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Author Topic: MFJ921 and 924 tuners  (Read 1249 times)
WB9OMC
Member

Posts: 10




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« on: July 25, 2002, 06:03:34 PM »

Anybody out there using or tried either of these?

FYI, 921 is for 144/220 MHz and the 924 is for 440.

I have one of each and they don't seem to work for squat, telling
me that ALL of my antennas have lousy SWR's which I am
pretty sure is NOT true.  I've tried them on at least 6 different
antennas for "pruning for resonance" and pretty much
destroyed 2 antennas trimming well past the point of absurdity.

My other two MFJ tuners (HF and 6 meters) work great -
so I'm hoping someone out there has some experience with
the 921 and 924 and maybe known what the magic cookies are
to make these things actually work!

Duane
WB9OMC
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WB2WIK
Member

Posts: 20542




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« Reply #1 on: July 26, 2002, 09:59:58 AM »

I've never used the 921 or the 924, although I've seen them on the shelf at HRO.  Cute.

Forgive me, but I just have to ask: Why in the world would anyone need an antenna tuner on 144 or 440 MHz?  Any devices used for "tuning" on these frequencies will have loss, which is the main reason that people don't use antenna tuners on VHF-UHF, but simply make antenna adjustments (on the antennas), instead, until they achieve a 50 Ohm match.

I can easily believe the directional coupler/Wattmeter properties of these products aren't very good.  It's not easy to make an accurate directional coupler for VHF at such a low cost.  A Bird 43 Thruline with a single 100-250 MHz element costs about $300, and is pretty much the "minimum configuration" for making reasonably accurate measurements at this frequency.

Lucky, accurate measurements aren't particularly important for us hams!  I settle for knowing when I've "dipped" my VSWR to the lowest figure possible, and that's all I want to know.  That figure will coincide with maximum power transfer, so measuring "results" of antenna pruning isn't difficult.

WB2WIK/6

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KD7QBF
Member

Posts: 1




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« Reply #2 on: December 13, 2002, 08:07:18 PM »

The 921 tuner has to be grounded before its meter will give any meaningful readings. I have used it with a home brew j-pole that only had 2 Mhz bandwidth below 1.5 SWR and was able to cover all the band I wanted to use at 1.2 SWR. There is some loss but that's the trade off.
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WA2JJH
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Posts: 523


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« Reply #3 on: August 08, 2003, 11:03:23 AM »

MFJ are not quality  products from the get go.
However, true the under 300W tuner/SWR meters do AN OK
on HF. The HF TUNERS are inexpensive, and the occasional capacitor and coil tap switch RF arcing will
extend bandwidth of just about any antenna, with some power loss.

  Like WB2WIK alluded to is that in a more commercial application, MFJ Like products are not used for VHF/UHF
for extending bandwidth.

  While you can get away with the el cheapo diode diretional couplers inside an MFJ for HF, FOR VHF/UHF
you are not going to get decent accuracy for a extending bandwidth. However as another poster said giving the 921 a very good ground will help somewhat.

  I have done VHF installations, where my tools of choice were the bird with proper slug, and I would just add a simple trimmer cap. I would tune the trimmer cap for max out fowared, and almost zero power reverse. UHF, many like to use tuning stubs made out of a small piece of coax in parallel. The end of the stub is for the most part shorted.

  It may just be a gap in my experience. Perhaps their
are other methods. You can bet you last dollar, somebody will check in with some little trick-o-the trade.

  Many HAMS, my self included go ga-ga getting a BIRD-43 cheap. Then at some hamfest you pick up the 3-30 slug, and slugs for frequecies and power levels you use. That is the key to the bird are the very specific slugs. Their are digital READOUT BIRD TOO.

  I know I have strayed out of the scope. The point that is being made is that thier are no decent combo
tuner/swr/power meters for VHF or UHF. I am sure you can correct some small mismatches. It will not do the dramatic job that a MFJ-949 would do for HF for example.

  I Think just about every ham has purchased some HAM radio periferal that the manufacturer says, this is THE
ANSWER for you. Sometimes there is a little trick to it, or it is just will not work in your situation.

  I could be wrong about your specific problem. What sometimes does happen in an ELMER thread is that someone will have some little thing they learned with the device you have.

  Hope this is that kind of case.
73 de MIKE

   
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WA2JJH
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Posts: 523


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« Reply #4 on: August 08, 2003, 12:14:24 PM »

just a clarification. I TYPED THE CAP AND BAND SWITCH RF ARCHING HELPS WITH MATCH!!!! TYPO DELUXE!!!!!
  I have found the MFJ'S to be OK tuners. One does get much for the money. However MFJ'S DO seem to have RF  arching problems in the caps and switch.

 IN fact I once really FRIED a MFJ 300 WATT tuner only using 100W! It is pretty cool the plastic on the meters even melted!

  What ever MFJ rates a tuner for, you could quarter
the max rating.(IMHO) HF,VHF,or UHF MFJ'S power handling capability(IMHO) are rated as if you have a perfect match.

  You might want to lower the power while tuning.
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