit reported 2.2db at 150Mhz and 3.5db at 450 Mhz (overall off about .6db).

I wasn't sure if this was the best accuracy I could get or whether the unit was properly calibrated in this area?>>

That's reasonably close for "one-port" loss measurement. The MFJ depends on the cable being exactly 50 ohms, and the unit having accurate SWR calibration. It measures cable loss by measuring unterminated SWR and defining SWR as "return loss" expressed in dB. Since transmission loss (one way loss)is exactly 1/2 of return loss (loss going out and back) the cable's unterminated SWR expressed as return loss is divided by 2 to indicate transmission loss.

A two-port S12 or S21 test is significantly more accurate, but most two-port test equipment is many thousands of dollars. The MFJ will be close, within about +- .2dB *plus* 10% of actual loss is typical when the cable is actually 50 ohm cable and the unit is calibrated. So at .7dB it would be +- .27dB tolerance. At 5 dB loss it would be +- .7dB. At 20dB loss +-2.2dB.
 
<<PROBLEM #3: Within the Advanced 2 area of the unit, I tried to perform the DTF functions. I used the same cable described above. I set the velocity factor at 84, and I carefully followed the directions in the manual to try and achieve an Xs=0 on the first reading and an Xs=1 on the second reading.>>

If the manual says that, it is a typo. You want X=0 or as close as it gets to X=0 near minimum Z on the meter for both readings!

<<The DTF result would come out to Thousands of Feet.>>

Interesting. That isn't calibration with an error that large. It is either firmware or operational error. The MFJ measures cable length by learning zero reactance crossings at minimum impedance. They repeat every 1/2 wavelength.

That means if a cable is 500 feet long electrically, minimum reactance low Z crossings would occur about 1 MHz apart regardless of analyzer frequency range.

Since you are reading thousands of feet, I'm going to assume the X=0 (or lowest)readings are only a few kilohertz apart. That tells me one of the following:

1.) After marking the first X=0 (or closest to it)low Z point, you are not tuning through a Z peak to the NEXT low Z point up or down and marking it at lowest reactance point.

2.) The firmware in the MFJ is totally hosed.      

<<Because, at the moment, I didn't have a satisfactory from the Elmer's route, I decided to once again talk to the MFJ Techs to see if maybe I was doing something wrong. The first tech I spoke to was upfront and said he really didn't know how to use the advance modes of the unit.>>

I am impressed. I'd hire that guy in a minute, he is very reliable. VERY few people can say "I don't know" or "I'm not sure"!

<He told me that the manual didn't go into great detail on this function and that the trick to doing the measurement, was to start at the highest frequency range and attempt to get an Xs=0 for the 1st reading. Then he said that you must switch the frequency range dial either up to the next higher range or down to the next lower range and then try to get an Xs=0 or Xs=1 reading for the second reading.>

The second tech is wrong. Since his idea of how it works is wrong, that is why he assumes the manual is wrong. He has misled you (and probably others).

Not good.  

 <I was able to do what he said, but the final DTF readings I got were between 3 and 4 feet (still on the 75ft cable).>

That's no suprise. He told you the wrong thing! What the second tech should have said is exactly the same thing the first tech said: "I don't know how it works".
   
<The Tech was perplexed and said that there probably is something still wrong with the unit. He asked me to send it back in. So on Monday, it will arrive back at MFJ Tech Service for repair or replacement.>

....and it probably did not need to go back.

<<So what I am really in need of is some clear step by step instructions for a few different, solid tests to show me that:

(1) the Impedence X&R Mode works to spec (both in the digital readout and the analog readout>>

You'd need loads, and the loads would have to be surface mount loads in modified BNC connectors to be reliable. You'd also have to verify harmonics are within spec.
 
<<(2) The Coax Loss Function works to spec>>

Same as above. You'd have to have known good loads.
 
<<(3) The DTF Function work to spec>>

MFJ analyzers depend on detector linearity within a few bits. Calibration has to be set accurately and in the proper steps. We also MUST be sure waveform is symmetrical and has minimal harmonics.

If there was a way to tell you how to check the 269, I'd sure tell you. Short of having a spectrum analyzer and 1% or better loads and my rewriting pages of engineering notes you'd have no way of doing that.

Hope it all works out for you. The unit is a good unit IF they build and calibrate it correctly, and answer questions accurately.

I have new expensive Network analyzers, and still use my 269's more than anything else. It is nothing for me to find a broken cable within a few feet, even when the cable is thousands of feet long.

73 Tom
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KK2QQ
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« Reply #21 on: August 23, 2003, 02:23:24 PM »

Tom:

Your feedback has been extremely helpful.  It looks like there is a lot of missing knowledge and poor education on how to use this 269 unit properly.  I realize it is quite a complex unit.

So right now I'm in a waiting game.  MFJ will receive my unit for the 2nd time on Monday.  I don't know whether they might escalate the service severity on the unit, re-calibrate it, re-burn the firmware or simply give up and replace the unit.  I'm not looking for a replacement necessarily, just a working, fully calibrated and tested unit, so I can get on with my projects which have been delayed (and I am now running out of summer).

For the DTF functions, it sounds like both the manual and the tech on the phone were incorrect in explaining how to use it.  

I also wish that either in the manual or on the MFJ website, that they clearly delineate the accuracies of the different functions (and analog meters).  It would also be helpful with regard to the meters that they say they are for approximation and visual trending, and to go with the digital output.   The techs won't say a word, which makes me always thing the numbers in the various functions (as well as the needles on the meters) should be dead on, or the unit is broken.

Even the calibration docs that MFJ posts online for the 269 looks very old, seems to focus on mainly Impedence X&R function measurement calibration and does not go into the detail you mention regarding adjusting detector linearity, verifying waveform symmetry and minimizing harmonics.

So maybe that part of the unit was faulty.  I don't know, and won't know until I receive a return package from them.

To test the unit from my end, I am interested in cobbling together the test loads you talked (and intrigued me) about.   Where might I get the parts?  How do I modify the BNC connectors?  Maybe I can order those parts and build them, as I wait for the unit to be returned.

As for testing the harmonics, that sounds like it might be out of my league.  Perhaps a few carefully, and correctly taken DTF measurements would help me prove/disprove accuracy there.   Maybe I could contact you offline for assistance in that area?

From my end, now I understand why some many folks who have bought this unit have posted frustrating and bad reviews of it on this site.  The unit isn't totally intuitive and, as you've discussed,  the manual is lacking some critical steps and information on accuracies.  Internally, however, it sounds like it is a good design and valuable piece of equipment that will serve hams well, once they understand how to use it properly.

I'll be excited when I receive my unit back from MFJ and can verify all these things and maybe put the unit to the good use I invested the $400 on.

Please let me know your answers to my questions, and, once again, thanks for the help.

Bradley
KK7QI

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W8JI
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« Reply #22 on: August 23, 2003, 10:27:23 PM »

Hi again Bradley,

First, I probably should say I don't work for MFJ, nor do I subcontract anything for MFJ any longer.
 
<So right now I'm in a waiting game. MFJ will receive my unit for the 2nd time on Monday. I don't know whether they might escalate the service severity on the unit, re-calibrate it, re-burn the firmware or simply give up and replace the unit.>

I'd wager nothing is wrong with it. Even if it is slightly out of calibration, they will probably just recalibrate the same way.

<For the DTF functions, it sounds like both the manual and the tech on the phone were incorrect in explaining how to use it.>

I just read the 259 and 269 manuals I have and they look fine to me.

Unless later manuals were changed, it should be right.

<Even the calibration docs that MFJ posts online for the 269 looks very old, seems to focus on mainly Impedence X&R function measurement calibration and does not go into the detail you mention regarding adjusting detector linearity, verifying waveform symmetry and minimizing harmonics. >

Don't know why. They were given correct info many dozens of times.

<To test the unit from my end, I am interested in cobbling together the test loads you talked (and intrigued me) about. Where might I get the parts? How do I modify the BNC connectors? Maybe I can order those parts and build them, as I wait for the unit to be returned.>

It would take hours to prepare a step-by-step instruction. I've been meaning to do that but I just haven't the time right now.

<As for testing the harmonics, that sounds like it might be out of my league. Perhaps a few carefully, and correctly taken DTF measurements would help me prove/disprove accuracy there. Maybe I could contact you offline for assistance in that area?>

The 259 instructions on my web site give an alternative method using the analyzer test menu voltage Vz and a 1/4 wl open stub. This proceedure works OK.

Hopefully someday in the next several months I'll get caught up with obligations and have time and energy to write a 269 test proceedure.

73 Tom
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W8JI
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« Reply #23 on: August 24, 2003, 12:17:51 PM »

One last thing. I just looked at the 269 calibration on the MFJ web page and, poor wording aside, it appears to not be correct.

It omits the critical harmonic null adjustment.

73 Tom
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KK2QQ
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« Reply #24 on: August 24, 2003, 02:46:20 PM »

Tom:

<It omits the critical harmonic null adjustment.>

That's what I was saying in my last note.   Thus, I might get my unit back from MFJ not *fully* calibrated -- just on the Impedance X&R part of the device.

So I might not be out of the dark just yet.

*Bradley
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« Reply #25 on: August 26, 2003, 12:34:21 AM »

Failure to properly adjust the harmonic null (bias) setting of the analyzers results in a slight skew of zero reactance or minimum reactance from the correct point.

The result is any lengths you measure, or anything that has to do with finding zero reactance in resonant systems, might be slightly skewed in frequency.

This would not produce a large error, it just means the unit is not as accurate as it could be. As an example a 90-degree length stub might show lowest X at 85 or 95 degrees. It might not even show X=0!

A mobile antenna might not show zero X at resonance, it might still show some reactance even though Vs,Vz,and Vr were all calibrated correctly on dummy loads. Reactance values will also be off a bit. It could also just shift the X=0 point off frequency.

Those are the type of errors from harmonics.

To me, it sounds more like service people at MFJ were telling you how to use the unit incorrectly. Just set the first low IMPEDANCE meter X=0 right in the center of X=0, and then tune up or down for the NEXT lowest impedance reading up or down and center on X=0.

If you can't get a X=0 the cable is lossy or the unit is not calibrated correctly.In that case tune for lowest X, rather than X=0.

You absolutely should **NOT** change bands, like the tech at MFJ told you. If you can not find two nulls in one band move to a higher band and look for two nulls!!!! The two nulls should always be adjacent, at X=0, and separated by an Impedance meter peak.

73 Tom
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K5DVW
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« Reply #26 on: August 27, 2003, 01:24:29 PM »

Whew, all this banter has pretty much strenghtend my position on MFJ equipment. I don't trust it and I think their quality control is unfortunately touch and go. I really labored over purchasing my 269 and went to a store and tested it for myself before I bought it.

I found the design information discussed here very informative!

Thanks!

K5DVW
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