The antenna consists of two sections. The bottom half which is a fibre glass tube with the loading coil and at its base has a standard 3/8" UNF thread, and the top half which is a stainless steel whip. Two grub screws hold in the whip and allows adjustment for tuning. As there is plenty of enamelled wire (22swg) in the bottom section, this can be reused to wind loading coils for other frequencies.
First stage of conversion is to carefully cut off the heat shrink plastic off the the lower section. Cut or desolder the enamelled wire from the base. Unwind the wire so it is straight and parallel to the bottom section, down to the base with the 3/8" thread and cut. Put aside the surplus wire for later.
Glue 5 bolts 40mm apart at the bottom of the bottom section, 10mm above the antenna base. These bolts will be used as taps for band selection on the loading coil. Araldite epoxy resin and 5mm diameter brass bolts was used.
On the highest bolt (from the base) connect the wire. This is the 28/29 MHz tap. You can solder or use nuts and washers to make this connection. Whatever method, don't forget to scrape the enamel off the wire otherwise you will have a bad connection!
Between the 28/29 MHz tap and the 27 MHz tap is 3 turns of wire.
Between the 27 MHz and the 26 MHz tap is 3 turns of wire.
Between the 26 MHz tap and the 25 MHz tap is 2 turns of wire.
Between the 25 MHz tap and the 21 MHz tap is 11 turns of wire.
Solder a short wire link from the antenna base and connect to the 21 MHz tap.
I wanted the whip to slide inside the antenna for storage. So the two grub screws on the top of the bottom section was removed to use a 4mm thumbscrew or similar. Unfortunately my local hardware store didn't have a thumbscrew, so I used a 4mm bolt bent into a L shape. I inserted 50mm (2 inches) of the the upper whip and tightened up the L bolt, to allow for future adjustments. Tape or use heat shrink plastic over the wire along the bottom section to the 28 MHz tap.
The antenna was tested on top of my van using a Sirio SO239 body mount with a 3/8" to SO239 adapter. As an experimental antenna, I used thin strips of tape to hold the windings in place. Wire jumper made from 160mm of flex with crimp connectors both ends. Connect one end of the jumper to the 21 MHz tap with a wing nut, and use the other end to connect to select the other taps, with a wing nut. For 21 MHz, remove the jumper wire.
Minor adjustments were made to the windings to improve the SWR. When you are happy with the SWR, glue or use self-amalagamating tape over the windings.
The SWR bandwidth is nice and wide. Measurements taken with a MFJ 269 Antenna Analyser which I recently purchased from Roy at www.pjbox.co.uk.
10M Section
30.0 = 1.5
29.5 = 1.4
29.0 = 1.3
28.5 = 1.3
28.0 = 1.5
CB27 Section
28.0 = 1.3
27.5 = 1.2
27.0 = 1.3
26.5 = 1.5
CB26 Section
26.5 = 1.3
26.0 = 1.2
25.5 = 1.3
12M Section
25.0 = 1.2
24.5 = 1.4
15M Section
21.5 = 1.1
21.0 = 1.1
As you can see, no need for an ATU (antenna matcher)!
Even though my original intention was to use this antenna static mobile, it could be used for portable work, backpacking, balconies etc. You will have to use a suitable groundplane. I would suggest three wires connected together; 3.40 metres for 21 MHz, 2.85 metres for 25MHz and 2.45 metres for 29 MHz. Better mention that I haven't tried this yet and the antenna may need adjusting.
With hindsight, the use of brass bolts wasn't necessary but looks impressive. A neater way of tapping the loading coil for the required band would be using banana plugs and sockets. Especially better for backpacking.
73's from Dave the Pixie, G7OPC.
