MFJ Magnetic Loop
If you want an antenna that really works, and takes up next to no space, then you can't do better than a magnetic loop.
The MFJ magnetic loops are one of only two commercially available magnetic loop antennas that are currently available. They are on the expensive side, but the other European-made product is even more pricey. If you want to try building your own, there are numerous websites available containing the information you need, but making a remotely tuned magnetic loop capable of withstanding 100W power level is not a simple project and certainly beyond the capabilities of this radio amateur. It's not an inexpensive project either, once the cost of tubing, a high voltage variable capacitor and motor drive are taken into account, making the MFJ not such a bad deal.
I have owned two of the MFJ loops. The first was the MFJ-1782, the model that covers 30m - 10m with the manual control unit, to save a bit of money. This was my first experience of a magnetic loop and I was very impressed. Even indoors, in the attic of my bungalow only 3m above ground level, it outperformed a Diamond CP-5 vertical by a few dB on 10 metres. The advantage over the vertical became less as the frequency dropped, so that on 20m the vertical sometimes had the advantage. Nevertheless, for such a small antenna the magnetic loop's performance is nothing short of excellent.
The MFJ magnetic loops are about 36 inches in diameter, made of aluminium, which is welded to the tuning capacitor to minimize losses. A flimsy and rather poorly made plastic cover protects the tuning capacitor and its motor and the coupling coil from the elements, if the antenna is mounted outside, but it seems designed more to let rain run out than to keep it out in the first place. There are gaps between the cover and the antenna, and others who have installed the MFJ out of doors claim to have experienced problems after a year or two and found insects or even wasps nesting inside! Installing the antenna inside the attic avoids such problems.
The MFJ magnetic loop may be mounted vertically or horizontally. However, the manual makes clear that it must be mounted high - such as on top of a pole above the roof of a house - to work properly in the horizontal position. Mounted vertically, height is not important, and it is claimed that magnetic loops can be used even at ground level. This makes it ideal for use in an antenna-restricted location. If permanent mounting is not possible, a magnetic loop can be mounted on a tripod stand, brought out when required and stored away the rest of the time. However, if this is done, care should be taken to keep it out of reach of fingers, since potentially fatal high voltages are generated on the surface of the antenna.
Mounted vertically, a magnetic loop is claimed to be directional, similar to a dipole. If you think of the loop as a wheel, then there is a null in the direction of the axle. However, the reality is not as simple as that. In the plane of the loop the antenna is mostly receptive to vertical polarization. However, round to the sides and at higher angles it becomes more receptive to horizontal polarization. I could not detect any significant directivity. When compared to a wire antenna, switching between the two, I found that the loop would sometimes have the stronger signal, and sometimes the wire. A little later, the situation could be reversed, even with the same station! So it is mostly down to propagation and how it twists the polarity of the signal.
The high Q of the magnetic loop means that the 1:1 SWR null is only a few kHz wide, and retuning is necessary whenever you QSY in frequency. Tuning the MFJ magnetic loop is not difficult, even with the manual tuner. I found that brush noise from the motor that drives the tuning capacitor provided an effective noise source when tuning the antenna, so it was possible to tune it close to spot on just by ear. This is good enough for listening. If I wanted to transmit then I could tweak the tuning it for best SWR using the metering on the radio. In common with many people, I found it necessary to bend the copper coupling loop (hidden inside the plastic casing) by trial and error to enable a 1:1 match to be achieved on all bands.
Some amateurs consider the need for retuning whenever you QSY to be a disadvantage of the magnetic loop. Certainly it is a disadvantage if you are able to put up an antenna that works as well and does not require constant retuning. However, the sharp tuning is a consequence of the magnetic loop's high Q, the factor that makes it so efficient for its small size. I'd rather live with that than use a broader tuned but much less efficient small antenna, which is the only alternative. It's a pity that MFJ could not come up with a computerized control unit that automatically tune the loop with the push of one button. I'm sure it would not be difficult to do with today's technology.
Soon after purchasing the MFJ-1782 I moved to a new QTH where outside antennas were not permitted. This was actually my reason for buying the MFJ loop in the first place. However, once there I found that the access hatch was too small to allow the antenna to pass through into the attic. I did not feel able at that time to make the necessary modifications to the house, so I sold the MFJ and used a wire loop in the attic for my antenna. This worked well for a number of years, until increasing noise levels made me consider getting a magnetic loop again. Due to being at the bottom of the sunspot cycle, this time I went for the MFJ-1788 model covering 40m - 15m with the so-called "automatic" tuner.
The tuner unit of the MFJ-1788 (and its 30m - 10m counterpart the MFJ-1786) is in fact only semi-automatic. It has the same two pairs of buttons as the manual controller for fast and slow tuning, but has in addition an SWR sensor that detects when an SWR null is reached and cuts off the power to the motor, by which time it has overshot the null a little. You then use the slow tuner button to bring it back to a perfect match. There is an illuminated cross-needle SWR meter to help with tuning.
The only difference between the MFJ-1788 and the MFJ-1786 / MFJ-1782 antennas is that the former has a larger tuning capacitor to enable it to tune down to 40m. Because the minimum capacitance of the larger capacitor is higher, the antenna loses the ability to tune down to 12m and 10m.
Despite the fact that the magnetic loop is electrically very small at 40m it still performs very well, outperforming my 20m loop of wire in the same attic, whether configured as a continuous loop or a "dipole bent into a square" configuration. On the higher bands it unsurprisingly performed just as well as the other model, outperforming compact trapped verticals and probably within a couple of dB of a full sized dipole.
The MFJ magnetic loop is my first choice recommendation for anyone who wants or needs to set up a covert or stealth ham radio station. It can be used outdoors, permanently mounted or on a temporary stand, or indoors. I know of no other antenna that is that small yet still capable of radiating such a potent signal.