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Author Topic: MFJ259B antenna test through long coax  (Read 903 times)
F6GNZ
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Posts: 11




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« on: September 17, 2002, 07:29:21 AM »

MFJ259B VSWR Analyzer Works great at the antenna terminals.
But: at the end of 40-meter 50-ohm 1/2" low-loss LDF coax feeding my HF beams, shows inconsistent VSWR values. My Drake MN2700 Matching box and a Bird Model 43 Power/VSWR meter show identical VSWR readings over the HAM band range of my beams, but the MFJ259B shows completely uncorrelated VSWR values, much higher than Drake/Bird (4:1 vs 1.7:1 for example) and 258B indicates lowest VSWR points at a completely different frequencies than Drake/Bird do! Checking on a 50 ohm dummy-load shows no false operation of 259B.

Question: what is wrong ?

My only conclusions may be that the power output of the 259B is so low, that the current arriving at the antenna through the coax is negligible, and it’s reflected part can not be detected/measured by the Analyzer. So, 259B indicates whatever… Is my conclusion correct or there is some other explanation?
Sent question to MFJ hoping they would respond, but after reading comments here, I doubt...
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WB2WIK
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Posts: 20611




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« Reply #1 on: September 17, 2002, 11:17:25 AM »

That certainly is unusual.  I've had a 259B for years and the only times I've observed an inconsistency between the 259B and a known good SWR bridge are when:

-There's something wrong with the 259B, like a loose internal connection to the RF input connector, which required my repair (that did happen, once); or,

-When the antenna I'm connected to (with the 259B) is receiving strong signals coming from other sources, so the little generator in the 259B is not strong enough to make the bridge detector work properly.  That happens pretty often, especially if I'm in the field at a mountaintop site which has other transmitters operating.

WB2WIK/6
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K0BG
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« Reply #2 on: September 17, 2002, 02:18:49 PM »

Steve didn't mention that MFJ offers a "tuner" for the 259. It all but eliminates out-of-band signals which cause false readings. It's worth the expense in my opinion.

Alan, KØBG
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F6GNZ
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Posts: 11




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« Reply #3 on: September 18, 2002, 10:37:51 AM »

W2WIK/6 and K0BG, thank you for comments. I just received a response from MFJ engineer (only couple of hours after my question ! ) telling me that +7 dBm of 259B output should be sufficient not to have reading problems with 40-meters of coax, and the problem could be in a defective unit. So, I will now check the readings directly on the antenna terminals and try to rotate antenna to see if readings change (which may indicate it was a result of interference). Also, will connect a 50-ohm dummy load through the same coax and see if I get some reactance readings. MFJ engineer says some problems were reported when used with remote coax antenna switches, and I do use one that grounds unused antennas. Will bypass it to se what happens too, hi !
73,
Radi F6GNZ
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WA9SVD
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Posts: 2198




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« Reply #4 on: December 01, 2002, 06:22:05 PM »

One simple thing to check:  Power!  Make sure either the 12 V. supply is truly 12 V., or make sure the batteries are good!  I had all kinds of problems and erratic readings with my 259, including not operating above 10 M; the problems "MAGICALLY" disappeared when I installed a fresh set of batteries!  (Sometimes we forget the most basic things.)
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