40m ‘Standard’ half-wave Dipole Antenna (2nd HF antenna project)
My 40m coaxial dipole antenna worked surprisingly well, considering how many compromises it was dealing with. However it’s performance was best on the higher bands and on 40m never achieved a reasonable SWR without an ATU to help. There were a number of problems, most of one leg was within 2′ of the house wall and a third of the other leg had a contorted 90 degree turn in it. After some fiddling with it, the whole thing came down which was a good opportunity to improve things!
In search of simplicity, I decided to make a standard 40m dipole using 16swg enamelled copper. First I moved the supports to ensure the antenna was in free space, away from the house, >18″ away from anything else and at least 18″ away from any metal in the supports (that metal being at 90 degrees to the antenna). Then the question, how long should it be?
Theory books say a half-wave dipole for 40m should be at least a half-wave above ground (66 feet for 40m) and each leg a quarter-wave long – for 7.050 MHz this is about 34’6″. The length calculation being 492/freq (in feet); 150/freq (in meters). Then there is something called “end-effect” which in practice makes each leg a little shorter at 33’3″. The length calculation is modified to be 468/freq (in feet); 2808/freq (in inches); 142/freq (in meters). However, this isn’t all because most of us can’t realistically get the antenna 66′ high in a normal UK garden. The lower the leg ends are to the ground, the lower the resonant frequency of the antenna; it becomes electrically longer, so perhaps it should be cut even shorter!!!
Taking all these factors into consideration, I first cut each leg (with a wire ring at the end) to be 32’9″ long; the wire ring can be attached directly to mono-filament fishing line acting as a guy. I then soldered 15″ bare copper wire extensions onto each leg, 6″ from the leg end. The extensions don’t support the antenna, they are designed to provide fine tuning; unfolded the leg is 34′; so by folding and unfolding, it should provide resonance tuning between 6.880 Mhz and 7.145 MHz. In practice, proximity to its surroundings will effect this, most likely by electrically extending one of the legs.
I first tried the dipole with about 10 feet of coax to the ATU/transceiver – 40m was 1.0:1 perfect with no ATU and the extensions folded back! The other bands tuned up ok with the ATU, including 160m. Patching the rest of the coax back to the shack I found 7.050 now had an SWR of 1.2 and SWR went down as I got to 7.100 (ie. the antenna had got electrically shorter!) – I fixed this by using the extending the adjustable copper pieces on the end of the dipole by 3″ on each end. However 160m seems not to tune up at all now and the feeder radiates on 40/80m just as the coaxial dipole did!
