5 element ZL special for 2m band



5 element ZL special


The booklet 'Out of thin Air' (Practical Wireless Publication ca. 1981) describes some interesting experiments with two, three and five element 'ZL special' Yagi antennas. Here I describe my experience making a five element beam antenna. The basic 2 element ZL special is related to the HB9CV antenna. Two folded dipoles are spaced a quaterwave apart and fed a quarter wave out of phase (using a twisted quarter wave feed line). This results in the waves adding in one direction (the 'forward' direction) but cancelling or nulling in the other. One dipole is usually made slightly larger than the other presumably to make the frequency response broader. The result is a two element antenna with a good forward gain (theoretically ca. 6dB) and excellent front to back ratio. Greater gains are possible by adding directors.



5 element ZL special


Construction
Two 1m lengths of fibre glass tube (10mm outer, 6mm inner) were covered in heat shrink tube using a heat gun. This was so that when I cut the tube I hope it would not split and also to protect the tubes from UV when mounted permanently outside. I used a Dremal ceramic grinder to cut the tubes to 98 and 90cm. These were then used to support the outer parts of each of the folded dipole which were made from aluminium wire. I had a long length of 4mm diameter aluminium wire that I had brought from a radio dealer years ago and thought it would be useful for making up the ZL style driven elements.

5 element ZL special


One length of the wire was used to make the longer 'reflector' element along with the quarter wave feeder (i.e. all out of one piece). Another length was used to make the smaller folded dipole. The wire was fed into the fibre tubes and simply bent around to make up the folded dipoles and feeder line (see photo below). The quarter wave line was twisted over in the middle and joined up with the smaller folded dipole using a large connector block. The coax also goes to this junction as does a small tuning capacitor for matching. The 50 ohm coax is directly connected but a quarter wave sleeve is added on the outside of the coax to act as a (bazooka) balun. The three directors were made from 1m lengths of 3mm diameter aluminium welding rod (brought in a pack of 50 from e-bay) cut to the correct lengths (see table below).

5 element ZL special

Dimensions and other spec:
Length of 'reflector' folded dipole: 98cm, 21.5 cm spacing to next folded element, 25.5 cm phasing line
length of driven folded dipole: 91 cm, 21.5 cm spacing to director
1st director length: 89 cm, 21.5 cm spacing to second director
2nd director length: 88 cm, 21.5 cm spacing to last director
3rd director length: 85 cm

Matching
If you are just using the 2 element ZL special (with no directors) you need 20pf or so to get a good match to 50 ohm cable but the capacitance you need decreases as more directors are included. In the 5 element (3 director) version I hardly needed any extra capacitance at all. As this the matched feed is 50 ohm the voltages on this capacitor wont be too high. We can calculate the voltage on the cap from power P = V2 / R and rearranging to get V = √(R x P). So for example if we have 50 watts into 50 ohm V = √(50 x50) = 50 volt, not too high for a decent pre-set capacitor. A bee hive type or Jackson air spaced capacitor would be ideal. Note: The PW article claims you can't add more than about 3 directors to the basic ZL special with pre-set capacitor tuning. Presumably you need a more complex matching arrangement for longer ZL specials such as a halfwave matching section, movable stub and 4:1 balun.

Testing and results
I mounted the ZL special antenna horizontally to compare it to my standard 5 element long Yagi used for SSB work. I found the 5 element ZL Special had similar gain but it has almost half the boom length (only about 1m). This antenna is small and light weight and would be a very useful antenna for portable operation. I have been using home-made antennas on the local 2m Worthing & District Radio club FM Monday night net for several months. At first I used a quarter wave ground plane (GP) antenna, then I tried a 5/8 wave vertical, then a 3 element collinear array (all partly constructed from 3D printed parts). As expected the 5/8 performed a little better than the GP but the collinear was best. Two of the net group are ca. 20-30 miles away (and the other side of the hill to me) and so I receive their signals very weekly. They are just strong enough for me to hear their carrier but not strong enough for me to hear what is being said. The collinear array brought these signals out of the noise. However because the collinear is omni-directional I then found I was having problems with the strong local Brighton repeater effecting the sensitivity of my reciever. The ZL special has a great front-to-back ratio which solved the strong signal issues. It was great to discover that with the ZL special mounted vertically these week signals were completely brought out of the noise so that two way communication was easily achieved.

3D printed
GP antenna 		3D printed
5/8 wave
antenna 		3D printed
3 element
collinear 		my radio
page 		my antenna
page 		3D page


THE CREATIVE SCIENCE CENTRE
Dr Jonathan Hare, The University of Sussex
Brighton, East Sussex. BN1 9QJ.

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