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Reviews Categories | Antenna parts, accessories, incl. baluns, hardware, etc. | MFJ-915 RF Isolator Help


Reviews Summary for MFJ-915 RF Isolator
MFJ-915 RF Isolator Reviews: 22 Average rating: 4.3/5 MSRP: $29.95
Description: 1:1 Current Balun to reduce stray RF on coax.
Product is in production.
More info: http://www.mfjenterprises.com/
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KY4TS Rating: 4/5 Jul 23, 2014 12:42 Send this review to a friend
It's MFJ, but it works  Time owned: more than 12 months
This unit really works when needing to remove (block) unwanted currents coming back into the shack. It did however take 2 tries to get a working unit. The first one isolated EVERYTHING! No signals were going anywhere. I returned it directly to an MFJ rep I ran into at R&L Electronics who agreed to replace it. After a couple months and a couple e-mails, the new one arrived, was tested and worked.
 
KD7RDZI2 Rating: 5/5 Jul 18, 2014 11:07 Send this review to a friend
Well made!  Time owned: 0 to 3 months
Revised review.

The name RF-Isolator may be a bit misleading as there is no DC isolation. There is continuity from one end to the other of the center conductor. There is continuity in the shield as well and no continuity between the shield and the center conductor. I believe it could be used with active antennas with DC voltage flowing in the center conductor and in this case I would put it close to the antenna.
According to the instruction sheet it is indeed a 1:1 current balun, i.e. a Guanella 1:1 balun. A current balun can be made in many ways. This is made up of many ferrite core beads placed on a quality coax cable. Connector have Teflon insulation. Overall mechanics seems to me very good!
Electrically I measured 84 microHenry as inductance between one and the other end of the outer braid. Also I measured the same inductance between the two ends of center conductor. This is in my opinion quite a high value and this explains why this product works remarkably well also on the lower HF frequencies. I tried it on VHF at 50Mhz band but SWR was higher. However the specs say 1.8-30Mhz and 50 Mhz is out of the band. I use a T2FD, which is balanced, as an inverted V and I noticed a slight change between 24-29Mhz. On unbalanced antennas the effects should be more important and on verticals I would put one unit close to the antenna and another close to the TX.

The price seems to be very reasonable. I have just made a Guanella 1:1 using a different approach using spare stuff I had in the drawer. Two Teflon PL connectors, some "Aircell 5" cable and a ferrite rod. The Aircell 5 is small as RG58 but is semi-rigid and keeps the shape around the ferrite rod. With 17 turns I got 12 microhenry as inductance compared to 84 microhenry of the MFJ-915, which means that the MFJ-915 has a better "isolation". Reading between the lines of the Jerry Sevick book my version should also work on HF. The moral of the story, you can make one at a little cost which may work however I would say the MFJ is a benchmark and I would buy two of them again!
 
KD7RDZI2 Rating: 5/5 Jul 17, 2014 12:27 Send this review to a friend
Well done!  Time owned: 0 to 3 months
The name RF-Isolator may be a bit misleading as there is no DC isolation: there is continuity from one end to the other of the inner coax. And there is continuity in the shield as well. According to the instruction sheet it is indeed a 1:1 current balun, i.e. a Guanella 1:1 balun. A current balun can be made however made in many ways. This is made up of many ferrite core beads placed on a quality coax cable. Connector have Teflon insulation. Overall mechanics seems to me very good!
Electrically I measured 84 microHenry as inductance between one and the other of the shield. Also I measured the same inductance between the two ends of inner coax. This is in my opinion quite a high value and this explains why this product works remarkably also on the lower HF frequencies. I tried it on VHF at 50Mhz band but SWR was higher, so I think this product is mainly designed for HF. On the instruction sheet I have not found any mention on which frequencies it should work. On unbalanced antennas the effects should be more important. I use a T2FD, which is balanced, as an inverted V and I noticed a slight change between 24-29Mhz. On verticals I would put one unit close to the antenna and another close to the TX.

I have just made a Guanella 1:1 using a different approach using spare stuff I had in the drawer. Two Teflon PL connectors, some "Aircell 5" cable and a ferrite rod. The Aircell 5 is small as RG58 but is semi-rigid and keeps the shape around the ferrite rod. With 17 turns I got 12 microhenry as inductance compared to 84 microhenry of the MFJ-915. Without the rod I got just 0.7 microhenry. Reading between the lines of the Jerry Sevick book my version should also work on HF. Overall, you can make one at a little cost, however I would say the MFJ is a benchmark!
 
KG4LLQ Rating: 5/5 Jun 26, 2014 08:09 Send this review to a friend
Great Asset  Time owned: more than 12 months
I had major problems tuning my IC-718 to my 16 ft. Force 12 Flag-Pole Vertical with my MFJ-949e. I inserted the 915 into my feedline where it enters the house. Problem solved!
 
K1FPV Rating: 5/5 Jan 24, 2014 14:35 Send this review to a friend
Great! Work As They Should!  Time owned: more than 12 months
I've got several of the isolators at use in my shack. Prior to using them, I had a couple radios that were affected by stray RF. Grounding, bypassing etc didn't work. As a last resort, I tried 1 of the RF isolators, and it worked.

I now use several. They are on the radios prone to RF problems. No more problems. A must if using high power. By the way, they will take a KW !
 
K6SDW Rating: 5/5 Jan 23, 2014 22:06 Send this review to a friend
Wroks fer me  Time owned: 3 to 6 months
1:1 balun seems well built and so far at 100watts it works on my dipole.....some day I'll try 1KW into it and see how it goes.

Cheers all
 
KJ6BBS Rating: 2/5 Aug 25, 2013 11:31 Send this review to a friend
Typical MFJ QA  Time owned: more than 12 months
Well, this is my 2nd MFJ product, and got burned again. I've had this isolator for less then 3 months, and the caps on either end came loose from the adhesive. It wasn't visible until I was packing away my set up, and gave a slight tug on the coax, and one cap came off, and separated the soldered wire from the connector. Too bad the PVC is stronger than the construction of the isolator. It would have been nice if they screwed the cap ends in instead of using glue. I don't spend money to have to fix shotty work. No more MFJ products for me.
 
K9DTH Rating: 5/5 Jan 16, 2012 12:49 Send this review to a friend
It works  Time owned: 0 to 3 months
I constructed a ocf dipole but my tuner would not tune the cw portion of 80 meters. I ordered the mfj 915 isolator and installed it close to the rig, in line with the coax and now my tuner will tune from one end of 80 to the other. I also noticed that there is less band noise. I am very pleased
 
KE5PDM Rating: 5/5 Feb 18, 2010 19:45 Send this review to a friend
RF problem solved!  Time owned: more than 12 months
My Motor Home steps would retract at times. The XYL was not very happy after the second nose dive in dark with no steps!!! Installed two of the MFJ-915, one at either end of transmission Coax and problem solved. Great product, buy a spare! As I recall, this is not a transformer(Balun or Unun). It consists of a of row of Ferrite beads.
I use them on G5RV, DiPoles and Vertical.
 
K7JBQ Rating: 5/5 Feb 18, 2010 17:40 Send this review to a friend
problem solved  Time owned: more than 12 months
The rig would hang up on CW on 40 meters at power levels above about 20 watts (condo station/balcony antennas). Inserted the 915 at the rig end of the feedline. Problem solved.

73,
Bill
 
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