A PORTABLE/COLLAPSIBLE QUADRAFILAR HELIX ANTENNA FOR THE 137 MHZ APT BAND

by Chris van Lint

Background

In the past, many articles have appeared in a number of publications singing the praises of Quadrafilar Helix (QFH for short) antennas. Those of us who have had an opportunity to try such an antenna will agree that this device appears to be optimum for APT, since it is truly circularly polarised, with more gain as compared to the classical Turnstile or Lindenblad types. Often these articles are accompanied by construction details to allow the home constructor to build a suitable device themselves.

The majority of these articles, however, employ an infinite balun matching scheme, which is not exactly constructor friendly to the average home experimenter. These antennas rely on using copper tubing filars together with a hard-line coax filar to form the balun. In some locations hard-line coax is difficult to obtain and expensive. QFH antennas using this construction method require that the filars are insulated from the support and often PVC piping is used for this purpose. Since the filars have to be shaped into the desired helix form, they have to be self supporting and some means of anchoring them to the support mast has to be devised. This is not an easy feat in addition to which the shaping of 1/4 inch tubing is not as simple as it seems. The interconnection of the filars has to be done carefully or the antenna will not work. Construction details which I have seen so far are rather confusing and they have put me off building a QFH of the infinite matching balun type. There are, of course, alternative matching schemes available and a vast improvement as far as the home constructor is concerned is the Slotted or Volute QFH. Such an antenna was described by Mark Pepper in RIG journal number 37; the most difficult part possibly being the slotting of the outer tube. Construction of this antenna, however, does assume that certain components, such as the end cap and 1/4inch copper tubing are readily available. Also the soldering of individual filars to the tube requires heavy duty soldering equipment, ideally a torch and again the shaping of the filars needs some practice. Notwithstanding these arguably minor problems I had no difficulty building two of these Volutes and both are performing well.

Ever since arriving in Hongkong I have been plagued by pager interference. Being a Crown Colony (at least until end June 1997) Hongkong uses the 137-138MHz frequency bands for paging applications as well as various others. In fact there is a pager transmission on 137.95MHz, the same frequency as Mercury Communications uses in the UK. This has made reception of certain APT frequencies misery and I can not get acceptable results on the 137.50 and 137.85 transmissions. After having tried numerous ideas to overcome the problem, including narrow band-pass filtering, I have decided that the only practical solution is to change location. This is of course easier said than done. It is not practical to relocate from my present abode for a number a reasons, not the least of which is the fact that any other residential area will not necessarily offer relief from the interference. The only areas where one could expect to get interference-free reception would be the more remote and less populated areas on the south eastern part of Hongkong Island and this would require some form of mobile set-up. Having convinced myself that the best possible APT antenna must be the QFH, I set my sights on constructing a portable version.

Description

I claim no originality for the antenna I am about to describe. As mentioned before there are numerous descriptions in circulation at the moment. A fundamental deviation from classical QFH antennas is the fact that in this design the filars are not supported directly by the mast but by PVC arms. This relatively simple but nevertheless brilliant idea came from Eugene (Buck) W3KH, who also provided the dimensions. My contribution lies in the idea of using removable arms and conventional flexible co-axial cable for the filars and matching line alike, which allows the filars to be folded against the support mast when not in use.

In contrast to the 3/4 turn Volute alluded to earlier, this antenna is a 1/2 turn, half wavelength device using a self-phasing big/small loop configuration. Impedance matching is achieved by an infinite balun arrangement in which one of the four elements is a length of coax to produce a characteristic impedance of approximately 40 Ohms. The antenna is constructed from standard 20mm PVC electrical conduit tubing for the support arms and a length of 32mm PVC tube for the support mast. To accommodate the support arms I used 6 PVC 2-way junction boxes. The relevant dimensions can be noted from figure 1 (see inside back cover). The dimensions for individual arms are always 1/2 of the total width when measured from the centre of the support mast. QFH antennas of this type use a Big and a Small loop and this is the reason why instead of 4 way, two 2 way junction boxes were used, mounted at 90ø one above the other This procedure creates a (B)ig and (S)mall loop. Each loop splits into 2 filars, which are supported by removable PVC arms B1, B2, S1 and S2.

Figure 2 (see inside back cover) shows the connection of the filars in relation to each other Note that the smaller loop is not connected at the bottom and for this reason filars S1 and S2 may be left joined so that only one length of coaxial cable of 221.6 mm is used. The diagram shows connection details as viewed from the top of the mast, connections at the bottom and the joining of the feed cable. Only one half or filar of the big loop is used as a coaxial conductor and those filars which do not require use as a coaxial cable have the centre conductors soldered to the braid at both ends. I used RG-6U coaxial cable, not because of its low loss, which is not particularly relevant at these frequencies, but because it has a continuous aluminium shield in addition to the braid, which gives the cable extra rigidity which is useful for shaping the filars once the arms are fitted. Since the APT satellites in which we are interested transmit their signals using right hand circular polarisation (RHCP), the filars must be wound in an anti-clockwise direction, when viewing the antenna from the top down along its vertical axis.

Construction

Start by cutting a suitable length of support mast. In my case I cut the mast just long enough to allow a socket to be fitted inside the lowest 2-way junction box, to allow the mast to be extended by inserting another length of 32 mm pipe. Cut holes in the bottoms of the junction boxes. The 2 junction boxes are cemented together at a 90° offset angle to form a cross and a total of 3 sets of assemblies are required. Slide the centre assembly onto the mast, followed by the top and bottom ones. For the centre assembly, a suitable hole may be cut in one of the junction box lids which is slid over the mast to cover the centre assembly. Note that no dimensions are given for positioning the centre assembly, as this is not critical and should simply be in the centre. Cement them into place initially with solvent based PVC cement. I find that this adhesive once a little dry allows some level of adjustment before it is completely sets. Once you are satisfied that the assemblies are correctly aligned and the adhesive is dry, apply generous amounts of an epoxy based adhesive to complete the mast assembly. Now cut the support arms. The actual length will depend on the physical dimensions of the junction boxes used. Make sure that you adjust the final length on the basis of the arms having been tightly fitted into the junction box sockets. Remember that the loop dimensions shown in figure 1 constitute the total width of each of the two loops and the width of the corresponding arms should therefore be half and measured from the centre of the mast. Allow a little extra length to accommodate 2 slots in the support arms in which the coaxial cable filars can be snugly pressed into place.

Each junction box assembly will have one pair of short and one pair of long support arms. Drill holes big enough to accommodate the coaxial cable on top of and as close as possible to the top arms. For the bottom arms do the same, but below the arms. Once completed, insert and twist the arms so that the slot slopes from top left down 45° to bottom right. The arms may now be removed and put aside. It may be helpful to put marks on the sockets and arms to indicate in which position the 45° angle is achieved. If the marks on both the sockets and arms are in exactly the same position there should be no need to put identifying marks on individual arms and sockets; i.e. the position marks should be sufficient. Insert the coaxial cable which forms the smaller loop into the hole below one of the short arms all the way so that the cable exits on the opposite side. Ensure that the length of cable at both sides is the same. Join the centre conductors with the braid. Cut two equal lengths of coax which will form the big loop. On one piece only join the inner conductor to the braid and insert one end in the hole above one of the longer arms. Do the same with the coaxial cable left over in the opposite hole. Solder the 4 cables in the top junction box according to the instructions in Figure 2. Insert the other ends of the coaxial cables in the holes situated 1/2 turn anti-clockwise on the bottom assembly.

Drill a suitable hole somewhere in the lowest junction box to accommodate the coaxial feed line which has been provided with a connector of your choice at the other end. end. Push the feed line through and join all cables as shown in Figure 2. Cement the top junction box cover and the cover for the bottom junction box assembly into place. Now insert the arms in their correct positions and adjust the slot angles, simply press the coaxial filars into the appropriate slots, moving anticlockwise from the top, completing a half turn by the time you get to the bottom. A little manual shaping of the filars to form the helical shapes and the antenna is finished. If the antenna is to be used as a permanent fixture rather than a mobile device you can drill a small hole through the arms, behind the slots at 90ø to accommodate a nylon tie to secure the filars into the slots.

Performance

I have compared the performance of my prototype against my other QFH antennas, including a commercial version under dynamic conditions, using a coaxial relay to switch from one antenna to the other. I am convinced that the performance of the portable QFH is a good as my other QFH antennas. There are no noticeable 'holes' and I get usable signals from AOS to LOS (on 137.62 Mhz which is the only frequency I can receive without interference. If you are plagued by interference from a single source, you may find the QFH more suitable than other types of antennas. The QFH produces a higher magnitude of gain on circularly polarised signals as compared to linearly polarised signals. This in effect attenuates linearly radiated signals in relation to the circular ones and reduces interference.

During field testing I noticed that the QFH appears to have distinct directivity in relation to linearly polarised signals. In my case there is a pager transmitter, which makes reception of signals on 137.5 MHz almost impossible. This interference is not intermodulation and it is there when no signal is being received from the satellite. I found that by turning the antenna when not receiving a satellite signal, it was possible to find a very distinct reduction or sometimes a "null" in the strength of the interfering signal. This has enabled me, for the first time, to decode NOAA 12 signals on 137.50 MHz. A similar improvement could be obtained when dealing with other forms of linearly polarised interference.