Introduction To Portable Masts And Antennas For RAYNET Use

By: Bernard Spencer (G3SMW)

Object

The use of a light-weight mast and antenna, carried by each Member in his car at all times, is not a new idea. It allows Members to be deployed directly from home or work to the scene of an incident, or to a desired relay point. Members of at least one Group carried a plastic-pipe mast and a modified Slim Jim antenna in their cars in the 1970's. When fibre-glass Roach Poles became available, some other Groups carried them, and have developed ingenious stands and fittings for them.

This Note is a brief introduction for the present (2005) use of portable masts and antennas for RAYNET, and suggests two easily made antennas for this.

The greater antenna height using a mast, compared with the height of a car-roof antenna, gives a greatly increased chance of setting-up a successful communication link. This applies in most terrain, urban or rural. While the height-gain from 1.5m to 3m in urban areas is about 3dB (6dB in open flat ground), getting the antenna above much of the natural and man-made clutter by using a 5m mast often results in a gain of 8dB to 12dB or more in VHF/UHF signal strength. If on arrival it is found that the car-roof antenna will not provide a link, the use of a modest mast often will.

Points to bear in mind are firstly, that simple light-weight antennas will not have as much gain as the commercial vertical colinear antennas used by Radio Amateurs on buildings, (but these require heavier masts). Secondly, that feeder loss should be kept to a minimum, as with all elevated antennas. Nevertheless, such simple antennas and light-weight masts, with a feeder as short and low-loss as practicable, will generally give greatly increased communication capability compared with car-roof antennas, and are quicker to deploy and erect than heavier masts and higher gain antennas kept in a central store.

Masts

What is comparatively new is the availability of light-weight and inexpensive fibre-glass telescopic masts, closing down to less than 1.2m (48in) in length.

They can be stored in most cars. Also, having a weight of less than 1 Kg they can be carried easily on-foot. The top section is very thin and bends easily in the wind, but that section need not be used. So, for example, a 6m telescopic fibre-glass mast without the top section will be about 5m in height, and sufficiently rigid at the top for a light-weight and low-windage antenna to be used without signal flutter in the wind. This height is quite a good choice, but similar masts up to 9m are available. A 6m fibre-glass telescopic mast can be bought for as little as £10.

Carbon-fibre masts should not be used because they are conductive, and this restricts the choice of antennas.

As these masts are very light and easy to handle, it is possible to tie them to a fence, car, or other convenient object with string. It is even possible for an operator to hold the mast vertical with one hand while using the radio. Of course a light-weight stand of some kind is useful.

Antennas

The majority of RAYNET communication is in the 2m band, and some may feel that a portable antenna for that band alone is sufficient. In that case a suitable and easily-made antenna is one of the 2m Band Ribbon-J Antennas such as that described by John Belrose VE2CV (1). This version can be rolled up when not in use. It incorporates a choke in the coaxial feeder, and this is desirable for most vertical VHF antennas. The dimensions given there are for the 144-148 MHz band, so for the 144-146 MHz band the lengths can be multiplied by 1.014. Larger and smaller versions could be made for other single bands.

On the other hand, links on 70cm and 'Talk-Through' on 2m/70cm are not uncommon now that most Hand-Held and Mobile Transceivers incorporate both bands. A dual-band antenna may therefore be preferred, if it is effective and can be made easily.

Some Groups have made and use the "Townsman" Dual Band 2m/70cm antenna (2). This is a rigid antenna, and has a Patented impedance matching arrangement at its base. The desired radiation pattern on 70cm is achieved with a cylindrical metal sleeve over part of the radiator.

A simple dual-band 2m/70cm antenna that is easy to make, does not require difficult adjustment, and can be rolled up when not in use, is the

"FBK 2/70 Portable Coax Antenna" designed by Frank Bremer PA0FBK (3).

It is made of RG58 coaxial cable, with some parts of the braid removed. The braid is retained over part of the radiator, for the same purpose as the rigid sleeve in the case of the Townsman.

We have added an RG58 "tail", part of which is coiled to form a choke. This choke is to reduce currents on the outside of the feeder that could tilt the main lobes of the vertical radiation patterns (VRP) of both bands up or down. It also avoids possible matching problems at the transceiver. This air-cored choke construction, winding the coax tail inside a PVC tube (wound with 6 turns of the PVC cable jacket touching), is satisfactory and reliable. It avoids the cost and difficulty of obtaining a suitable ferrite choke.

In the design of dual-band 2m/70cm vertical antennas generally, the harmonic relationship makes it relatively easy to obtain acceptable impedance matching on both bands, but it is not easy to obtain the desired VRP for the 70cm band.

It is difficult to avoid an upward or downward tilt of the main lobe, or a null in the horizontal plane. The braid on part of the vertical radiator of the FBK antenna considerably improves the 70cm VRP.

The details of the FBK antenna are shown on the attached graphic, together with the tail/choke. In (3) PA0FBK mentions his previous use of the Townsman. We have made the FBK antenna and have tested it for impedance match and VRP on both bands, with the choke (4). We checked VRP's on a large flat open area (Playing Field). Whilst the 70cm VRP is not quite optimum, it does have one of its maxima in the horizontal plane and this is within about a decibel of the others. After initially trimming the top, the VSWR was below 1.5 at the centre of both bands, and below 2 at the band edges. The antenna performance is considered quite acceptable for its purpose on both bands. It is recommended that in the first instance the top (unsheathed) section of this antenna be made about a couple of centimetres longer than the figure given. By checking the VSWR in both bands (with the antenna attached to a fibreglass mast using sticky insulating tape), the top can be trimmed, say 2mm at a time, for the best compromise VSWR on both bands. It is best to tape the antenna to the mast at only three or four points, so that it is not held tightly against the mast over most of its length and therefore will not be affected appreciably by the dielectric of the mast. If desired, the ends of the braid can be covered with short pieces of heat-shrink tubing as a seal, taking care not to melt the polythene dielectric.

We have designed and made other flexible Dual Band 2m/70cm antennas, with the radiator centre-fed through the coax. These work well, but they incorporate traps that require accurate setting-up, and so are not as suitable for general DIY construction. No doubt other designs will emerge.

Flexible antennas can also be used conveniently without a mast. For example the top can be tied to a branch of a tree. If operating near the top of a building of several storeys height, it is sometimes possible to open a window and let the antenna dangle down outside from the window sill, or just to use the antenna inside the room. And so on...

If using any antenna on 70cm, remember that half a wavelength is only about 35 cm (approximately one foot), and that there may be a horizontal standing-wave pattern in space due to reflections, as well as a vertical pattern. Moving the antenna horizontally a few inches may then result in a considerable change in signal strength. For fixed commercial Point-to-Point links at similar frequencies, the antenna is often mounted on a "swan-neck". Antenna position then is adjusted horizontally and vertically during installation.

1. Technical Topics (Radcom May 1995)

2. B.Hewlett G3JAM (Wireless World February 1980).

3. Frank Bremer PA0FBK www.qsl.net/pa0fbk/hampage_uk.htm

4. Gerry Hussey G4PGZ assisted with the antenna tests.

Bernard Spencer G3SMW

Marlow April 16^th 2005

Text modified 4th April 2008

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