What can a SOTA fanatic do with a broken plastic coat hanger? Good question you ask!
During a recent visit to Melbourne I broke the hook section of a plastic coat hanger, which at the time I cursed as I didn’t have a spare. For some bizarre reason I threw the broken coat hanger in my luggage for the return trip to Canberra.
Monday evening I was about to throw the coat hanger in the recycling bin when it occurred to me the coat hanger material has a round profile 6 mm in diameter, 1/4 inch in the old scale. Hmm perfect lightweight material for a link insulator to compliment my SOTA 6/10/20/40m 1/2 wave link dipole by including a link for 30m 10.130 MHz. Also the coat hanger material can be used to make a short insulated rod to couple a variable capacitor to a 6:1 reduction drive. Enough of variable capacitors for now, let’s get down to business and modify the SOTA HF link dipole.
The rod diameter is 6.8 mm measured with a vernier caliper
Cut two 40mm lengths and drill 2.5mm holes in each end and debur.
Next I laid out the link dipole antenna along my drive way and cut each half at 7.2 metres, between the 14MHz link and the extant end insulator. I started with a 100 mm length as the fold back to create the join to the 7 MHz section. The length of the fold back section will need some trimming as the antenna is adjusted for resonance at 10.130 MHz.
Thread the end of the wire through an insulator and folded back 100mm. Repeat this process for the second element. I now have two elements 7.1 metres long. A cable tie is used to apply light pressure on the wire, the cable tie will be tightened later.
The modified antenna with 6/10/20m links closed is supported by a 7 metre squid pole with elements set as an inverted V. The ends of element are about 2.5 metres above ground level. The idea is to test the antenna at home ensuring the shape closely matches the shape when deployed on a mountain peak, it doesn’t have to be perfect. The squid pole is supported by my homebrew ‘QTH Squid Pole Support’. With each length of the dipole cut at 7.1 metres the antenna is resonant at 9.65 MHz, further adjustment is required to shorten each half of the dipole. How do you calculate the amount of wire to cut from each half of the dipole?
F1 = Target frequency 10.130 MHz
F2 = Resonant frequency 9.65 MHz
Frequency Ratio = F1/F2 10.130/9.65 = 1.049
New element length: 7.1 metres/1.049 = 6.768 metres
Element difference (mm): 7100 – 6768 = 332mm.
I trimmed 300mm of wire from each half of the dipole to bring the resonant frequency to 10.130 MHz. After trimming I laid the two halves side-by-side to make sure the element lengths were equal, one side was 10mm longer than the other. After making a small adjustment to even out the elements, I raised the antenna to its normal position on the squid pole and checked the VSWR.
Next lower the antenna and tighten the cable clamps add your choice of electrical connectors, I prefer to use automotive spade male and female crimp connectors. After securing the connectors restoring the 7 MHz extension then check the VSWR and resonant frequency with all links closed. Note I had to add a short length of wire 200mm at each element end to replace the 300mm section removed when adjusting the 10 MHz link. Remember 100mm of wire forms part of the 30/40m link.
Have fun building or modifying your homebrew SOTA antenna. I will keep the remaining coat hanger material for the next antenna or ATU project. 😉
On air performance. Friday 4 Dec 15 I deployed the modified antenna at the summit of Mt Ainslie VK1/AC-040. Operating a Yaesu FT-817 (QRP) on 10.130 MHz, SOTA chasers were Rick VK4RF, Ron VK3AFW, Nev VK5WG, Rob VK2XXM, Ian VK5IS, Gerard VK2IO, Tony VK3CAT and Ian VK1DI. 🙂
Last update: 14 December 2015.