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Review of MFJ-1786 Magnetic Loop

By Ian DJ0HF

March 2013

Hi,

my name is Ian, my callsign is DJ0HF/G3ULO and this is a review of the MFJ-1786 Magnetic Loop antenna for the Active-Elements web-site.

The MFJ loop is approximately 1 metre in diameter, made from thick aluminium tubing and the tuning capacitor and motor of the loop are enclosed by a thick black ABS plastic housing which also allows the loop to be bracketed onto a mast and it can be used on any frequency between 10 and 30Mhz.

The coaxial cable from the loop carries both the RF signals and the voltage to drive the motor and tune the loop and connects with the controller unit which is mounted in the shack.

I have owned the loop for a number of years and have it mounted in the loft on a short wooden stub mast, my loft is not very large but I have mounted it as far away from any metal as possible and oriented for maximum radiation in the East/West direction. My loft doesn’t have any metal foil lining in the eves so there is just the heavy concrete roof tiles between it and the outside world.

For those who have never used a loop, they are usually mounted vertically and if you look at the loop rather like a polo mint, then the minimum radiation occurs through the hole which for me is North/South and the maximum radiation off of the ends of the loop (for me East/West) and the radiation pattern has the typical dipole figure eight pattern which I can confirm is the case with the MFJ loop.

The advantages of a magnetic loop are that you get a fairly effective antenna in a very small space (1 meter diameter), the disadvantage is that you have to tune the loop very accurately for the frequency you are working on. Though for reception you can listen plus or minus 100Khz or so on most bands and still hear most signals but for transmitting you do need to get the tuning on the nose for good results.

The tuning is done using the remote controller in the shack which requires a 12 volt power supply, which normally comes with the controller and is for 220V AC. The controller has 7 buttons and a crossed needle SWR meter. There is a power on button and lamp on button to illuminate the meter. There is also a Range High/Low button to set the SWR meter for 300W or 50Watts full scale deflection. The maximum power allowed for the loop is actually 150 Watts. I did once try 300 Watts and the loop was fine but the controller didn’t like it and started to smoke. Normally I use 100 Watts which is no problem.

The other 4 buttons are 2 coarse up/down and 2 fine up/down buttons and there is more than one way to tune the loop but this is how I usually do it.
On receive I depress either the coarse up or down button which locks depressed and the motor starts and begins rotating the butterfly capacitor on the loop. In the receiver I hear a weak interference signal from the motor (typical motor hash) and as the loop approaches resonance the interference gets louder and louder on my receiver normally ending up around S8 to S9 and I then press the coarse up or down button again to release it. I’ve usually then gone just past resonance so I press and hold the fine tune up or down button to go back to the resonant point where the S meter reading is highest and that’s it for receive. On transmit I then send a carrier and just use the fine tune up or down button to reduce the SWR to minimum, usually below 1.2:1 and I’m ready to transmit. Of course if I wander up or down the band or change bands then I need to retune the loop but that’s the price you pay for using a Magnetic loop.

The other way to tune the loop is to transmit say 10 watts or so of carrier through it and depress either the coarse up or down button depending on the direction you want to travel and wait until the controller issues a tone indicating it has found the resonant frequency, it then automatically stops the motor and you just have to press the coarse up or down button again to release it. In reality you may still have to jiggle the fine up and down buttons again to get the minimum SWR. A VI operator would need some sort of audio indication of minimum SWR to be able to tune the loop effectively.
There is an adjustment for the speed at which the motor rotates so you can set it to whatever you are comfortable with. A slow speed will make tuning easier but takes a bit longer. I tend to use a relatively slow speed though too slow and the fine buttons may not start the motor at all so there is a minimum you can realistically use.

But the important thing is how well does it work in practice and my first comment would be surprisingly well. Obviously if you can get up a multi-band 3 Element beam then you are not going to bother with a Magnetic loop antenna but many of us are not that lucky and in this situation the Magnetic loop can be a useful solution.

Here I have an FD4 Windom antenna running down the garden, this is an 80 metre dipole but fed offset (about 1/3rd of the way along it’s length) with a step down Balun to match the 50 ohm coax and has a fairly low SWR on 80, 40, 20 and 10 metres. Though works on the other bands with an ATU. I also have a 10 Metre Dipole in the loft.

The lowest usable frequency with the MFJ-1786 is 10Mhz and on this band it is usually around 2 S points or so down on the FD4 at best which is not surprising. Though as 10Mhz is nearly all CW the 12db drop in signal strengths still allows lots of contacts around Europe and beyond, but obviously I normally use the FD4 on this band.

The loop starts to do much better when you get to 14Mhz on this band the signal strength on the Loop is normally around the same as the FD4 in the East/West direction but a couple of S points down for stations located North or South, showing that the loop has real directivity. Whereas the FD4 being really an 80 metre dipole has lots of lobes and becomes almost omni-directional on all the HF bands from 20 Metres up.

In the direction of the loop East/West signals are rarely much down on the FD4 and sometimes up to 3db stronger on the loop. This applies to most signals around Europe and also most of the USA, though I notice that the loop does less well on the very distant DX such as Japan etc. Indicating that the radiation angle of the loop is not really low enough for this DX. I had thought about trying to tilt the loop to lower the angle of radiation but have never got around to trying it. I’ve worked many W/K stations on 20 Metre SSB with the loop.

On 15 metres my best DX is VK and again the loop works as well or better than the FD4 on this band in the East/West direction. On 10 metres it is the same though the 10 metre Dipole usually out performs the loop by 3 to 6db.

Commercial loops are not cheap and the MFJ-1786 is no exception but it really can allow you to put out a reasonable signal on 10 to 30Mhz from a very small space indeed. If you mount it somewhere where it can be touched then be very, very careful indeed as there are thousands of volts generated on the loop which can result in very nasty RF burns even on quite low power. If you really want to get the maximum out of the loop then being able to rotate it through 90 degrees is the way to go and you can either peak the signals in a particular direction or null out signals you don’t want to hear. I’ll get around to doing it one day.

Would I buy the MFJ-1786 loop again if I was looking for a compact HF antenna, the answer for me is a most definite ‘yes’. Just don’t expect it to compete with Kelvin’s (M0AID) SteppIR.

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