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Very few of us these days have gardens that will allow the “traditional” 40m long dipole (for the 80 Meter band) to be erected. Fewer still have space for the double sized dipole for “Top Band”. So, essentially, this means for many of us that 80 Meters is totally out of the question. Or such a full sized antenna has to be bent into various contortions to get it to fit into the available space.
The antenna I am describing here was intended to allow operation on both 80 Meters and 40 Meters in less than half the space of the traditional dipole. However, it will, in most instances, give a performance equal to, or greater than its full size cousin. The antenna will also give a good account of itself on 14, 21 and 28 Mhz being electrically similar to the W3DZZ dipole.
The antenna system will almost certainly require some impedance matching to suit rigs with solid state power amplifiers – again, this is just like the W3DZZ antenna. Antenna purists will often tell you that an antenna of this type requires radials, or a sophisticated earth system for optimum performance. But in practice the antenna works very well with a modest earth system, although this is dependent on soil conditions.
The prototype antenna was constructed from heavy copper earth wire. There is a single 7 Mhz resonant trap to make it more efficient on that band. The general layout of the antenna and the theoretical lengths of the antenna are shown in Fig. 1. The antenna is fed with 50 ohm coaxial cable, with the coaxial screen connected to the earth rod. This connection is secured using an earth clamp intended for earthing water pipes. The center core of the coaxial cable is connected to the antenna via a single 15A “chocolate block” connector. Cover all connections with a waterproof tape. No balun or other matching network is needed for 80 Meters and 40 Meters as the antenna’s feed point impedance is close to 50 Ohm. The 7 Mhz trap is constructed from 11 turns of RG58 coaxial cable wound on a 100 mm piece of 40mm PVC pipe as shown in Fig. 2. In this type of trap the coaxial cable acts as both capacitor and inductor and is capable of working at power levels in excess of 400 watts.
It is imperative that screen and center cores of the coaxial cable are parted as close to the point the cable passes through the hole in the pipe as possible. This is to ensure the correct value of capacitance and inductance. The center core of one end of the coaxial cable is soldered to the screen at the other end.
As with the feed-point, the ends of the coaxial trap and other joints must be weather proofed. The capillary effects of coaxial cable are legendary and water ingress will total ruin your trap. The height at which the antenna folds over from vertical to horizontal is not critical but generally the higher it is the better. Extra height, not only aids the DX performance of the antenna but also significantly reduces the amount of horizontal space required. Tuning the antenna is quite simple but it is imperative that it is done in the correct order.
Firstly cut both sections of the antenna about a half a metre longer than the dimensions shown in Fig. 1. To start the tuning operation, begin on 7 Mhz and trim the wire length at the end nearest the earth connection 50 mm at a time until he lowest s.w.r. is achieved. I managed an indicated s.w.r. of less than 1.2 : 1 over the whole of 40 Meter. Then move to 80 meters and repeat the process, but this time trimming the side of the antenna furthest away from the earth i.e the side nearest the shack in Fig. 1. The S.W.R. on the 3.5 Mhz band should be less than 2:1 over the whole of the band falling to about 1.2 to 1 at the point of resonance. So it is worth setting the lowest S.W.R. at the section of the band you normally use, if you have a preference.
ED: I have had a lot of fun using the antenna and thoroughly recommend it to anybody not having enough garden room to erect a full size dipole in the optimum direction.
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