This dipole is multi-band for SOTA and covers 12 metres, 20 metres and 40 metres. I was going to use links on the dipole sections to select the band, but thought that switches would be an interesting alternative.
The switch “off capacitance” was a concern and for that reason I was going to use slide switches, but I had a bag of small toggle switches with a compact toggle. The dipole starting measurements were based on Peter, VK3ZPF’s design.
Tuning was started from the 12 metre section, then the 20 metre section and lastly the 40 metre section. While tuning, a centimetre of scrap wire was soldered to the PCB and the antenna section was looped through the PCB holes (strain relief), twisted to the scrap wire and then a VSWR measurement made. The antenna section was cut as required, passed through the holes on the PCB, retwisted and measured again. When the VSWR was OK, the next outer section was tuned in the same manner.
As it turned, out the dipole behaved as expected when tuning and with switches on and off. VSWR is perfect on the three bands. The dipole centre feed was made from single sided PCB as well as the link sections. Lightweight Jaycar hookup wire was used for the initial tuning and will be replaced with lightweight and strong Aramid core wire later when it is proven itself in the field a few times.
The switches are sort of surface mount soldered to the PCB mounts and the two locating pins for the switches straddle the edge of the board. I might cover the switches with neutral cure silicone to help with water protection. I didn’t bother with a balun to keep the weight down and keep it simple. The whole dipole is wound on SOTAbeams antenna winder. These are great and have an elastic wrap to keep it together.
Now, if only I can remote control those switches…….
The weight of the dipole all wrapped up is a tidy 172 grams!
Yesterday, I changed the wire on the dipole from the Jaycar hookup wire to Ultralight Stranded DX Wire with an Aramid core of the same lengths. This wire has 6 x 0.25mm strands (~0.3mm²).
The hookup wire was 13 x 0.12mm (~0.15mm²).
On the first activation, I noticed straight away that the SWR was higher than usual for some reason on all 40m and 20m. The antenna was mounted in the clear so it didn’t seem to be an environmental issue. On both bands the antenna was tuning high.
On returning home, the antenna was checked in the back yard with the same results. I had to add about 12cm to the 20m leg and the same to the 40m leg on both sides.
After retuning, the antenna was tested in the inverted V configuration with the apex at a height of 7 metres, 5.5 metres and 4.2 metres to check performance on a smaller squid pole. As the height came down, the resonance dropped down slightly although the tuning was fine for field use at all these heights.
The antenna weight has dropped about 15 grams although the breaking strength is now 700 N.
An improvement to this design is to make the switched sections a little smaller and streamlined. The toggle switches could be prone to damage. Some slide switches from Ebay were purchased at $4.00 for 10. These were then mounted on some 1.6mm PCB at a size of 10mm x 50mm.
The copper was taken off the board as the wire will terminate directly on the switch lugs after being strain relieved. This time, the switches will be closed when the toggle points towards the dipole centre. There is then no need to look at the switch or a label indicating the closed position. They also look a lot neater than the toggle switch arrangement. The switches are 240 VAC rated at 0.5 Amp.
The two holes at either end of the PCB are used to strain relief the wires from the switch. After connecting the wire, heatshrink was used to cover the whole PCB with a small slot cut out for the switch actuator. Weight is now 155 grams.
An advantage with the switches over push in links, is that with one hand, I can throw the switch while reaching up high and not having to drop the squid pole down as far or at all for the outer switches. The finished wire section lengths came out at:
inner section 4830mm
middle section 2130mm
outer section 4830mm