Hardware Help


24 cms AERIALS

By M. Walters G3JVL.


The Alford Slot antenna, which has been developed for l.3GHz by G3JVL, is an easy means of obtaining an omni- directional radiation pattern with horizontal polarization . The antenna has a gain which depends principally upon its length and is typically 5 to 9 dBi . This is a better performance than other simple omni-directional antennae commonly used such as halos or whips.

It is particularly suitable for a beacon or repeater antenna where an omni pattern is required with as high a gain as possible. In this application it is possible to stack two such antennae end to enc3 (as used at the beacon GB3IOW) and nearly double the gain. with higher path losses on 23cm compared to 2m and 70cm the extra gain makes it particularly useful as a mobile antenna.


The antenna consists of a length of slotted tubing as shown in figure l. The width and length of the slot, the wall thickness and the diameter of tubing are all related and much experimental work has been done by G3JVL and G3YGF to evolve some working designs, details of which are given below.

  Tube Dimensions               Slot Width                 Slot Length

31.8mm OD, 20swg wall           4 mm                  510 mm

35.8mm OD, l.lmm wall           8 mm                  510 mm

38.1mm OD, l6swg wall          ll mm                  510 mm  


The dimensions cover three common sizes of tubing available (copper, brass and aluminum materials are all suitable). If they are not followed exactly then some experimentation will be necessary for correct operation. In any case, it is advisable to check the field distribution in the slot as explained later.

The length of tube beyond the slot is completely uncritical and the same tube could be used both as a mast and as an antenna! This includes the length of tubing above the short, so that either a simple short across the slot or a disc covering the top can be used, or the tube can be extended upwards in a similar manner to the bottom.


The feed impedance of these antennae is approximately 200 ohms. A convenient method of feeding from 50 ohm coax is to use a 4:1 balun which is fabricated from semi-rigid coax, as shown in figure 2. It consists of a piece of 0.141 inch (3.6mm) semi-rigid with two slots cut along opposite sides of the outer. The two leaves formed by the coax outer form a twin wire transmission line which; is a quarter wave long, and short circuited at one end. This quarter wave resonator is excited by connecting the coax inner conductor to the end of one of the leaves. The two sides of the semi-rigid a and b are connected to the feed point of the slot (see fig:1 and 2). A convenient method of doing this is to attach small solder tags to the cable so that small screws can then be used to attach the balun assembly to the sides of the slot.


The cable should be bent round after leaving the feed point so that it sits somewhere between the back wall and the center as it passes down the tube. The exact arrangement is uncritical so long as the cable does not come too close to the slot and upset its operation (apart from the feed point of course).  

It is not necessary to connect the cable to the inside of the tube as it passes out of the bottom. However, a convenient method of mounting is to fit d shorting plate of some description across the bottom with an N-type plug or socket in it. The antenna can be mounted entirely by the N-type connector as shown in figure4. This method is particularly convenient for mobile use where the N-type can be screwed on to a female back to back bulkhead fixed to the roof. This feedthrough in the roof can of course be used for other bands as well. Obviously many other methods of mounting are possible.  


1)    The slot in the tubing can be cut with a hacksaw blade and filed to size. It will be neces5ary to drill a few holes to start off with.

2)    If the tubing used is a pluming material (e.g. 35mm copper central heating piping) , then other fittings will be available. In particular a pipe blanking cap can be used at the base which will solder or clamp to the tube and in the center of which an N-type connector can be mounted to bring the coax into the tube from the outside world.

3)    The semi-rigid coax for the balun can be held in a vice and bent slightly while the cuts are made. Care should be taken not to cut into the dielectric too much. The leaves should be kept in contact with the PTFE dielectric, and not bent apart at all.

4) At the feed point two holes can be drilled and tapped to fasten the solder tags. Alternatively, the tags can be directly soldered to copper or brass tubing and the balun fastened to these later (a blow torch being needed for the first operation, a soldering iron sufficing for the second).

5) The presence of moisture on the inside of the tube will not affect its operation, apart from the balun getting wet, which will introduce a slight loss. However, water will accumulate in the tube and this is not desirable. The slot can be sealed with PTFE adhesive tape. An alternative approach is to enclose the whole assembly in a container such as a sealed length of plastic drainpipe. This method has been used successfully at GB3IOW.


Slot antennae are not new - a vertical half wave slot is equivalent to a horizontal half wave dipole and produces horizontal polarization. The novel feature of the Alford is that by making the wave travel up the slot faster than light it is possible to obtain a dipole type field distribution over its length which is many times longer than the free space half wavelength value. The net gain is similar to that obtained by feeding several dipoles in phase, but is obtained without the need for a complicated phasing harness. The gain obtained is directly proportional to the length of the slot in free space in half wavelengths.

The idea that waves are traveling faster than light would at first seem impossible, but in tact it is only a standing wave pattern that appears to travel at this speed; the actual wave travels at a lower velocity than light.


The slot behaves like a transmission line shunted by inductive loops (the solid cylinder is equivalent to an infinite line of closely spaced loops) . Cut off occurs when the shunt inc3uctance resonates with the capacitance of the slot. Below the cut off frequency waves cannot propagate at all. At the cut off frequency, the velocity (anc7 hence wavelength) is infinite. Above the cut off frequency the wavelength eventually c3ecreases to the free space value.


In principal, any velocity factor could be used, but the higher the velocity factor (longer the slot), the more critical the dimensions. Velocity factors greater than about 10 are impractical for this reason and the normal operating range is around 5 to 15% above cut off, i e. with velocity factors of 2 to .5. In the designs given, the velocity factor is approximately 4 and the bandwic3th 100MHz at l.3GHz. The gain achieved for the c7imensions given will be about 8dBi.


The dimensions are, to a certain extent, interdependent. The velocity factor will be increased by decreasing the tube diameter, or by increasing the slot width. The wall thickness also has an effect since it determines the capacitance across the slot so that a thinner walled material will also increase the velocity factor. Thus, if a slightly smaller diameter tube was chosen than one of the designs, then this could be compensated for by using a slightly narrower slot so that the same velocity factor is achieved. Alternatively, the length of the slot could be decreased. The antenna would then operate with a lower velocity factor, but this would give a lower gain. For l.3GHz antenna, the tube c3iameter should be within the range of those given, any tube much beyond these limits will not operate correctly.


It is important that the operation is checked, particularly if any of the original design parameters are changed. This may be done by feeding the antenna with a signal at various frequencies and looking at the voltage distribution using a power meter, detector or analyzer with a small probe to pick up the radiated signal. The probe should be held close to the tube, but not directly in front of the slot (hold it 20 or 30 degrees round from the edge) and moved along its length. The diode current meter described in the microwave newsletter (08/81) would be suitable for this purpose.


The balun works by taking the voltage on the unbalanced 50 ohm line and producing two output voltages relative to earth (the cable outer) which are equal to the input voltage but are 180 degrees out of phase with each other. The balanced load is connected between these two outputs and sees the difference between them, which is twice the 50 ohm voltage. Hence there is a 4:1 step up in impedance. The balun has a comparable bandwidth to the slot, about 10 to 15%. Note that the length of the cuts in the semi-rigid must be an electrical quarter wave long . Since the space between them inside is PTFE and the space around them outside is air, this gives an effective velocity factor of about 0.36. Thus the length is 0.86 times the free space quarter wavelength. If there is a significant gap between the leaves and the PTFE , then the velocity factor will be slightly higher.



The antenna represents a very practical means of realizing horizontal polarization with an omni-directional pattern and high gain on l.3GHz. The bandwidth is sufficient to cover all of the band so that it would be suitable for any modes including TV. The circularity is very good (ratio of max to min gain) being typically ldB. This type of antenna has also been used on other bands successfully - G3JVL has used it on 2m, 70cm and l3cm. For further details contact Mike Walters G3JVL, or ,Julian Gannaway G3YGF', or the RSGB Microwave Committee.