Lengthening a cheap CB antenna for the 12m band
Jonathan Hare, G1EXG

The antenna comes marked for assembly producing an antenna resonant for the top end of 27 MHz (e.g. 27.8 MHz). The vertical metal radiator is 525 cm long consisting of 5 tubular sections held together using hose clips. The tubes range in diameters from about 3 cm to 1 cm. The base fits into a 20 cm U shaped bracket that allows the vertical tube to be insulated from a mast. The bracket has two plastic insulators, the bottom of which is hollow and contains space for a matching coil system. This is an auto transformer that converts the relatively high impedance of the end-fed half-wave vertical to 50 ohms for direct connection to coax cable. The feed impedance of a thin half-wave of wire is very high (1000's ohms) but drops (100's ohms) for when relatively large tubes are used. The auto transformer consisting of 5 or 6 closely wound turns of 12 SWG enamelled copper wire on a piece of 3 cm diameter Tufnol tube. The start of the coil goes to 'earth' (the bracket) while the other end is wired to the radiator. The transceiver / feeder connection taps in at about 2 turns from the earthed end.

Lengthening for 12m band
In order to get the antenna to be resonant on the 12m band (24.9 MHz) we will need to increase the length to: 27.8 / 24.9 x 525 = 585 cm, i.e. increase the length by about 60 cm. I simply cut a slot into the end of the existing top section of the antenna and, using a hose clip secured another smaller (ca. 8mm diameter, 1m long) tube to lengthen the radiator.


Results
I mounted the antenna a quarter of a wave (ca. 2.5 m) above ground and used about 9m of RG58 50 ohm to feed the antenna. Measurements with a MFJ-269 antenna analyser at the end of the coax found low reactance and good match to 50 ohm (low SWR) at: 28.0 MHz when unmodified, 26.0 MHz with a 50 cm extension and 25.0 MHz with a 90 cm extension. In each case the 2:1 SWR range (at the transceiver) was about 2 MHz. SWR and reactance measurements at the antenna and at the transceiver are shown below. The antenna is resonant and gave a good SWR [3] over the 12m band. As a result I didn't attempt to adjust the auto-transformer / matching coil system and simply decided to keep the 90 cm extension in place for the 12m band.


12m band measurements made at the antenna:
(antenna 1/4 wavelength agl)

Frequency	SWR	Rs	Xs
24.80	1.3:1	66	6
24.85	1.3	64	5
24.90	1.3	63	5
24.95	1.2	61	6
25.00	1.2	60	7

12m band measurements made at the transceiver:
(antenna 1/4 wavelength agl + 9m of RG58)

Frequency	SWR	Rs	Xs
24.80	1.3:1	39	5
24.85	1.2	40	5
24.90	1.2	41	5
24.95	1.2	43	5
25.00	1.1	44	6

Where the Rs and Xs are the resistance and (modulus of) reactance respectively



Ground planes and radials
Like the J or Slim-Jim antenna the half-wave end fed antenna does not in principle need an earth radial system. So far I have not tried adding 1/4 wave radials to the antenna but if I decide to raise the height I might try using quarter wave radials (with insulators and guy ropes) as guy wires.


Results ... so far
In the first week of experimentation (January 2011) the 12m band has been very quiet. My only contacts so far (using about 25 Watt) have been with a local Brighton station and then immediately afterward RZ3FS (Moscow) who gave me a 57 …. so I am QRV on 12m!


Up-date
March 2011 - 12m really getting busy now, had some great days on the band. Worked: Indonesia, Russia, Greece, Malta (Gozo), Crete, Hungry .... heard lots of W's, K's, India etc.
I seem to be able to work what I can hear, depending of course on QSB and noise / QRM etc.


References
[1] Worthing and District Amateur Radio club AGM, December 2010.
[2] e.g. on e-bay look for 'silver rod' CB antenna, ca. £20 new, less than £5 for second-hand (but usually only if “buyer can collect”).
[3] Understanding SWR by Example, Darrin Wailraven, p. 37, QST, November 2006