Hardware Store 40m Magnetic Loop Antenna for Regional and EMCOM Use
Richard Bono – NO5V
QST Antenna Design Competition – 80 through 10 meter entry
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
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.
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 well‐
known “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:
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)
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.
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
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
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
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
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.
PVC Pipe Threaded Rod
Lock Nuts Washer and
The following is the bill of materials required to complete the antenna:
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
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
11.) With the PVC support arms and capacitor laying flat on the ground, connect one end of the coax
to the capacitor. Wind