There are 2 versions of the Battle Creek Special shown - a wire version. which will work out first. The traps are made of coax cable as described by W1FB. The second version is a 'real' vertical make of tubes complete with a description of the traps... I hope to unsecret the mystery of the Battle Creek Special and the construction of it.

Other authors have written articles about coaxial traps and their use. A computer program for accurately designing coaxial traps is one of the many contained on the VE3ERP 3.5 inch diskette for DOS, which is available for $5. (Note: download here).

There is another nice tool for coax trap design. Tony Field, VE6YP, released a Windows based program producing reliable and proven results. Therefore, I will not include the design procedures for coaxial traps in this article. Instead, let's discuss trap performance and the mechanical aspects of building these devices. Coaxial traps that are wound with RG-58 cable will safely accommodate the full legal amateur power level. They may be made more compact and lightweight by using miniature RG-174 coax and smaller coil forms, but the maximum safe power level is 500 watts for the smaller coax. RG-174 coaxial traps are desirable for portable multiband antennas.

Conversely, RG58 cable requires a larger and heavier coil form, which must be a consideration if several traps are contained in one antenna. It becomes obvious that strong, thin-wall coil forms are better for these larger traps. The weight of the coax for 160 and 75 meter traps, especially, is a tad on the hefty side to begin with PVC plumbing pipe is readily available and works nicely as coil-form material. Try to select the thin-wall variety. The traps pictured in this article are wound on thick wall pipe (3/16 inch wall thickness) with a 2 3/8 inch OD, simply because that's what I had on hand. The 1.9 MHz trap weighs 14 ounces without the end caps and eye bolts installed. Fiberglass, or thin-wall cloth phenolic tubing, would reduce the weight substantially. The 3.85 MHz trap weighs 11 ounces with the 3/8 inch Delrin end plugs and other hardware in place. Wooden end plugs, boiled in canning wax, would decrease the weight by a couple of ounces.

The plastic plugs are drilled and tapped for 6-32 screws (three for each plug). If wooden plugs are used, you may use flathead wood screws to affix them. It is wise to seal the open ends of the trap cable with epoxy cement, GOOP brand sealant, or Coax Seal 9(r) putty. This will prevent moisture from migrating inside the shield braid and contaminating the RG-58 cable.

The traps described in this article are wound with RG-58AIU. Other small 50 ohm coaxial cable is suitable. If you want to add a margin of safety for high power, you can use RG-8X coax. The Radio Works, Inc.s catalog lists their top-of-the-line RG-8X as CQ-8X M/M. It is a marine-grade cable that is high-temperature stable and contains solid dielectric crushproof insulation. Since RG-8X has a greater OD than RG-58, a longer coil form would be required. Fig. 2 shows the assembled 160 and 75 meter traps. The medium-size unit with the end plugs is for 3.85 MHz. Included in the photo is a 1 inch OD mini trap for 20 meters that was built by my son, Dave DeMaw, N8HLE, for use in his QRP trap dipole. This tiny trap is wound with RG-174 and weighs one ounce. It is coated with tool-handle dip. Some of you are probably wondering where an amateur might use a 160 meter trap. Traps for that frequency are useful in a 75 meter fullwave loop to permit 1.8 MHz operation. Half frequency loops (e.g., a 75 meter loop on 160 meters) perform miserably. They provide substantially less gain than a full-size dipole. However, if the loop is opened electrically opposite the feed point by means of a 160 meter trap, the antenna performs as a 160 meter " scrunched" dipole and works quite well. The trap effectively opens the loop at 160 meters, but closes it for the bands below 160 meters. Multiband operation assumes that tuned feeders are used with the 75 meter loop. Note that the center conductor at one end of the coil is connected to the shield braid at the opposite end of the coil. The trap turns are close-wound.

The following electrical constants are based on a coil-form diameter of 2.375 inches and RG58A/U cable for the 3.85 MHz trap described here. An effective capacitance (28.5 pF/ft.) of 241 pF and an inductance of 7.14 uH (Xc, XL = 172) results from using the required 104 inch length of coax cable. The coil length is 2.53 inches and the form factor is 0.98:1. There are 12.45 close-wound turns of cable wrapped around the 4 3/4 inch length of PVC tubing. Table I lists winding information for a variety of coaxial traps. The referenced QST article by R. Sommer. N4UU, contains charts that can be used to design traps for any HF band when using various coil form diameters. If you have an IBM-compatible computer, you can utilize the aforementioned VE3ERP software to design these traps precisely. The recommended form factor (diameter to length ratio) for these traps is from 1:1 to 2:1. This range ensures the high Q that makes the traps effective for divorcing the unwanted parts of an antenna.
 

MHz	Form O.D.	Form L	Turns No.	Coax L	Net C	Net L
7.15	2 3/8	4.0	11.85	66.51	152.24	3.28
3.60	2 3/8	4 1/2	13.20	110.25	255.00	4.00

The letter L in this table (other than the right-hand column) refers to length, which is given in inches before the shield braid is separated from the inner conductor The braid at one end of the coil must be long enough to reach to the center conductor at the opposite end of the trap. The remaining shield braid and the two center-conductor stubs are approximately 1 inch long Resonance of the completed traps may be checked with a dip meter while monitoring Ihe dipper signal with a calibrated receiver.