Here is a trick you can do to make a 40 meter dipole be resonant on 15
meters at the frequency you want it to be. First build a 40 meter
dipole, about 68 ft. Then at 7 ft either side of the center, add a 20
inch wire and let it just hang down. It should now be resonant close to
the center of the 15 meter band, but you can prune the length of the 20
inch wire to move it where you want.
This does raise the impedance of the dipole (close to 85 - 90 ohms at
most heights), but it is a very good match for 75 ohm line on both
bands. Dimensions are for bare wire.
An interesting solution for matching to 50 ohms occurs if you use 104 ft
of 75 ohm line (Vf=1). Correct the length for the velocity factor of
the cable you are using, e.g. most FT677 type TV cable is Vf = 0.82 so
you would need 85.3 ft. Then connect this to 50 ohm coax. The SWR
relative to 50 ohms should then be below 1.5 across all of 40 and 15
meters. (That's with coax loss =0. Actual SWR measurements should be
even less.)
Jerry, K4SAV
Barry Kirkwood wrote:
>Interesting topic.
>As the old saying goes, a good dipole is hard to beat.
>
>A 40m dipole approximates 3 1/2 wavelengths on 15m, giving two bands for the
>price of one, if you can tolerate somewhat higher SWR on one of the bands.
>Have been told that adding small inductances either side of the feedpoint
>can give low SWR on both 40 and 15 but have never bothered to model or
>experiment along these lines.
>BTW, the cloverleaf pattern from a three half wave dipole is a pretty useful
>pattern in most parts of the world.
>Any one remember the famous RCA 'Spiderweb" antenna, widely used by SWLs and
>commercial receiving stations through the 30s and 40s.
>Could be interesting to rework one for the hf amateur bands. Has the
>advantage of separating the dipoles with minimum of hardware.
>73
>Barry ZL1DD
>
>
>
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