A crossed-dipole turnstile antenna for 2-meter ARDF
I came up with this antenna design for use with hidden transmitters on
international-rules ARDF events. The rules call for an omnidirectional
horizontally-polarized transmitting antenna at a height of two to three meters
above ground level. Here in Southern California we have typically used
vertical polarization in the past. This same antenna could be used for
satellite or space communications as well with the addition of an appropriate
The electrical design is a classic crossed-dipole with 75-ohm phasing section.
W4RNL has an excellent article on the electrical characteristics of this
antenna with schematics and patterns
online here. This basic design has a slight mismatch due to the difference
between the 36-ohm characteristic impedance of the antenna and the 50-ohm
feedline. This results in an SWR of about 1.3 to 1.
The parts used are relatively inexpensive. It will probably be cheaper to
build several antennas at once due to the burden of buying some items in
- 2 ea. 1 1/4 inch trade size PVC pipe caps
- 2 inches long 1 1/4 inch trade size PVC schedule 40 plastic pipe
- 1 ea. 1/4-20 eyebolt
- 2 ea. 1/4 inch internal star lockwashers
- 2 ea. 1/4-20 hex nuts
- 2 ea. 36 inch long 1/8 inch diameter uncoated bronze welding rod (see text)
- 4 ea. 8-32 x 1 inch long hex spacers (see text)
- 4 ea. 8-32 x 3/4 round-head machine screws
- 4 ea. 8-32 KEPS nuts
- 4 ea. #8 locking solder lugs [Mouser 534-906]
- 5 ea. ferrite cores [Mouser 623-2643002402]
- 16 inches RG-179 75-ohm teflon coaxial cable [Mouser 566-83264]
- RG-58A/U coaxial cable of desired length for feedline (see text)
- Suitable connector for your radio to fit above cable
- Cable ties
- Heat-shrink tubing
- Silicone sealant
- PVC solvent cement
The case of the antenna is made from PVC plumbing pipe and fittings.
- Drill a 1/4 inch diameter hole in the center of each end cap.
- Cut a piece of pipe approximately two inches long. Fit it snugly
into one of the end caps. There should be about a half inch protruding.
- The next step is drilling the element holes at 90 degree spacing
around the circumference of the end cap. Aligning these can be tricky,
but the following shortcut works well. Cut a narrow strip of paper long
enough to wrap around the end cap at least once. Make a mark on the
paper where it overlaps. Remove it from the end cap and fold it in half,
aligning the marks. Then fold in half again. Unfold and make a mark at
each crease. Now re-wrap it around the end cap and transfer the marks
to the end cap. They will be at 90-degree spacing. Mark the end cap in
four places, 3/8 of an inch from the lip of the cap where the pipe protrudes.
Drill four 11/64 or #16 clearance holes for 8-32 bolts at the marks, going
through both the end cap and the pipe snug-fit inside. It is not necessary
to glue the pipe to the cap. The bolts will hold them together.
- Thread a nut and lockwasher on to the eyebolt. Apply a small amount
of silicone sealant to the threads and push the eyebolt through the hole
in the top of the end cap/pipe assembly. Thread a second lockwasher and
nut on from the inside and tighten with a socket wrench.
- Thread five ferrite beads on to the jacket of the RG-58 feedline.
- Thread the feedline through the center hole of the other end cap.
- Prepare the matching section of RG-179 by stripping both ends and
separating the braid from the center conductor. Stripped length should
be 1 1/4 inches on both ends, with 13 1/2 inches of jacketed cable inbetween.
The velocity factor of RG-179 is 0.69. If you substitute a different
cable, adjust accordingly for a 1/4 wave section.
- Attach solder lugs to one end of the cable, insulating with heat
- Prepare the end of the RG-58 feedline in a similar manner, stripping
1 1/4 inches and separating the braid and center conductor.
- Solder both the shields of the RG-58 and the matching section to
a solder lug, insulating the shielding with tubing.
- Similarly, solder both center conductors to a solder lug.
- Using longnose pliers, thread the two lugs on the loose end of the
RG-179 phasing section through two holes 180 degrees apart from each other.
Secure in place with KEPS nuts. Tighten the nuts securely while holding
the screw head from turning. Take care with the center conductor as it
is rather fragile.
- Similarly, secure the two lugs on the feedline/phasing combination
through the two remaining holes.
- Secure a cable tie to the feedline below the point where the jacket
is removed to act as a strain relief.
- Carefully fold the RG-179 cable into the pipe, watching for possible
- Press the end caps together on the pipe and perform an ohmmeter check.
You should see continuity between the two studs connected to the shield braids
90 degrees apart from each other, and continuity between the two center
conductor studs, also 90 degrees apart. The pairs should not be shorted
to each other.
- If all checks out, spread a blob of silicone sealant around the cable
tie on the feedline to seal against moisture, apply some solvent glue to
the protruding pipe and quickly push the two end caps together until they
butt against each other. Hold in place for a minute or two until the cement
The elements are made from 1/8 inch diameter bronze rod, commonly available
at welding shops. This material comes in 36 inch lengths and is referred
to as material 15, uncoated. I have used it for making numerous antennas
and other projects. At one time it was sold by the piece. The last time I
went to purchase it, I was told that it was only sold in sealed containers
by the pound. One pound has eight pieces, enough to make four antennas.
Apparently the vendor wants to ensure that the buyer receives warning
notices about the dangers of welding, etc. and thus it is now only sold in
sealed containers. Your experience may vary. Similar material is likely
available at hobby and hardware stores at higher cost, but you won't have
to buy a pound of it for one antenna. The 1/8 diameter material is a
slip-fit inside the 8-32 spacers. Cut the rods exactly in half, yielding
two rods 18 inches long. This length plus the added length of the spacer
and the case gives a good match in the 2-meter amateur band.
On my first production run of these antennas, I purchased plated brass
spacers. I inserted the rod about 3/8 inch into the spacer and soldered
them in place. The Mouser part number for these spacers is 534-1474E.
A substantial amount of heat and a good flux is required. Consider the
use of a torch if you have one. Take care not to get solder into the
opposite end of the spacer where it will attach to the antenna case.
When making another batch, I found that the local supplier only had aluminum
spacers in stock. Soldering aluminum is problematic at best. As an
experiment, I used a prick punch and hammer to crimp the spacers on to
the elements. I found this to be very strong and not have the problems of
solder wicking into the remaining threads. Over time this method may
develop issues with elements becoming loose but so far it looks very good.
I made six crimps to each spacer, three near the center and three near
the end on alternate faces of the spacer. The Mouser part number for
aluminum spacers is 534-2219.
- Slide the ferrite beads down the feedline until the center bead is
19 1/2 inches from the element studs. Secure the beads in place with
cable ties above and below. The beads act as a choke balun.
- Attach a connector to the other end of the feedline to match the one
on the radio with which you will be using the antenna (PL-259, BNC, etc.)
The feedline length is not critical. Note that ARDF rules specify a height
of two to three meters above the ground, and the pattern will be distorted
if the antenna is too close to the ground, so it's better to be a little
long than a little short.
- Thread the elements on to the studs. You may want to put plastic
balls or wire-nuts on the ends of the elements to reduce the possibility
of injury if someone were to run into the antenna.
- Hang the antenna away from obstacles using the eyebolt, check SWR and
performance. Note that the design SWR is about 1.3 to 1.
Improvements and options
Note that the materials and dimensions in inches are based on what is
commonly available in the USA. Metric hardware could certainly be used.
There is nothing special about the dimensions other than the element and
phasing section lengths.
There is no reason that the same antenna could not be adapted for other
bands, just scale appropriately.
If you choose to paint the housing to reduce visibility, consider the
use of some of the newer spray paints designed for plastics. Regular
paint doesn't adhere well to PVC. You might also want to mask off a
small area around each element stud. I'm not sure of the RF characteristics
There is a huge variety of configurations available in the way of PVC pipe
fittings. Alternative mounting options like a threaded pipe mount from
below as opposed to or in addition to the eyebolt are possible.
Jay Hennigan, WB6RDV 2004
Please report problems to: email@example.com