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Reviews For: MFJ-915 RF Isolator

Category: Antenna parts, accessories, incl. baluns, hardware, etc.

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Review Summary For : MFJ-915 RF Isolator
Reviews: 30MSRP: 29.95
Description:
1:1 Current Balun to reduce stray RF on coax.
Product is in production
More Info: http://www.mfjenterprises.com/
# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
00304.3
NU9R Rating: 2022-07-06
Not real happy Time Owned: 0 to 3 months.
I bought two of these. At the time , they were the only ones I could find. While they do work a little bit, they were not what I expected. They are poorly constructed. One end cap came off of both of them. I suppose there is a reason they are not glued on or permanently attached. I just don’t know that reason. They became very hot with just 500W. One of them even showed discoloration of the label from the heat. Simply put one ended up in the trash because it came apart and I’m not sure I trust the other one!
AC7A Rating: 2022-04-17
Very good common-mode choke Time Owned: 0 to 3 months.
I have a new Hy-gain, Hy-tower 80-10m vertical I have been working to tune on each band. That is a very time consuming task and I could write volumes about that experience alone.

When it came to tuning 15m my RigExpert was indicating a feed-point impedance close to 50 Ohms and an SWR of about 1.1:1 at the antenna base. However, when I checked the impedance and SWR through the feedline in the station the impedance was off by quite a bit from 50 Ohms and it was also reactive. The SWR was now 2.2:1 minimum instead. Transmission line theory with a feedline having loss indicates that shouldn't happen. That result was indication that there was a common-mode current flowing on the transmission line. That of course is not the goal or desirable.

I tried a couple of homemade current chokes I had made from some -43 ferrite cores and coax. They helped, but the antenna resonant frequency and SWR varied between them. I decided to purchase the MFJ-915 isolator and see what results I would have.

Once added between the antenna and the RigExpert at the antenna base I found my 15m resonance moved up a 100 KHz, but the SWR was very nearly 1:1 at that frequency. When I checked the impedance and SWR in the station they closely agreed with what I was observing at the base. I then retuned the 15m wire element for the desired CW point and all is good. No evidence of the common-mode problem, and now 10m is behaving as well. All is good on the other bands so I think the choking impedance is quite effective on all the bands the choke covers.

I've swept the MFJ-915 from 1 to 30 MHz with the RigExpert and a 50 Ohm resistive load on the choke output. An impedance close to 50 Ohms showing little reactance is observed across frequency.

Someone mentioned some rattling inside the MFJ-915 enclosure. Mine does make some noise too and I am not sure why, but it doesn't seem to affect the performance. My choke sits at the base of the antenna so it isn't getting tossed around.
WB0OAF Rating: 2021-10-07
These are super units-THEY WORK! Time Owned: more than 12 months.
You really need to install a couple of these. I put them Outside and inside, on Feedlines, on DIPOLES, on LOOPS, inside the house, Keeps the RF off of the FeedLine Jackets.

Even stabilizes and reduces the noise as well, quiets the the receiver Hash..

A necessary item in the HAM World.

One TIP you might consider, Sand the PVC around the ends where water can enter, if you're gonna use these outside, Silicone the Water points for humidity and moisture entering!

I pulled the ends apart once since they didn't use enough PVC Glue!

One problem I seen was these units when taken apart, are wrapped up over the top in stuffed into this short little PVC..Not good electronics design practice. There's about 35 Beads on a little piece of Coax inside that when stretched out to about 15 inches, they perform much better! If you don't take something apart and make it a better product, you're not a real HAM!

I know Ferrite isn't increasing in value! HI HI
K2PBO Rating: 2021-02-15
Solved RF Problem with G5RV Time Owned: 0 to 3 months.
I'm using a standard G5RV in the attic with the ladder line snaked around in the air and tacked to studs. I drive it with about 35' of 50 ohm coax to the ham shack. I feed that with a Dentron 3000A tuner connected to an Elecraft 500A.

I have noticed that a tremendous amount of RF floats around my ham shack, tripping the USB port of my computer connected SDR. Ferrite cores on all USB leads did not solve the problem. Tuning the antenna, especially on 40M is also difficult, especially on the lower half of the band.

After reading up on this, I decided to try a 915. What a difference!!!!! The USB no longer trips, and the antenna tuning (tried on 40 and 20) is easy and predictable all over the band. My educated guess is that the USB cables and various monitor and other wires to my computer had become part of the antenna system! This was a great investment and came in 1 day via Amazon Prime.

KZ4ZK Rating: 2020-05-24
Good product Time Owned: 0 to 3 months.
When I transmitted through an Ameritron 811H my house TVs went crazy and I lost my wifi to the ham shack. I had to reboot the computer any time I transmitted. I took a chance on the MFJ-915 (39.95now) and inserted it into my coax on the radio side of the antenna switch. PROBLEMS SOLVED!!! No more RF problems. For the price I suggest before you do anything drastic try this RF choke.
N1FDX Rating: 2020-02-19
160 Meters Time Owned: more than 12 months.
I have a home brew OFC dipole with a Icom IC-756 Pro3, could never get on 160m. as an experiment put the MFJ-915 in line and I can tune anywhere on 160m. Performance is compromised but I'm on the band. Many FT-8 contacts thanks to MFJ.
K1FBI Rating: 2020-02-19
MFJ got this one right! Time Owned: 0 to 3 months.
I moved to a no antenna complex, so I'm stuck with indoor antennas. I use two EndFedz (20 and 17) meters. Anything much over 5 watts was causing RF in my audio monitor and giving me bites from the microphone. I bought two 915 isolators, placing one at the rig and the other at the antenna. Everything is fine business now!
KK4OBI Rating: 2019-06-03
Changes antenna frequency Time Owned: 0 to 3 months.
The antenna is a home brew version of the MFJ-1790 vertical only for 20 meters. It resonates at 14.3 mHz but, with the MFJ-915 RF isolator attached, it resonates at 15.0 mHz. Unuseable.

A type 43 Guanella or clip-on ferrite chokes were satisfactory and had no effect on tuning.
K4FY Rating: 2019-05-09
Works Well Time Owned: more than 12 months.
I don't know the value for common mode isolation is but it is high enough to be effective in stemming common mode currents that may cause RFI. It works well in my OCFD and 4:1 dual core current BALUN. I run 700 watts SSB/CW
VE3TMT Rating: 2018-09-07
Solved my RFI Problems Time Owned: 0 to 3 months.
Over the years I've been having various RFI issues in the shack that were resolved with ferrites, and other methods. However, two issues I could not resolve were one of the TV's upstairs would mysteriously turn on when I operated on 40m, and my outdoor driveway lights would come on every time I transmitted. For the past couple of months, since acquiring a TenTec Pegasus, I have been plagued with RF in my mic audio. I tried mini mix 31 cores on the mic leads which helped a bit, but didn't totally eliminate the problem. In desperation, I purchased the MFJ-915 line isolator. Installed at the antenna connector of the Pegasus, it has pretty much eliminated the mic RFI, the TV no longer turns on, nor do the driveway lights. It also seems the receiver is quieter than it was before. Not as much noise. I am totally satisfied with the results.