A shortened 40 meter rotary dipole

For the PACC contest I need (several) antennas for every band. Up to 40 meters it’s possible to make these antennas rotatable looking at the size of the antennas (and my backyard). After using verticals for 40 meter and last year a delta loop this year I choose to make a rotatable dipole. The small crank up tower against the barn makes this possible.

The antenna is made from 2 fishing poles on a fiberglass pole in the center.

The antenna is:

  • Easy to make
  • Cheap
  • No traps
  • Light in weight
  • An easy antenna project
  • Great for field day use and as ‘extra’ antenna at home for additional bands (or contests).

Needed materials are:

  • 2 fishing rods (of better) as long as possible (up to 10 meter length)
  • 1 meter center support pole (to slide the fishing poles over)
  • Mast to boom support
  • Some tyraps and/or electrical tape
  • Feed point insulator with 1:1 balun/choke
  • Some cheap wire as antenna wire
  • Some tools

Construction
The center support is a 1 meter fiberglass tube bought on a flea market. These poles most often come in a package of 5 for 25 Euro and are former military masts. The center support could also be a small aluminum pipe but this is what I had in the barn.  The mast to boom clamp will attach the center support pole to the mast (above the rotor if available).

[Picture center support needed with only center support pole]

The two fiberglass (fishing) rods do slide over both ends of the sturdy center fiberglass pole. The center pole is thickened on the end and almost at the center with electrical tape (on 4 places) in such a way that the fishing poles have a snug fit on the center pole. Not shown in the picture is a rope from one fishing pole to the other to prevent them from sliding of the center fiberglass pole.

The fishing poles should be as long as possible. A 40 meter dipole is around 10.10 meters per side. As the fishing poles I had lying around were 8.70 meters this is what I used. Most of the mechanical construction is now ready.

Wire length and support
The length of the fishing rods is 8.70 meters and yes, too short. There are several solutions to this.

  • use a longer center tube and do not slide the fishing poles up to the mast to boom clamp. In my case a 3 meter center support pole would have given me around 10 meters (with 50 cm room for the fishing rods to slide over the center support).
  • Use coils. Not easy to make and how much windings do you need. This depends a lot on the fysical place along the antenna wire.
  • Use linear loading. Easy to make, just wire. You have to sort out how long the linear loading should be, where to start and where to end.

The linear loading solution is what I choose and made.

The antenna wire starts in the center from the balun and goes over the fishing pole to around 6 meters, then goes back about 2 meters and then back again to the end of the fishing rod. The length from the center of the antenna and the start of the linear loading should be as long as possible (4.1 meters in my case). The linear loading part is 2 meters long (between 4 and  6 meters on the fishing rod). There are 3 wires on the fishing pole between 4 and 6 meters. The end of the antenna is again a single wire from 6 to 8.7 meters (tip of antenna).

The wires are each one piece with a length of 13 meters and taped (or use tyraps) to the fishing pole. The picture below shows the linear loading at around 7 meters on the fishing pole.

The distance between the wires was in my case only a few centimeters as I taped the wire on the fishing pole (so around 2 cm). Of course you could make the distance wider by using small PVC supports like I used for the multiband vertical but I saw no need for it.

The picture below shows the complete linear loading wires (before taping is all up..)

Feed point and Balun
In the center I used a center connector bought on a flea market. This was before I had my baluns ready. The center pin of the coax is attached to one end of the antenna and the shield to the other end of the antenna. When I used the antenna a choke balun of type W1JR which was attached direct to the connector of the antenna.

Adjusting
When all is done the final check is if the antenna is in the 40 meter band. Wow, it was just below it (around 6.8 MHz). This was 2 meters above ground. Once is was on 11 meters in the small tower the resonance frequency went up to 6.95 MHz. The total bandwidth is very wide and even at 7.2 the SWR is way below 1 on 2.

The place of the linear loading on the fishing rod and length (and distance between the wires) will influence the resonance frequency. So pruning the length of the linear loading and the place on the fishing rod will take some time to get the antenna in resonance on 40 meters. In my case it was just luck that I was that close and really needed no pruning at all.

Using this technique using different lengths of fishing rods, different lengths of wire and different lengths (size)of linear loading will make is possible to use it on other bands as well.

Conclusion
The antenna was easy to put together and took about 2 hours in total. Placing it in the small tower was easy as the antenna is very light in weight. It is wide however and some room is needed. The picture below gives a view of the antenna on top of the small tower. During the PACC contest the antenna worked well. The height of course could have been better but this is what I have.

Note: Above the rotary 40 meter linear loaded dipole is a N6RK receive loop (which works very well).

73’ Thomas
PA1M
8 august 2012