Eugen Berberich, DL 8 ZX
Easily Assembled UHF - VHF Antennas for the Radio Amateur
VHF Communications 1/1996
Radio amateurs are often faced with the question of whether to buy antennas for special purposes ready-made or make them themselves.
1. INTRODUCTION
Standard antennas for the VHF - UHF bands are available at reasonable prices, so there is little to be gained by building your own. It's a different matter if you're looking for antennas with special characteristics. For example, anyone who wants antennas for beacons or transponders with omnidirectional characteristics will often fail to find them on the market. Home-made antennas can be of assistance here, using materials from builders' markets or from domestic shops. Anyone with some talent for handicrafts can thus construct effective antennas at a reasonable cost. For directional antennas for GHz bands as well, home-made equipment can often yield better results than those obtained by comparable industrial products.
2. DIRECTIONAL ANTENNAS FOR 13 CM
As a first example, I'd like to describe an antenna with directional characteristics for the 13cm. band. It is considerably easier to make than a long Yagi, as it doesn't have the many elements of a Yagi with precise but varying lengths.
With regard to the short element lengths, attention should also be paid to the boom, which accounts for a large part of the length of the elements of UHF / VHF "Yagi" antennas. Here we are dealing with a short-backfire antenna, with somewhat less gain than a long Yagi. Publications [1 to 7] provided the basic information for this antenna.
This antenna is very narrow-band and so has not been used much in the entertainment electronics industry, but it can be used to great effect in the amateur radio field. Above all, the use of a broader-band radiator, as described by DL7KM in relation to hybrid antennas (standing 8), means this antenna is very suitable for GHz bands. Moreover, jamming is also possible in an enlarged resonator body [1].
In an earlier construction project, I made the resonator back from epoxy resin printed circuit board material and the collar from a strip, which was soldered on. So the assembly required quite a lot of expense! However, this form of antenna still required a lot of construction, so I pondered on a simpler construction. While out shopping in the household equipment department of a big store, I came across a "sprung cake tin", the dimensions of which I liked. For this had a diameter of exactly 2l and a collar height of l/2, which is what is required for a short-backfire antenna. It didn't even cost me DM 10.00 to have this component immediately erected and converted to function as a VHF antenna (Fig.5). When I displayed this antenna at a club meeting one evening, it was immediately baptised the "cake-fire antenna" by a member who liked a "joke". Unfortunately, I have yet to find any suitable cake tin or pizza pan with the dimensions (37.5 cm. diameter) of a meteorological satellite antenna.
As the radiator used is symmetrical and the antenna "squints" without the need for any special measures, I used an l/4 circuit, open on one side, for matching to an unbalanced coaxial jack. Lean styling can be used to tune the eight-element radiator to 50 W. Ehrenspeck gives a gain of 13.1 dB [1] or 18 dB [7] for this antenna, with a normal dipole as radiator, but unfortunately with no source in either case! In comparison measurements using an industrially produced 25-element Yagi, with a specification of 16.8 dB, the short-backfire's results were 5 dB lower, i.e. to obtain the same gain, four such short-backfire antennas have to be wired up in the same circuit, or else housed in quite a big joint resonator body. I also included two other known types of antenna in the comparison measurements. In comparison with the 25-element antenna referred to, the well-known "Münchner Gruppe" equipment scored 10.5 dB and the DL7KM radiator 6 dB. The literature referred to also mentions additional directors in front of the l/4 resonator disc, but I have not tried this yet. So this antenna gave a similar gain to that for a "Yagi" with about 15 elements. Special-steel cake tins are also on the market, for constructions with a higher mechanical stability.
3. OMNIDIRECTIONAL ANTENNAS
There follows a description of omnidirectional antennas for 70, 23 and 13 cm, as I constructed them as indoor beacon antennas for the DFOANN beacon.
3.1. 70cm Antenna
The first omnidirectional antenna assembled and tested was a Maltese cross antenna [8]. Of course, the omnidirectional characteristic was unsatisfactory. I then started to build a big wheel, the technical specifications for which can also be found in [8]. The biggest difficulty in making this myself turned out to be the mechanical construction. A detailed description can be found in [10] VHF Communications 2, 1995.
3.2. 23cm Antenna
Experiments were carried out with a big wheel for the 23cm. band as well, but because of the poor reproducibility another path was selected. As the patterns were already very manageable at this high frequency, the radiator was constructed as a printed circuit board. I used an aluminium plate at a distance of 1/4 lambda as a reflector. The radiator was fastened to the reflector with four distance bolts.
Two pieces of sheet metal, bent into the shape of a U or screwed together in the form of a lantern, were used for the omnidirectional antenna. For this antenna design, a case wave barrier was also provided, which can be constructed using a l/4 stub. To obtain omnidirectional radiation, whichever antenna is opposite must be powered through a 180° angle [8]. Tuning to 50 W is done through transformation circuits housed within the structure. An N-socket is mounted on the front face of the lantern structure. At these high frequencies, open chassis built-in jacks should be replaced by cable built-in jacks to avoid reactive components. Before group antennas are hooked up, it is advisable to tune all individual radiators to 50 W first, for good matching. For double-hybrid radiators, tuning can be accomplished by altering the distance from the reflector, by lean styling the eight-element radiator (wire format), or by L/C wiring at the connection point.
3.3. 13cm Antenna
Once the hybrid quad radiator form had proved itself in the 23cm band, a similar structure was assembled for the 13cm band. While I was cogitating about the antenna carrier, I made myself a coffee. This drew my attention to the coffee tin, which would certainly be a more suitable antenna carrier (reflector) than the "lantern format". After dividing up the "antenna carrier" into four segments, I attached the radiator, made from Cu-Ag wire, to ceramic holders, at a distance of lambda/4, and attached them to the coffee tin with distance bolts. As already mentioned in publication [9], the transmitter dimensions can be scaled to correspond to the frequency ratio. I then measured and calibrated the individual antennas. With the structure I was using, I had to wire a 3.3 pF capacitor up in parallel (standard value), with lengths of wire each 1.5 cm. long at the connection point. A trimmer can also be used for calibration here, and you can use NWA for measurements, if available. The high-frequency connection I selected was an N-type built-in cable jack with single-hole fixing, which was mounted in the tin's base. The individual antennas were inter-connected through precisely dimensioned transformation circuits, and then the voltage standing wave ratio was checked. So-called "Philips winding trimmers" (as used in radio in the post-war era) can be used to calibrate at the best SWR at the connection points of the matching stubs, which may be bared only for very short periods. With standard trimmers, you can't normally obtain C-values < 1 pF. The assembly is carried out as follows. A piece of Cu-Ag wire app. 5 to 10 mm. long is soldered onto the "hot potential". A few turns of enamelled or plastic-insulated thin wire are then wound around these supports to act as the stator of the capacitor. The taped external conductor is earthed at one end. The loose end can now be coiled on and off, using a pair of tweezers or a little plastic rod (do not attach with wax or the like, as this changes the dielectric). As only very low capacitance values are required with UHF, the inductance created by coiling on has no great part to play.
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