MFJ Enterprises Original MFJ-915 RF Isolator 1.8-30 MHz, 1500W PEP, SO-239

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.

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.

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 replaced the 1:1 "ugly balun" (air-wound coax choke) at my broadband HF antenna's feed point with this, and what a difference it made! I run an unbalanced random wire, which the laws of physics dictate will generate significant common mode interference at power. Before applying a choke, LEDs would illuminate on powered-off devices and my speakers would howl upon keying my radio. This was significantly reduced after installing a DIY air-wound choke, and eliminated by the MFJ-915.Keep in mind that these chokes are far more effective at lower frequencies. They'll theoretically help to some degree by the time you hit 6 meters, but there's a reason they're advertised as covering 160-10. As you move upwards through the VHF spectrum, more and more RF will continue flowing backwards on the coax shield.I've read complaints of poor QC with regard to MFJ's products, but I own a few of them and haven't had the same experience. This choke's build quality is solid. The unit is weatherproof, with drain holes at both ends of the cylinder, and the caps are epoxied in place. As always, the PL259 connections should be wrapped with self-amalgamating tape (aka 'coax seal') if this unit will be used outdoors.Inside the PVC cylinder is a short segment of RG-213 clamped end-to-end with ferrite beads. You could certainly build this as a DIY project, but if you don't already own the components, the end cost will be the same.

All my stray RF problems went away as soon as I put this neat isolator in line with my antenna. I am running a DX3000 to an Ameritron AL811H amp to a multi-port antenna switch. Then it goes to a MFJ-962D tuner and next to this MFJ line isolator. The final destination is an OCFD antenna. The RF in my shack was getting into my on air audio, shocking my finger, and ringing my phone. Now all that is gone and I put out a great signal. I keep the MFJ-915 inside the shack.One side connects to the antenna lead in coax and the other end attaches to the tuner by way of a 6-foot coax jumper.

I bought this because it was recommended by MFJ for their 40/80 dipole. I have tried the dipole with and without and can hear or measure no difference. I've built and used dipole antennas for many years, always without a balun or isolator. I recommend trying your antenna first and only buying one of these if you detect a problem.

Bought this to cut down on Rf. Works as it should. Made my 40 and 20 meter much quieter. And helped on 80 meter. Good buy.