Restoring Diamond F22A amateur radio antenna
Diamond F22A antenna is a two element 2m 7/8 wave high performance FRP (Fibre Reinforced Placstic) radome vertical antenna. This type of antenna is inductor loaded (it has loading coils) to bring it to resonance a small capacitor is needed at the base of the antenna to tune it to its designed frequency.
The original specifications states that the antenna is designed on the amateur frequency 144-148Mhz @1.1:1 SWR at the center frequency 146Mhz.
VSWR Less than 1.5:1
Max. input power 200W
Max. wind resistance 50m/sec (112.5MPH)
Mast diameter accepted 3-62mm (1-1/5″ to 2-2/5″)
Length 3.2m (126.0″)
Weight 1.3kg (2.9lbs)
Connector UHF Female
Type 7/8 wave two-element
The usual problem of this antenna before if it stops to work are the frequent symptoms of intermittent VSWR and the sudden high VSWR read out after some time. This usually indicates that the loading capacitor is now broken and must be replaced. However another problem encountered by majority of users of this antenna when the capacitor fails is the availability of the replacement capacitor which are often not easily accessible on the market. The capacitor value for this type of antenna is rated at 6pf, but you can use a capacitor in the neighborhood of about 15-to-20pf and the antenna will still work reasonable well. But for the purist you may stick with the 6pf and just be happy with it :). The usual problems of capacitor availability can be fixed by substituting a portion of coaxial cable to replace the capacitor. You need to check the coaxial cable datasheet and compute the nominal capacitance per foot or per meter to estimate the length of the coaxial cable you need to cut.
In this case we have 31.08 pf/ft for the RG174 to compute for the length of the coax we need, convert ft to cm.
1 ft x 12 in 1 ft x 2.54 cm /1 in = 30.48 cm
So to estimate the length of the coax we need, divide
31.08pf/30.48cm = 1.036pf/cm, so if we need 15pf of capacitance that is
<= 15cm coaxial cable length.
Feel free to cut about 15cm of RG174 coax and just trim it slowly when tuning. In this case I have use about 15pf less than 2cm for the soldering points so that is about 13pf.
If you need more accurate capacitance value you may also use a capacitance meter to measure the length of the coax before you solder it to the antenna as capacitor replacement.
Frequency response of the antenna before and after the coaxial cable replacement