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Small Transmitting Loop Tuners from MFJ

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Small Transmitting Loop Tuners from MFJ

MFJ-936B_image_frontThis is from the “I’m not paying attention department”.  When I wrote the article on the AES IsoLoop I didn’t know that MFJ Enterprises has a wide selection of small transmitting loop antenna tuners.

The biggest challenge in building the small transmitting loop is not the wire loop – it’s getting the tuning capacitor.  The images I posted in the AEA Isoloop article showing homebrew loops showed that most of the capacitors used in those homebrew loops were “ham fest specials” or from the junk box.

MFJ Enterprises solves the problem of getting the capacitor for your loop antenna.  They have an number of loop tuners from $160 to $260.

The $160 model covers 160m to 10m and handles 50 watts.  The $260 model which also covers 160m to 10m can handle 300 Watts and has a built-in SWR Meter as well as loop current meter. I don’t know what significant parts are inside the $160 model other than the tuning capacitor but $160 seems like a lot of money for a box with a capacitor and a few other parts and no tuning indicator (loop current meter).

I can’t remember exactly how much I paid for the AES Isoloop.  It was about $329.  So. for another $70 or so what I got in addition to the (expensive) tuning capacitor is the stepper motor which allows remote tuning.  Why remote tuning?  Watch out for the RF!

RF Exposure in Loop Tuning

MFJ-935B_LoopTuner_RFExposureAn issue with the MFJ small transmitting loop tuners is that you have to be literally 1-2 feet or less from the loop while tuning if you use it in their preferred configuration.

For these tuners, the expectation is that you mount the frame for the loop directly on the top of the tuner.  If you inspect the images for these loops you will see a fitting on the top to receive a PVC pipe that is the loop framework.

Being this close to a loop antenna to perform the tuning can be dangerous.

The MFJ manuals for these loop tuners do in fact start out warning you about RF radiation hazards and they include a handy table.  Be aware that at 100+ Watts into a loop at resonance can generate 10’s of thousands of volts in near proximity to the loop.

If you look at some of the resources I posted with the article on the AEA Isoloop you will see a fluorescent light glow a few feet from a transmitting loop in operation.

So, a few advantages of the stepper motor approach to loop tuning are:

  1. No need to be in close proximity of the loop while tuning – keeps you away from the RF.
  2. Remote operation – you can place the loop where you want it rather than in a place of close physical access.  The AEA IsoLoop came with a 50 ft control cable.
  3. Precision tuning.  I don’t own any of the MFJ loop tuners but my experience with the AES IsoLoop is that the Q is so high on this tuned resonance circuit that even the smallest change in capacitance is going to produce a large affect.

This is not to discourage you from the MFJ loop tuners.  The principle is that same, but you can see the value of the stepper motor tuning approach – and the added expense to realize this value.

Sizing the Loop

The MFJ manual for these small transmitting loop antennas have some valuable information on the size of the loop you will need for a particular band.  Using the software in the references in the posting in the AES Isoloop you can theoretically calculate the loop sizes.  Starting from that point, you can easily experiment based on actual measurements of SWR, loop current, and reception reports.

Here are the loop size tables from MFJ to give you an idea of loop sizes.

Single Band Loop Lengths


Multi-Band Loop Lengths


Technical information in the MFJ Manuals

I suggest that you take a read on the MFJ-936B manual for the technical information that it has on small transmitting loops and how to use the their tuner in actual practice.

Here are a few paragraphs

A small loop antenna is one that is characterized by low-noise reception, works
well even when mounted at ground level, and has a conductor length or
circumference of less than 1/3 wavelength.  The ideal small transmitting antenna
would have performance equal to a large antenna, and a small loop antenna
approaches that performance. Bandwidth is quite narrow due to the extreme hi-
Q of the tuned-circuit configuration when paired with a capacitor.

The components in a resonated transmitting loop are subjected to high currents
and voltages because of the large circulating currents found in the high-Q tuned
circuit formed by the antenna. It is very important that capacitors used in this
antenna have a high RF current rating. Even a 100-W transmitter develops
currents in the tens of amperes, and voltages across the tuning capacitor in
excess of 10,000 V. This consideration also applies to any conductors used to
connect the loop to the capacitor. A piece of #14 wire may have more resistance
than the entire loop conductor! The best electrical connections possible, are
those using soldered or welded joints.

The heart of the MFJ-935B Loop Tuner TM is the “Butterfly” loop-tuning capacitor,
which has no rotating contacts. When coupled to a low-resistance loop
conductor, such as a copper strap, it provides a high efficiency-transmitting loop.
As the loop antenna is elevated, its efficiency improves accordingly. When
traveling, a room at some elevation above ground level makes for a better
portable operation experience with the MFJ-935B Loop Tuner TM. At very low
heights, close coupling to the ground causes detuning and losses due to current
induced into a mirror image of the loop below the surface with resistance of the
image loop proportional to soil resistance. Another loss component is due to
current flowing in the soil via capacitance between the loop and soil surface.
An operational height equal to 1/2 diameter of the loop antenna is recommended
to prevent detuning and excess ground losses when using the MFJ-935B Loop
Tuner TM loop antenna system. This means the tuner should be at that
recommended height, since it is connected to the bottom (ends) of the loop,
whatever the loop antenna configuration: Circle, Square, Hexagonal, etc.

For operation on the 14 MHz band and higher, ground losses are a minimum
near ground, so it is fine to operate on the ground floor. For the 7 MHz band and
lower, ground losses become significant on the ground floor. To reduce ground
losses, operate on a second or third floor.

For minimum ground loss when operating near ground, the loop should be
mounted vertically. For higher elevations (relative to the wavelength), horizontal
mounting will also give low ground losses.


As most folks know, there is a ton of activity on 20 meters morning, noon, and night – as long as the propagation gods are with you.  According to MFJ,  ground losses working the 20 band and above are minimal even at ground level.  So even if you don’t have a tower, or roof mounted antenna, you can work these bands from a near ground floor location.  In addition, no radials, counterpoise, other such “image” is needed for a loop.

The recommended size of the loop you will need, based on the table provided by MFJ, is only 7 feet for 20 meters and 4 feet for 17,15, and 10 meters.  So that’s 2 feet on a side or less to work all these bands.  Everyone should have enough space to set that up – even in a condo or apartment.

MFJ will also sell you the PVC to frame your loop – but you can get that from you local home improvement store.  So, all in all, MFJ will meet you half-way or all the way with the full loop setup or just the tuner.

I have had a lot of fun with my AEA Isoloop that I bought a decade ago.  With the MFJ small transmitting loop tuners starting at $160 and some cheap PVC pipe and wire – you are on your way.

Loop antennas would also make a good field day antenna or emergency preparadness antenna since its so easy to set up an very portable.


Limited Space Antennas – The Small Transmitting Loop Antenna – How to build your own.  Or, using OPL – the AEA Isoloop

MFJ-936B Manual


Written by frrl

June 7, 2009 at 12:30 pm

One Response

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  1. Great Article. While looking for information on loop antennas and the MFJ loop tuner I came across your atricle and learned what I needed to make a good decision on the purchase of the loop tuner.
    Walter KI4UCJ

    Walter Marrone KI4UCJ

    June 20, 2010 at 10:29 pm

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