This is what I like about Amateur Radio . . . Being able to look at to what for many would be a pile of scrap metal, and see a potential project. That’s what happened last year (2004). However It’s only recently that I have finally done something about it.
Once upon a time, a friend of mine had the misfortune of having his 15m mast topple over. Actually he has had this misfortune on no less than three occasions but that’s another story . . .
On one of these occasions, a rather nice 10m beam was sadly written off and the debris stowed at the side of the ‘garden’. Last year, whilst helping my friend to clear his garden of various items of detritus, we unearthed the remains of the 10m beam and I instantly thought “I could turn that into a multi-band vertical”. Having just made a multi-band dipole for 30m thru’ 160m employing coaxial transmission-line traps, I was already geared up for the job. The original plan was to employ a coaxial transmission-line trap, until I found a design for a 10m/15m Trap Vertical in the ARRL Antenna Handbook. This design is simpler, forming the trap out of a few turns of heavy gauge wire and a length of coax as the resonating capacitor across the inductor. Although more lossy than the transmission-line trap, this design is mechanically less complex and is very simple to implement into a dual band vertical. The coil is wound on a length of 25.5mm grp tubing and the coax, RG316/U (used because I have miles of the stuff) is fed up the inside of the tubing and connected across the coil. See figure 1, below.
I still had the length of ‘hard-line’ (FSJ 2-50) in place, that used to feed my modified HF6-V, and with the ‘Bypass’ connector on my Vectronics VC-300 un-used, I decided to make an antenna for 17m and 12m. Both these bands are very narrow, only 100Khz, so once the antenna dimensions had been fixed, no tuning would be required. This kind of antenna requires radials and two on each band would double as support guys. Although the initial tests were carried out with only one per band, the results were encouraging, with the resulting VSWR across both bands not exceeding 1.7:1. I decided to press ahead with the construction.
One option was to leave the trap inductor exposed, but since it is wound closely onto the tubing, it would inevitably be compromised as moisture collected between the turns. The second option was to simply weather-proof the inductor by applying self-amalgamating tape. This too was dismissed since I anticipated the possibility that the tape itself would have an effect on the inter-turn capacitance. Therefore I opted to house the trap assembly inside a 2-inch length of plastic piping.
See figure 2, right.
To make the enclosure, I made rings from 31.7mm grp tubing to support the plastic tubing at either end. As can be seen from figure 3 on the left, the inside diameter of the plastic pipe is still greater than 31.7mm. However pieces of matchstick fitted into the gap at four positions resolved this issue beautifully, and the excess gap was then filled with epoxy resin. The finished enclosure is seen below.
Fig. 4. Now to make the mounting bracket. . .