I am not a regular operator on Top Band so, at this time of year when the RSGB Club Calls Contest arrives, it sees me dragging the tangled mess of wire that is my 160 meter dipole out of hibernation in the shed and getting it back into active service.
Like many urban dwellers, I do not have the requisite amount of space to hoist up a full size dipole for top band so have constructed a shortened version with loading coils in the centre of each leg. This affords me a resonant dipole but with a much restricted usable bandwidth, but, at least it gets me on the band and able to score some points for my Radio Club (S&DARS).
My prefered mode of operation is CW and most of my antennas are resonant dipoles that are tuned to the low end of any band. As the Club Calls Contest is a Phone one it requires shortening the antenna to tune it for the Phone segment of the band. This is usually simply a matter of folding back the ends of the dipole and fastening with a few cable ties. But by how much?
My usual approach to shortening a dipole for the phone section of the band is to use the trusted magic formula; 468/Freq to calculate the theoretical length of the antenna at two different frequencies; that at the low end of the band for CW and that of the new required frequency in the Phone section and noting the difference between the two lengths. This normally gives a fairly good guide on how much to fold back before fine tuning with an analyser but, with my loaded dipole, this proved to be wildly inaccurate.
Using the magic formula, the difference in length of each leg between an antenna resonant at 1.818 and 1.920 MHz is 6.8 feet, or approximately 82 inches. In practice, for my shortened dipole with quite hefty loading coils at the centres, the actual difference turns out to be just 22 inches.
I realise that the effects of a loading coil are more complex than simply one of wrapping some of the wire into a coil. For me though it makes the use of an antenna analyser so much more valuable. At the time of purchase I did baulk at the cost but have subsequently found its value to be inestimable.
73 and hope to work you on the bands, whatever the mode.
