Hardware Store 40m Magnetic Loop Antenna for Regional and EMCOM Use Richard Bono NO5V QST Antenna Design Competition 80 through 10 meter entry Overview: This describes a field deployable magnetic loop antenna for 40m band using (mostly) materials from the local hardware store. The magnetic loop antenna is deployed close to the ground and provides a directivity pattern suitable for NVIS use making it a field expedient EMCOM HF antenna with a very small footprint. Key to this design is a high voltage cylindrical capacitor that is also an integral part of the antenna structure. Design Points: I used AA5TB s magnetic loop antenna spreadsheet to determine the basic design with the loop conductor composed of a 22.5 foot length of RG 213 coax. RG 213 had a braid diameter of 0.314 inches. This cable is terminated with a PL 239 connector on each end. A loop of these dimensions gives the following design points: :
The estimated efficiency is 41% with zero additional losses. The loop is greater than 1/10 wavelength but smaller than a ¼ wavelength. The bandwidth is estimated to be 13.1 khz. Capacitor Requirements: I based this design around operation at 100W although in a field or EMCOM scenario 20 25W watts would likely be suitable for regional communications. At the 100W design point, the capacitor must withstand 4.3kV. Rather than using an expensive air variable or vacuum variable capacitor, I opted to build a homebrew cylindrical capacitor made with copper plumbing pipe. This is a similar to the wellknown trombone capacitor with some important differences. First, the capacitor is a single telescoping section as opposed to a trombone capacitor, that has two sections in series. The ends of the copper pipe have SO 239 chassis connectors soldered directly to them so that the RG 213 coax connects directly to it. This makes the capacitor an integral part of the antenna structure and minimizes losses. A threaded rod is used to manually adjust the capacitance by telescoping the pipe nested lengths. The design spreadsheet shows that a capacitance range of 42.7 48.1pF is needed to tune the 40m band (after subtracting out the 18.5pF of distributed capacitance from the loop itself). This is a view of the capacitor:
Capacitor Design: The capacitor itself is composed of nested copper plumbing pipe. The outer pipe is ¾ and the inner pipe is ½ pipe. The gap between the two pipes is filled with polyethylene sheet to form the dielectric. The capacitance per inch of a cylindrical capacitor is calculated from the following equation: Where = permittivity of dielectric (PE = 2.25) 0.614 log 1 2 D1 = inner diameter of outer pipe (0.811 inches for a ¾ copper pipe) D2 = outer diameter of inner pipe (0.625 inches for a ½ copper pipe) This gives a capacitance per inch of pipe = 12.21 pf. The gap that needs to be filled is 0.093 inches. The PE film I used is 4mil thick clear plastic sheet that requires 23 turns around the length of the inner copper pipe. This represents 51.5 inches of plastic sheet wrapped around the inner conductor. The length of pipe was selected is 7 inches long to provide room for mounting it. The tuning range for the 40m band represents less than a half an inch of travel. PE has a dielectric strength of 18.9 kv/mm. This provides considerable headroom against the 4.3kV needed for 100W operation. Capacitor BOM: Item Qty Supplier 1 1/4" x 1/2" PVC reducer 1 Home Depot 1 1/4" x 2 ft PVC pipe 1 Home Depot 1 1/4" PVC Coupling 1 Home Depot 3/4" type M Copper pipe 2ft 1 Home Depot 1/2" type M copper pipe 2ft 1 Home Depot 5/16" x 12" nylon threaded rod 1 Mcmaster Carr 5/16" lock nuts (10 pack) 1 Home Depot 5/16 nuts 1 Home Depot 3ft x 50ft 4mil PE Sheet 1 Home Depot SO 239 Chassis mount 2 Ebay Plastic cutting board 1 Dollar tree Washer 1 Home Depot sheet metal screw 2 Home Depot cable tie (small) 1 Home Depot Electrical tape Home Depot
Capacitor Construction: 1.) Cut length of 1 1/4 PVC pipe 4 1/2 long 2.) Drill through two holes 1 1/2 apart. One hole is 0.811 inches in diameter and the other is 0.50 inches in diameter. 3.) Cut copper pipes to 7 total length and deburr the edges. 4.) Solder one SO 239 chassis connector to one end of each pipe. I recommend using a vise to secure the connector then place pipe on top. Then heat the joint with a propane torch for about 30 seconds then run solder around the joint. 5.) Insert the larger copper pipe into the PVC drilled earlier such that about 1 inch protrudes through. 6.) Cut a piece of plastic cutting board that is 1 1/2 by 2 1/2 in size. Mine is about 3/8 thick 7.) Drill two holes centered on the board cut in #6 above and 1 1/2 apart. One hole should be.625 in diameter and the other 0.50 in diameter. 8.) Insert the smaller pipe into the large hole on the board until the chassis connector is nearly flush. 9.) Wrap a 6.5 x 51.5 piece of 4 mil sheet around the smaller copper pipe. Use scotch tape to hold the initial wrap in place and wrap as smoothly and tightly as possible with the ends as flush as possible. Use a small piece of scotch tape to hold the sheet n place. Note that the sheet will extend off the end of the pipe. Use a cable tie to secure the sheet to the pipe up against the plastic block. 10.) Drill two small holes just above and below the smaller hole on the PVC pipe. 11.) Jam a 5/16 nut into the ½ hole in the PVC pipe. Place a washer over the opening and use two small sheet metal screws to secure the nut in place. 12.) Thread a 5/16 lock nut onto the threaded rod such that about and 1 1/2 protrudes. 13.) Insert the threaded rod through the small board and then apply a second lock nut and tighten against the board just enough to allow the rod to turn. 14.) Wrap electrical tape around exposed sections of the larger diameter copper pipe. 15.) Line up the threaded road with the 5/16 nut and start running it through. When the smaller pipe enters the larger pipe, help guide it in while continuing to engage the threaded rod. Continue until about 1 of the inner rod is left. 16.) Install the 1 1/4 coupler to the PVC pipe on the side closest to the copper pipes. 17.) Install a 1 1/4 to ½ reducer to the coupler.
Lock Nuts Washer and captive Nut PVC Pipe Threaded Rod SO 239 Chassis mount Plastic Board Electrical tape wrapped around ¾ copper pipe PVC Coupler Tie Wrap Inner Copper Pipe wrapped in PE Sheet PVC Reducer
Antenna BOM: The following is the bill of materials required to complete the antenna: Item Qty 1/2" PVC Pipe 40 Inches long 1 1/2" PVC Pipe 46 inches long 2 1/2" PVC tee 3 Hi voltage Capacitor Assembly 1 22.5" RG 213 patch cable assembly 1 BNC female to dual binding adapter 1 BNC male to UHF Female adapter 1 FT140 43 Toroid 1 18 Gauge solid insulated wire. 1 RG 8X feed cable length as needed. 1 String or twine 1 Antenna Construction: 1.) Cut ½ PVC pipe to length indicated in BOM 2.) Cut ½ wide slots on two of the three ½ PVC tees 3.) Insert the modified tee onto each of the two longer PVC pipes. 4.) Insert one of the longer pipes into the center of the remaining (center) tee 5.) Insert the remaining pipes onto the ends of the center tee 6.) Cut a length of 18 gauge wire and wrap 5 even turns around the FT140 43 ferrite core. 7.) Attach the wire ends to the binding posts on the BNC adapter.
8.) Attach the BNC to UHF adapter to the BNC adapter 9.) Install Hi voltage capacitor to PVC pipe on end without tee. 10.) Run the RG 213 cable connector through the toriod and position roughly halfway along its length. 11.) With the PVC support arms and capacitor laying flat on the ground, connect one end of the coax to the capacitor. Wind the cable through the top tee then through the side tee then over to the other end of the capacitor and connect. This forms the loop 12.) Adjust the toroid so that it is supported by the side tee. 13.) Using string or twine, tie one end to the top tee and the other end to a convenient tree branch. Slowly pull the antenna up so that the loop is vertical. The capacitor will be in the 9 o clock position and the feed point ion the 3 o clock position. The loop at the 6 o clock position can be just a few inches off the ground. 14.) Tie off the support string.
Antenna Deployment Here is the antenna deployed in a low hanging tree branch:
Here is the antenna ready for deployment: The antenna when deployed is adjusted manually with one of two methods. One is to use an antenna analyzer to adjust the capacitor for minimum SWR on the frequency you with to operate on. The adjustment is done with the nylon threaded rod which has 18 turns per inch of sensitivity. The second method is to set your transceiver for the desired transmit frequency and then adjust the capacitor for maximum receive noise. Fine tuning is then done with a low power signal and an SWR meter.
The antenna is very narrow band and frequency changes require re adjustment. This is particularly important when running SSB mode. Antenna Test Results: When first deployed I ran the antenna on 40m WSPR with one watt of power. Here is the WSPR result after 24 hours:
The WSPR data indicates that the antenna is working well for its intended use as a regional NVIS antenna but also does a decent job on DX. This is not unexpected as the magnetic loop antenna has directivity pattern that supports both usages. Next tests were with PSK31 and JT 65 running 25 watts of power during morning hours. Made solid contacts with stations within 400 miles of the QTH. Further testing on JT 65 yielded contacts to Guatemala and Cuba. Last test was to check in to the daily Texas Traffic net on 7.285 Mhz on 100W SSB. The antenna was tuned to this frequency using an antenna analyzer. Net control in Kaufman, Texas (~560mi) copied my check in and QSO without issue. The antenna performed well at 100W SSB. Antenna Dimensions:
Hi Voltage and RF Exposure Estimates: Care must be taken to prevent contact with this antenna when in operation. At 100W of transmit power 4.3KV is present on the antenna conductors! This is an estimate of the RF exposure condition of operating this antenna at 100W on 40m. Note that I am using dipole gain. Summary and Future Work: The design goal of building a highly portable and field deployable 40m magnetic loop antenna for regional and EMCOM was achieved. Most of the antenna components including those for the hi voltage capacitor can be sourced from local hardware stores. Most of the RF components either are in your junk box or easily obtained from online sources. The capacitor design is also suitable for use in smaller coax loop designs and has been successfully tested on a 40m 15m loop and a 15m 10m loop. This design also lends itself well to the addition of motorized control of the capacitor. A small DC or stepper motor can be easily coupled to the threaded rod for direct drive.
References: 1.) Amateur Radio RF Safety Calculator v1.2 (2015-08-18) by Paul Evans, VP9KF, Hintlink Technology - http://hintlink.com/power_density.htm 2.) Small Transmitting Loop Antennas Steve Yates AA5TB http://www.aa5tb.com/loop.html 3.) A Home Brew Loop Tuning Capacitor Bill Jones KD7S QST November 1994 4.) The Trombone Trimmer Jay Rusgrove, WA1LNQ QST November 1975