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Author Topic: Still Looking for Assistance: MFJ 269 Working or  (Read 3129 times)
KK2QQ
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« on: August 20, 2003, 02:07:38 PM »

I'm really desperately trying to get some assistance for this issue. The original thread got mangled by EHam, so I am reposting my followup to my original question:

Although a lot of you have replied to use various resistors to check the unit -- I simply don't have the variety of them (and wouldn't know where to go to purchase 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 got the unit calibrated from MFJ.

What I was interested in finding out from you Elmers is why the Coax Loss and DTF functions seem out of whack. All the references that you have provided 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"?


You can reference the original thread at :
http://www.eham.net/forums/Elmers/45538

Thanks again in advance for your continued feedback and help.

*Bradley
KK7QI

 
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K5DVW
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« Reply #1 on: August 20, 2003, 02:13:52 PM »

Hey I have that unit and the DTF and loss seems to work fine for me.

I seem to remember you have to set it up right and tell it the velocity factor and impedance of the coax line. Maybe one other thing, I don't remember...Did you do that? If you're using an AC adapter instead of batteries, then this information is lost every time you disconnect the power (a real pain).

On a 500' spool of 9913 I measured, it was only off a few feet, so I thought that was pretty accurate. Short pieces of RG58 seemed very close too.
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K5DVW
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« Reply #2 on: August 20, 2003, 02:21:55 PM »

One other thing... when you measure the DTF, what you're doing is measuring the 1/4 wave length, and the 1/2 wave length of the coax. SO, the frequencies you select for measurement have to be correct for your coax.

For instance, a 6 foot piece of RG58 (assume Vp=0.66)should be around
1/2 wave at 54 Mhz
1/4 wave at 27 Mhz

Did you measure with frequencies near these?

(I think I did this right from memory, I dont have my analyzer with me)
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N2MG
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« Reply #3 on: August 20, 2003, 02:31:12 PM »

Not sure how it effects DTF or cable loss measurements...but these types of instruments are very sensitive to nearby RF - do you have any AM broadcasters nearby?

Mike N2MG
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KE4MOB
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« Reply #4 on: August 20, 2003, 02:59:21 PM »

I'm just wondering...what's the accuracy of the velocity factor you're using?  

Velocity factor can vary from one foot of a coax cable to the next (but on a bulk basis conform to spec), so you may want to find the actual electrical wavelength of a line by experimentation (remember the SWR on a line will repeat itself every 1/2 wavelength) and recalculate the velocity factor--you might be surprised.  Then remeasure DTF and loss based on the real velocity factor, rather than what's stamped on the reel.

Just a suggestion.

Steve, KE4MOB
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K5DVW
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« Reply #5 on: August 20, 2003, 03:17:17 PM »

NOte: VSWR on a line doesnt repeat every 1/2 wavelength, it's a constant along the length of the line! Impedance repeats every 1/2 wavelength. VSWR and impedance are not the same thing.
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KE4MOB
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« Reply #6 on: August 20, 2003, 03:44:46 PM »

Thanks for the catch Darrin, it is impedance that repeats, not SWR.  Was thinking one thing and typing another!!

Steve, KE4MOB
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KK2QQ
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« Reply #7 on: August 20, 2003, 04:13:26 PM »

Well, I want to thank all that replied so quickly.  I tried a number of calculations for frequency of half wavelengths for the 75ft length of coax  and the "calculator" functions of the 269 agree with those figures.  I tried a bunch of other things but it still doesn't seem to work correctly on the DTF and Coax Loss functions.

I spent an hour on the phone today with some of the MFJ Techs.  The first guy really wasn't familiar with the Advanced Functions (DTF and Coax Loss) of the units, so I asked to speak with someone else.  

The second guy said that there is a step that is missing from the manual that requires you to change the band range between the 1st and 2nd measurements for the DTF calculation.  The resultant answers for a 75ft piece of brand new, never used coax still came out to 3 or 4 feet (instead of several thousand feet without changing the band range each time).

He said this should come out correctly and the fact that I couldn't get the Xs measurement low enough for the calculations might indicate another problem with the unit.

Also, in speaking with the Techs, I asked if the Analog Meters were designed to ONLY APPROXIMATE (not coincide exactly) the Digital Readout.  This is what my unit does -- only approximates with the analog meters.   This guy wouldn't say Yes to that.  He seemed to feel they should be dead on.

So MFJ decided to have UPS pick up the unit and then they'll test it or replace it.   I said, I don't really want another unit of the warehouse shelf that is out-of-whack.  I really need someone to verify it works perfectly before they send it to me.  They said they'd accomodate that.

I'm frustrated, but think the $400 investment I've made should result in an accurate instrument.  The only thing I'm losing is time -- and my projects will just have to wait.

Thanks again for the feedback.

*Bradley
KK7QI
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K5DVW
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« Reply #8 on: August 20, 2003, 04:44:57 PM »

Well, maybe you did get a bum unit. I once bought a VHF VSWR meter from MFJ that didnt work out of the box. I sent it back and the second one didnt work, neither did the third or fourth one. In all cases they showed no better than 1.2:1, even into a dead short. Couldnt get MFJ to affirm or deny the product had a problem. I gave up and got my money back, minus all the shipping of course.

YOu asked where to get resistors.. if you have a radio shack, check there. It's a good way to check the VSWR function.

25 ohms = 2.0:1
30 ohms = 1.6:1
50 ohms = 1.0:1

And so on.

Good luck with your meter.
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AD6JN
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« Reply #9 on: August 21, 2003, 11:05:48 AM »

I have made a study out of the MFJ-259B for about 4 years.  Using the unit's "advanced functions" requires an undersanding of the test as it is not a single reading device.  The instruction book appears to be not clear at first.  Again it requires more than just a sinlge test data point.  My instruction book contains a comment to that effect.  After many hours of use (and note keeping) I have an unstanding of how to use the unit.

Take the batteries out and use the external power supply.  Low batteries can give inconsistent readings.  I only put fresh batteries in if the tests are in the field.

You will need some termination resistors and a few lenghts of calibration coax.  Run initial tests and keep the data intact with the test articles for future reference.

Bob
AD6JN




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KK2QQ
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« Reply #10 on: August 21, 2003, 12:50:15 PM »

Bob,

Can you be a little more specific about using the advanced features, specifically DTF?

I understand your point about the batteries.

Thank you,

*Bradley
KK7QI
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WB6BYU
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« Reply #11 on: August 21, 2003, 03:54:23 PM »

And if you STILL have problems finding resistors for
calibration, I'll be glad to send you some from my
junkbox.  I'm sure others would, too.
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WB2WIK
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« Reply #12 on: August 21, 2003, 05:12:54 PM »

I haven't used the new 269, but own an older 259 and never found its DTF function to be helpful.  It might be very helpful if you have a fault within one (electrical) wavelength of the meter, but if the fault's that close, I wouldn't need a meter to help me find it!

Typically, if I'm looking for a fault, it's located "somewhere" in a 300 or 400' length of coax.  The 259B doesn't get nearly close enough to start digging, so I'll assume the 269 doesn't, either.

But then, I didn't expect a TDR or Network Analyzer for $300....

WB2WIK/6

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KK2QQ
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« Reply #13 on: August 21, 2003, 06:22:22 PM »

WB2WIK or anyone with a working 269 or working 259B:

Can you post the steps necessary to get an accurate reading on DTF function (e.g., determining the length of a spool of coax)?

If posted, perhaps I'll be able to try those steps on my 269 when it gets back from MFJ Service

*Bradley
KK7QI
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KT8K
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« Reply #14 on: August 22, 2003, 11:02:03 AM »

The MFJ 259/269 measure capacitance.  Can't one measure the capacitance between the shield and center conductor of a foot (or other known length) of cable and then divide that value into the capacitance measured for the unknown cable length?  Wouldn't that come out within a couple of feet?
Just a thought.
73 de kt8k - Tim
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