MFJ-1788 SUPER HI-Q LOOP ANTENNA© 1999 by T. J. "Skip" Arey N2EI EQUIPMENT REVIEW MFJ-1788 SUPER HI-Q LOOP ANTENNA MFJ Enterprises, Inc., P.O.Box 494, Mississippi State, MS 39762. This is probably as much a story about camping as it is about an antenna. Historically, it seems, QRP and camping activities go hand in hand. Many fine articles have been written about backpackers climbing to dizzying, often frigid heights carrying rigs that weigh mere ounces feeding all manner of wire configurations. Still, there are other campers. Folks that can be found in any of hundreds of campgrounds on any given weekend. I’m talking about people like me. Pop-up Tent and light trailer campers. I’ve done my share of adventurous deprivation backpacking throughout my college years. I’m more settled now and don’t mind pulling a bit of weight behind my mini-van in search of a woodland weekend. While this commitment to trailer camping allows me to carry radio gear of less modest size (and a decent sized battery to keep me in QSOs for a long weekend) it does have other limitations. Family campground culture has its own set of rules. The first obstacle to serious radio fun is that many campsites formally prohibit stringing wires in the trees, no matter how carefully they are placed. Campground culture frowns on things that bring the possibility of “clotheslining” someone on the way to the “two holer” in the night. So tree tossed dipoles are often out of the picture. Then there are vertical antennas. Then again, even the fewest number of radials needed to begin to get a few electrons to leave the surface of a traditional vertical could also come under scrutiny in a family campground. What else can a campground camper use to keep the temporary neighbors from complaining? Enter the MFJ SUPER HI-Q LOOP ANTENNA. Measuring just 36 inches in diameter and requiring no additional ground wires, the antenna attracts no more attention at a campground than any of the dozens of portable TV antennas and satellite dishes that sprout from various trailers. Let’s take a look at the antenna specifications and then I’ll give you a run down on how this set up works in the semi-wilderness of campground camping. Small loop antenna theory has been discussed by folks with far more engineering background than I have. The short version is that an antenna of this type functions primarily because of its ability (some would say need) to be finely tuned to any particular operating frequency within its design range. This is accomplished by an air variable capacitor mounted right at the radiating element. Accurate adjustment of this capacitor to bring the antenna into resonance is critical and could be described as the biggest liability to such a design. Well, Maurice Ju and his folks down in Mississippi have got this problem licked. In their design, the capacitor is tuned by way of a motor drive connected to a control head that also allows for monitoring of SWR. This remote control unit in and of it self is larger, weighs more than any of the transceivers I hook to it. At 6mA standby and 20-55 mA of active current would be considered a bloated power hog by many of our backpacking compatriots. Ah, but as a campground camper, I am not constrained by what I can carry in my Kelty. If I want to get really lazy I can even hook up the supplied “wallwart” power supply to (shudder) line current. In my case this is not often a problem. My various home brew rigs are rather miserly and my gel cell is rather generous so the current needs to keep the 1788 happy are not out of line with my regular radio practices. The 36 inch loop and capacitor/motordrive come as a single unit that can be clamped to the mast material of your choice allowing for a diameter of 1 through 1 ½ inches. This radiating unit is well designed using welded construction that should hold up under just about any practical application. The capacitor/motordrive and the entire center mounting section is covered with a high impact plastic. This is not only aesthetically pleasing but prevents those accidental bumps up against the surprisingly substantial RF present at those points. (Do the math and you’ll choose to keep the covers on during operation) The radiator unit can be mounted in either a vertical or horizontal polarization position. Your decisions in this area will require a bit of time with the Instruction Manual as they are critical to loop efficiency. For example, horizontal polarization of the loop will yield an omnidirectional pattern that will sacrifice some skywave effectiveness but produces a null directly below the antenna that will reduce interference to other electrical devices. An important consideration as TVI is a problem in campgrounds too. The tradeoff here is that you need to get the antenna at least 20 feet above any metal roof or other ground plane. This can best be accomplished in a campground environment by any one of the various nested masts that are available commercially or can be creatively homebrewed out of hardware store materials. Most folks who chose to use this antenna in a nomadic setting will probably go with vertical polarization of the loop element. This introduces some directionality but given the portable nature of the installation, adjustment by the “armstrong” method should do just fine. The Instruction manual pointed out something I hadn’t thought about with vertical polarization of a loop antenna. While the antenna exhibits the low angle of radiation of any vertical, unlike a traditional single element vertical antenna, a vertically polarized loop also radiates straight up and down from the antenna. This leads to a high angle of radiation that can be used very effectively to cover shorter distances by way of skywave propagation. If you check over your logs, this probably reflects a good chunk of your casual communication. When I’m camping, I’m relaxing and, while I won’t turn down a DX QSO, I’m just as happy to ragchew with some folks in the region. If you feel obliged to reduce this “up and down” radiation component, the manual has a few suggestions. I’ve never felt obliged to take those steps as the antenna meets my needs without such measures. Incidently, the folks at MFJ take pride in their equipment being designed “by hams for hams”. In this spirit, the instruction manual goes into a great deal of detail about the subjects of practical propagation and use of this product in real world conditions. Also, the MFJ folks have respect for amateur radio operator skills and they are not afraid to tell you to occasionally “lift the lid” and make a few reasonable adjustments to improve overall performance. This attitude is very refreshing and is reflected in several entries throughout the Instruction manual. Primary installation instructions lean toward a permanent installation, either outdoors or in a restricted space environment such as an attic. Given that my uses and needs were slightly different, I still found the information useful in planning how I would adapt the 1788 to courteous campground use. If you are going with a more permanent installation, I would give some thought to sealing the radiating unit a bit more. In OEM. trim, the plastic cover does leave a few small openings that would make attractive entry points for the various insects and arachnids that are so often seeking shelter and nesting places. Any plastic friendly, RF transparent sealant would solve this problem in short order. However, the manual does warn that you to leave adequate drainage for condensation so make the same kind of balance of choices in this area that you would apply to such things as beam traps. The biggest adjustment a user needs to make is in adjustment of the antenna. Small loops have very sharp tuning. A first time user would be well advised to follow the step by step tuning instructions in the manual. This is relatively important as it is remotely possible to do things wrongly enough to cause damage to the control head. However, a session or two tuning this antenna up will turn most of the effort into second nature. The control panel of the unit gives the user a great deal of information. LED’s monitor the movement of the motor drive and a twin needle SWR meter helps the users to bring the loop into tune. This meter is calibrated in 300/30 Watts forward, 60/6 watts reflected. On the surface this would seem to be a bit too generous for QRP work, however, since you are tuning for minimum SWR in all cases, the 6 watts reflected scale gives adequate deflection in all but the most minute QRPp power levels. The manual recommends 1 watt of tuning energy minimum. For many of us, that is running full tilt. If you have been known to resort to QRO power levels, the 1788 can handle up to 150 watts by design. The Super High-Q Loop comes with everything you need except coaxial cable and a mast suitable to your application. As with most any antenna application, high quality coax with the lowest practical losses is the most desirable. The radiating unit weighs in the neighborhood of 12 lbs. according to my bathroom scale so the manual’s recommendation of galvanized steel or thick walled aluminum masts is a good one. The manual also gives very specific directions concerning guying such masts to assure antenna efficiency. If your application requires guy wires, you will want to study this information carefully. My personal preferences are for a pair of 10 ft. steel mast sections with vertical mounting of the radiating unit. I secure the masts along the side of the camper. Since I can take the unit down when not in use or in high wind conditions, I do not worry myself with guying as the mast is extending only about 10 feet above the camper. In the future I may consider trying to get high enough to utilized horizontal polarization but that will require more thought. Okay, so how does the thing work on the air. Lacking a proper antenna measuring range and adequate test equipment to give true information here, I must resort to the highly subjective but fairly practical “fun” meter. The folks at MFJ claim performance just slight below par for a dipole hung under similar conditions. Well, given the camp ground environment which is full of nearby metallic surfaces, electromagnetic noise and overall antenna heights well below optimum, I would still have to say that this antenna works “good enough”. My goals are simple when I am on a family camping excursion. A couple of hours of friendly ragchewing is all that I ask. I really don’t care if the person I am in QSO with is on the other side of the world or on the other side of the campground. Subjectively compared to dipoles I have used under similar conditions. I find the loop design much quieter in terms of received QRN. As I have said, campgrounds can be very electrically noisy places. As with most MFJ products there is a one year “No Matter What” unconditional warranty and a 30 day trial with full refund policy. Arguably on of the better deals in the hobby.
Given the antenna’s $429.95 price, this does not represent a casual purchase.
Many of you are probably saying you can get a beam for less and you would be
right. However, many folks find themselves in the unenviable position of needing
to put up an antenna on minimal real estate and with the lowest “eyesore”
quotient possible. Folks subject to such limitations could spend many hours
and dollars trying to come up with solutions that will allow them to continue
to enjoy the radio hobby under less than ideal conditions. Back |