Suitcase Portable SLOOP (Shielded LOOP) Receive Antenna for 160 or 75 Meters

By KØFF

For MF and low HF (160, 80,75) fixed station use, the Beverage is probably the first choice among receive antennas. When space does not allow such a large antenna, the Shielded Loop or SLOOP made from inch CATV cable is another proven performer (see my article on the SLOOP on eHAm). When traveling to a remote DX location for an expedition though, it may not be practical to transport such large loop of coax, nor is its ruggedness necessarily needed for a limited operation. For these instances, I have developed the Suitcase SLOOP, which uses flexible RG-8U coax cable, and can easily be assembled and broken down again.

Shown here is an example from which you may draw ideas.

The subject of the shielded loop is covered in great detail by ON4UN in his excellent book : Low Band Dxing

So look to those original articles for the theory behind this type antenna. Briefly, it is a loop of coax cable, less than 1/10^th of a wavelength, that has the shield split at the midpoint by a gap. Most designs simply resonate the loop with a capacitor, and run it directly to a coaxial feedline to the receive-only port on the station rig.

I have always figured that the operating impedance of such a high Q loop is being swamped by doing such a simple feed scheme, so a coupling transformer has been included. By using a transformer made from an Amidon BN 73-202 Binocular core, with a loop side winding of 12 turns, and a feedline winding of 2 turns, a very good match is achieved using an MFJ Antenna Analyzer, and also looking at it with an Autec RF-1. These figures were arrived at by the back door method, that is by winding a series of transformers that got closer and closer to a good match, until a set of figures could be determined, and then the final turns count was calculated. This ratio is 6 to one, and since the impedance transformation is the square of the turns ratio, that would indicate that the loop itself is about 36 x 50 or 1800 Ohms. This makes sense to me, as the antenna is very narrow, having a 2:1 SWR bandwidth of about 40 KHz on 160.

Any other interpretations will be welcome.

Now, on with the mechanical layout.

We start with 20 feet of RG8U coax cable for 160 (10' for 80) to which a quality PL259 if soldered to each end. Once that is done, fold the length in two, and mark the exact center. At that point, remove approximately of the outer black insulation, and also the braid below. By doing so we form the gap, which is the heart of this antennas' success as a noisebuster. To strengthen and weatherproof the gap, wrap a few turns of Scotch 33+ tape over the wound, and top that with some heat shrink tubing.

For the main housing a cast aluminum outdoor electrical box called a Three Hole. Single Gang- , Suitable for Wet Locations is utilized.

Into the top hole is screwed a threaded to PVC fitting which has been strengthened by a brass plug. The plug also serves to waterproof the fitting.

A plastic 5/8 button plug is inserted fully into the bottom hole, to seal it from weather, and then a galvanized close nipple is screwed all the way in that hole. When mounting your finished antenna, this piece of threaded pipe will adapt to the larger pipe needed to fit the rotator, or other mount.

On the back side of the box is the third hole, and a standard chassis mount SO-239 fits nicely over that hole, for eventual connection of the feedline. Two more SO-239's need to be mounted on the blank sides as well, and there is where the loop coax with attach. Holes will need to be drilled for these.

Inside the box, between the loop coax connectors is where the resonating capacitor (s) is mounted. In the example shown, it is pre-tuned for the CW segment of 160, and therefore a variable is not used. In some DXpedition activities, the operating frequencies are well known in advance, and a variable may change position during transit, so we used fixed Silvered Micas to eliminate on more source of problems. You will undoubtedly want to include at least some variable capacitance in your unit for tuneup experiments. Then if you wish, they may be measured, and replaced with a fixed unit.

With Polyfoam RG8U, which has a stranded center conductor, it will take 800 pF to resonate at 1825, and 400 +/- for 75 meters. Each specific type, brand and even lot number will change this figure a bit.

A frame is constructed using PVC schedule 40 waterpipe, and cross(4 way) fitting . The 4 pipes are cut to 40 inches in length, and inserted into the cross to form a big X and the bottom is seated into the adaptor on the top of the box housing. The other three ends are terminated in TEES through which the coax loop is fed, and which hold the loop's shape.

In the example shown, I used TEES instead of , because the PL259 will easily fit through the ends, and it makes the wind profile a bit smaller. Some extra work was needed, because no adaptor could be found to go between the sizes, so I simply took a piece of PVC schedule 40 pipe, and reduced an inch of it's length for a slip fit INSIDE of the frame members. Into to top of this adaptor is inserted a 9/16 Nylon Hole Plug, to further limit the ingress of water into the framework.

All that is left is to assemble and tune the unit. There will be a sharp decrease in the SWR at the exact resonance point, and the impedance should read 50 Ohms.

Of course an antenna like this is used to reduce the local man made noise and improve the signal to noise ratio of the desired incoming signal. There is a tradeoff of ultimate signal strength, so a preamp may be in order.

If there is enough interest, I may make a partial KIT available, which would consist of the finished boz and all machined parts. You would have to supply the coax cable and PVC pipe. Don't forget that this construction techniqe should be used for temporary activities only, for fixed station use, the regular SLOOP is highly recommended.

Happy Homebrewing

Geo> KØFF