Proposed Crossed-Wire HF Antenna for the NLO
D. W. Knight, Radio Group, Norman Lockyer Observatory, 9th May 2006.
Prior to the building of the new lecture theatre, the existing wire antennas use by the Radio Group were in a state of disrepair, and were of a design and construction which was arguably not in keeping with the public image of the Observatory. The removal of those antennas now gives us an opportunity to install an effective and scientifically interesting antenna system; capable of transmitting signals over the entire 1.8 to 30MHz HF range, and capable of receiving signals at much lower frequencies also. To this end, we propose the installation of a crossed centre-fed wire system, consisting of four thin wires brought to a central mast mounted on the main observatory building. It is proposed that one pair of wires should run exactly North-South, and that the other should run exactly East-West. The preservation of an exact 90 degree difference in the orientation of the two pairs of wires is required for best technical outcome (electrical symmetry), but the cardinal orientation is negotiable in the event or objections by other interest groups.
Five masts are required for the antenna system, and we expect that 50mm diameter tubing will serve for this purpose. The use of thick-walled aluminium poles (theatrical scaffolding poles) should eliminate the need for guy-wires on the outlying masts. We propose that each mast should consist of two 6m poles clamped together with a straight-coupler. Aluminium will be used for the bottom half of the central mast, but the top half will be made from fibreglass to avoid compromising the electrical symmetry of the feeding arrangement. Fibreglass is more flexible than aluminium, but the four attached wires will act as guys. The central mast will be mounted on T&K brackets on the North facing wall of the main building, and its top will be about 13m above the ground. Additional small VHF and UHF antennas may be attached to the lower section of the central mast as the need arises. The four outlying masts will be sunk into concrete to a depth of about 1m, so their tops will be about 11m above ground. Visibility will be minimised by painting the central pole grey, and the outlying poles in green and brown camouflage. The new installation will be constructed using stainless steel (A4) fasteners wherever possible and will therefore remain permanently in good condition. Existing masts, with their rusty fittings and steel guy wires, represent a public hazard and will be removed.
For optimum efficiency at the lowest transmitting frequency (1.81MHz), and avoidance of an inconvenient resonance in the 3.5 – 4MHz region, the electrically conducting part of each antenna wire should be about 45m long, measured from the tie-off point on the central mast. The four conductors must all be of exactly the same length to prevent feed-line radiation and minimise interference pickup from other electrical equipment in the building. The free ends however, should not be tied directly to their respective masts. In order to minimise resonant absorption by the metal supports, there should be at least 5m of non-conductive line between the end of each wire and its mast. Hence it is proposed that the conductor should terminate at a plastic insulator at 45m, thereafter being attached to a stainless-steel tension-spring (to prevent breakage in high winds), and thence to a plastic halyard of at least 5m length leading to a pulley at the top of the outlying mast. This arrangement gives considerable flexibility in the placement of each outlying mast, since the plastic line may be of arbitrary length (greater than 5m), provided that the length does not become so great as to give rise to unacceptable sag.
Proposed sitings for the masts are shown on the attached plan. The important technical stipulations are that there should be an exact 90 degree difference in direction between adjacent wires, and that the outlying masts should be placed outside a circle of 50m radius centred on the feed-point (shown dotted on the plan). Note that the halyard for the West wire passes over the extension of the Kensington Dome, and that the North wire passes close to the Victoria Dome. Halyards in each case however will be brought over a pulley to a cleat mounted low down on the mast. Thus, in the event of an optical obstruction, it will be the work of less than a minute to slacken or drop the offending wire. The West mast may alternatively be placed to the East of the Kensington dome, not far from the position of one of the original masts, but it will then stand in the clear instead of being hidden amongst trees. Note that the erected antenna wires, insulators, and tensioners are of light weight, and will not cause damage or constitute a public hazard in the event of wire breakage.
Connection to the antenna will be by means of a 4-wire ladder-line, i.e., the four wires dropping down to the radio room will be maintained in square formation by means of plastic spacers placed at intervals along the line. The spacing between two wires on a side will be 10cm. Such a line must not be allowed to come within 32cm (absolute minimum) of any metal or waterlogged objects (i.e., masonry), and close proximity to fibreglass (a lossy dielectric) is also best avoided. Hence the attachment to the central mast will not be directly at the top of the pole, but to a fibreglass spur of about 1m length and 38mm diameter protruding at right-angles to the pole. The spur should be at 45 degrees to the two wires it goes between, and so might point in a South-Westerly direction. A further spur lower down will be used to keep the feed-line taut in its vertical drop. From the lower spur, close to the central joint, the 4-wire line will be taken across the roof of the building (route shown dotted on the plan), sloping downwards and held reasonably taut, to a fibreglass stub-mast, mounted on small T&K brackets on the south elevation of the building just outside the radio room. The stub mast only needs to protrude just above the roof level, to keep the feed-line away from the roof. The four wires will then be brought to terminals on a grey ABS box screwed to the wall and covering a hole through which cables are brought into the building.
On the inside of the building, the antenna wires will be brought to four user-accessible terminals via four high-voltage single-pole change-over relays. The relays will be powered by a master switch such that, when the power is turned off, all four antenna wires will be connected to a stout cable which passes outside and down the wall to a 1m long copper spike hammered into the ground. In this way, equipment will be protected from lightning discharges when the station is unattended, and a direct lightning strike may melt the antenna wires without causing significant currents to flow in the mains wiring. The user terminals should be covered by a hinged or sliding panel made from clear polystyrene or acrylic sheet, so that members of the public may not accidentally touch them.
In the event that we are able to address the various technical and practical issues outlined above, the capabilities of such an antenna system are manifold. Some of its properties and advantages are listed below:
1) 100% Operability:
The existing HF beam antenna (mounted on the lattice tower at the south side of the building) operates only on the 14, 21, and 28MHz bands. These frequencies are generally unsuitable for communication within the UK, and at certain times of day become unsuitable for long-range communication. Hence, with only this antenna available for HF working, we have found ourselves at times unable to demonstrate the radio station to the public. The proposed wire antenna however can be adjusted, by means of a matching network, to operate on any short-wave frequency. This means, in general, that both local and long-distance communications will be possible at any time of day or night and in any atmospheric conditions.
2) High efficiency on low short-wave frequencies:
Given its location on Salcombe Hill, the operation of a 90m long wire antenna at about 11m above flat ground will make the Observatory installation extremely effective on the 160m and 80m amateur bands. Licenced operators often have difficulty in erecting efficient antennas for these bands at their home locations, and so the facility should help to attract visitors and new members.
3) Accessibility to operators of varying technical ability:
Notwithstanding subtle properties to be described shortly, any pair of wires can be selected and matched to a transmitter using an antenna tuner. This means that any qualified operator will be able to demonstrate the radio station. We may also, at some future date, obtain an automatic antenna tuner, making the station operable by those who do not feel confident adjusting a manual tuner.
3) Electrical steerability:
On higher short-wave frequencies, most of the energy radiated by a long-wire emerges at angles close to the wire axis. This gives the antenna considerable gain, but the fixed radiation direction can be disadvantageous. By using crossed dipoles, it is possible either to optimise communication by selecting wire pairs, or to steer the radiation pattern by varying the ratio of the feed currents to the two members of the cross.
4) Directionality:
The antenna may also be considered as two V-shaped dipole antennas mounted back to back. By feeding power into the pair of wires leading to one V, and connecting a suitable load reactance to the other, it is possible to vary the electrical length of the spare pair of wires and turn it into a reflector or a director. This practice will allow transmitted energy to be concentrated broadly in one of four directions (NE, NW, SE, SW if the proposed orientation is adopted).
5) Elliptical Polarisation:
Local (European) communication at lower frequencies is via Near-Vertical-Incidence Sky-Wave (NVIS) propagation. This operating mode is of great interest to Broadcasters, the Military, and Radio Amateurs, and is still the subject of some research. NVIS communication is subject to fading, which is partly due to changes in polarisation caused by the atmosphere, and partly due to phase cancellation caused by multiple propagation paths. The production of circularly polarised vertical radiation has the potential to eliminate polarisation fading and separate it from multi-path effects, giving improved communication and information about the atmosphere. The crossed dipole antenna can easily be made to produce elliptically polarised vertical radiation (i.e., a good approximation to circular polarisation) by the expedient of feeding the two members of the cross with signals which are 90 degrees out of phase. This can be done by means of a power splitter, a length of coaxial cable in one of the feeds to give a time delay, and two antenna tuners. We (the Radio Group) would like to conduct experiments in this field and publish the results.
6) Thunderstorm Tracking:
Wire antennas of the type proposed work as atmospheric electrometers. Smaller antennas than this can light glow-discharge tubes (neon bulbs, etc.) fairly brightly during rainstorms, and cause discharge tubes to flash in the event of lightning at distances of several tens of kilometers. With more sensitive equipment (e.g., an oscilloscope) it is possible to detect thunderstorms at hundreds of kilometers, and by comparing pulse arrival times at the four antenna wires of a cross-dipole, the bearing of a storm can be determined.
In summary, an electrically-symmetrical crossed-wire antenna is a device which offers high performance in HF communications practice, and maximum versatility in relation to experiments involving phase or time differences between received or transmitted signals. It can be used in a simple manner by treating it as a set of wire antennas, or it can be used for experiments by connecting it to various reactive or time-delay networks and measuring devices. We propose to build the installation to a professional standard, using high-quality materials, and it will not compromise the operation of any other equipment used at the observatory. It will be a somewhat unusual installation, of greater technical significance than an ordinary wire antenna, and should attract public interest.
David Knight. 9th May 2006.
Post script:
Above is the original proposal placed before the management committee in May 2006. The aluminium masts proved unable to withstand the severe weather conditions which sometimes prevail at the NLO site, and have been replaced by wooden utility poles kindly donated by Western Power Distribution.
