MFJ RF Isolator Balun MFJ-915 Choke For Ham & CB Radio HF Antennas

easy to install , helped greatly

I’ve got 1 in my shack and 1 outside connected to my G5RV. I am very happy with the performance off this balun. If using outdoor in the weather, then you must seal the balun.

The simplest possible isolation choke - a bunch of toroids strung on a length of coax, inside a piece of plastic sewer pipe for physical protection. Thus, its insertion loss is very low, and there is no chance for mis-winding, etc., during construction. MFJ is notorious for poor quality control, and I've read of a few of these needing repair by purchasers, but I've bought 3 so far and all were mechanically and electrically good (no loose screws,no loose glued endcaps, and no sign of poor soldering to the connectors inside when I did electrical tests. Test #1: very low DC resistance end-to-end on center and shield and not shorted either. Test #2: running over the entire HF band up to 30 MHz, testing them with an antenna analyzer and a 50 ohm resistive load, they show essentially no reactance (1:1 SWR or very close). As you get into the low VHF, things start to fall apart a bit (usable up to maybe 6 meters as I recall). A Balun Designs isolator, built with a single wound toroid, did just about as well. Another brand, (which I won't mention) with a single wound toroid of the same design, started misbehaving between 7 and 14 MHz. I'd not use that one above the 20 meter band, and it was marginal there (1.3:1 or more SWR, JUST DUE TO THE ISOLATOR, from its reactance. Finally, using a portable antenna notorious for feedline RF, (always changing SWR as I touched the radio, etc.) - an MFJ-915 at the radio stopped that (and likely would have stopped RF burn too, if my power levels were high enough for it to be a problem). Yes, it's MFJ - but it is so simple that even THEY (usually) can't screw it up! NOTE: the SO-239's on each end are mounted inside the end-cap, and stick though a drilled hole. Thus, they are somewhat recessed. Depending on the PL-259's on the end of your cables, it can be a little bit of a tight fit, getting them fully tightened. None I've found are too bulky to tighten (which would require some reaming of the end-cap hole to fix), but a couple were close, so MAYBE it could happen with some really large ones with big knurled rings.

It was used to stop RF from returning into the radio when transmission is complete. It is inserted into the coax line by cutting the transmission coax in half and soldering a male PL-259 connector to each section of the cut coax and then connecting the 1.1 balan which ha a female SO connector at each end of the balan.

It does what it is supposed to do. I have never had stray RF leak past this device, and I have used several of them. The only complaint I have is that one was shipped to me with a missing screw on the connector, so it was a bit loose. These cannot be disassembled, so I just used it that way. It doesn't affect the electrical characteristics of the device, it's just poor QC. DX Engineering makes a similar device that is built into an aluminum case, and its build quality is higher. But it also three times the price. For a good choke on budget, this is a good choice. For high power applications (e.g., 1kW or more), I would go with the DX Engineering part.

I used this to in prove the tuneup on a Windom short antenna. It worked as expected. Others have complained about the build quality. Mine was just fine. I am going to miss MFJ.