Placing the ground radials of the monopole antennas usually used with 80 m fox hunting (radio orienteering) transmitters such that the runners do not risk getting entangled in them is time consuming. So it would be good if it were possible to eliminate these radials by instead using dipole antennas.

There is a design published on Open ARDF for such a sleeve dipole. It uses a coax to feed the signal to the center of the antenna and has an additional sleeve on this coax as the bottom element of the dipole. At first I intended to build this design, but it turns out that it is hard to get the required toroids from Amidon (at least for me), the additional sleeve or alternatively a triax cable is quite expensive and I also did not fully agree with all the design decisions of this antenna. But it inspired me to develop something similar, but different.

The design I came up with is shown below.

Figure 1. Schematic antenna design. The parts in the blue box sits in the middle of the antenna while the parts in the red box are at the bottom.

I opted for using a shielded 2-conductor cable (LIYCYP1X2X1.00, sold by TME) as the bottom part of the antenna and thus to feed the signal differentially to the center. I also discarded the 470 pF silver mica capacitor, which reduces the required matching inductance, but increases the amount of current flowing through the inductor and capacitor. Instead I just used a 31 µH inductor in series with each antenna element and a 1:4 (impedance ratio) step-up transformer to feed the matching network. Both of these were empirically determined after measuring the impedance of an antenna hung next to the trunk of trees. Theory says the inductance should be more than twice as high and the impedance ratio different, but the tree has a large effect on the antenna impedance and forces us to use values like these for good matching.

Another change from the Open ARDF design is that I moved the tuning inductor to the bottom where it can be easily reached after hanging the antenna in a tree. The tuning ideally needs to be done after the antenna hangs in a specific way in a specific tree, so having the tuning knob 4 m up in the air is not optimal. Tuning can be accomplished by measuring the antenn using e.g. a LiteVNA or similar, or maybe the transmitter has a feature to measure the amount of signal that is reflected back from the antenna.

Instead of using the mechanism of a lip balm tube, I designed and 3D-printed a coil former. 12 turns of 0.6 mm copper wire gives an inductance range of 1 – 4.4 µH. See below.

Figure 2. Trimmable inductor using a 4061287011 ferrite rod.

The threads are M10 and with a little grease they run smoothly while staying put when left alone.

The main matching inductors are wound as 19 turns on 5961004921 cores. To ease the winding and ensure consistent results, I designed a jagged washer with a notch for each turn. There are 24 notches, so 5 should be left without a wire turn when 19 turns are placed on the toroid. See below.

Figure 3. The main matching inductors. 19 turns on 5961004921 cores with the assistance of a 3D-printed washer with 24 notches.

The transformer at the bottom of the antenna converts the single-ended signal from the transmitter to a differential signal suitable for the pair of wires in the cable and also functions as a choke that prevents the antenna from coupling strongly to the transmitter and its cable. The Open ARDF design does not have such a choke, so there the transmitter and the cable to the antenna form uncontrolled parts of the lower element of the antenna.

Both transformers are wound on 5943000221 cores. The lower one has 6+6 turns (0.4 mm wire) while the one at the center has 6+12 turns (0.4 and 0.25 mm wire respectively).

To help with the build, I designed two simple 2-layer PCBs. The gerbers can be downloaded via links below. The PCBs fit inside 32 mm sewage pipes that serve as weather proofing.

Figure 4. Assembly drawings (top and bottom) and copper patterns (top and bottom) for the board at the bottom of the antenna. None of the capacitors should be mounted.

Figure 5. Assembly drawings (top and bottom) and copper patterns (top and bottom) for the board at the middle of the antenna.None of the capacitors should be mounted.

I also 3D-printed several other parts for the design, like strain reliefs, parts helping to hold the matching inductors, “funnels” to seal between the cables and top of the pipes and a dust cap for the BNC connector. The funnels and dust cap should be printed using flex material.

Figure 6. Top side of the board at the bottom end of the antenna, showing the 1:1 transformer.

Figure 7. Bottom side of the board at the bottom end of the antenna, showing the adjustable inductor.

Figure 8. Top side of the board at the center of the antenna, showing the 1:4 transformer and one of the matching coils.

Figure 9. Bottom side of the board at the center of the antenna, showing the other matching coil.

Figure 10. The two PCBAs enclosed in 32 mm sewage pipes.

All 3D models can be downloaded via a link below.

This antenna seems to be about as efficient as a normal 6 m tall monopole according to preliminary tests, but is much quicker to put up and take down. It does however require a transmitter made to drive a 50-ohm load. Many existing fox hunting transmitters might have built in matching networks for monopoles and such outputs are not a good match for this antenna. If a transmitter like that is to be used with this sleeve dipole, the matching network in the transmitter must first be removed or redesigned.

Zip-file with the 3D-models:

https://axotron.se/blog/wp-content/uploads/2026/08/Sleeve-dipole-3D-parts.zip

Zip-file with the gerbers for the lower board:

https://axotron.se/blog/wp-content/uploads/2026/08/Antenna-Balun-C_PCB_package.zip

Zip-file with the gerbers for the board at the center of the antenna:

https://axotron.se/blog/wp-content/uploads/2026/08/Antenna-Match-C_PCB_package.zip