C A I R O
CommunicationsAudioInterface for Remote Operations
Talk-Through Relay Installations
It is unlikely that a Raynet Group will make use of Talk-Through Relay operations as a matter of routine because these modes are available as a "concession" and should NOT be used indiscriminately. However, when they are required for emergencies the facility must be configured swiftly and safely, and must deliver a reliable service for the using outstations, from the outset. Thus, a Talk-Through Relay would be established while the rest of the communications team is being mustered and deployed to the various duties on the emergency (or exercise) ground. The transceivers are taken to a Strategic Site as the normal vehicular mobile stations of their owners, one of whom must also be in possession of a Grab-Bag containing all the TTU hardware.
This scheme for inherent flexibility, places some extra demands on the members of a Raynet Group, since anyone (having CAIRO transceivers) could be deployed to implement the facility and so avoid the delays which might otherwise occur were it necessary for key members, with specific expertise, to become available. To some extent then, the underlying plan sets an agenda for some further operational planning to accompany the engineering construction of TTUs.
RI .1 Strategic Sites
The terrain of a particular district is a fixed factor in operational planning and so it is reasonable for a local communications team to have identified a small selection of Strategic Sites for use in their VHF/UHF relayed operations. In the first instance, sites can be identified from a consensus of the membership and later confirmed by experimental evaluation during exercises *. In particular, sites which, by virtue of their geographical prominence, already have permanent radio relays (for other services), should be very carefully evaluated to determine any RFI hazards which might compromise temporary operations from there.
Once found and agreed, the details of Strategic Sites should be documented, perhaps as an appendix to the Group Manual. Whenever split-site in-band talk-through relays are anticipated, the plans might well show the exact standing places for the two vehicles to be 1 mile apart (approximately) and at similar elevations. (This separation distance is 'defined' by the 2m Relay requirement - the limiting case - and is more than sufficient for a 70cms configuration.)
In rural areas, a strategic site might be a piece of relatively high ground having easy and public access, in all weathers, for at least the two main vehicles. Given a choice between a pair of sites which meet these primary criteria, secondary factors like the travelling distance for most members, or the presence of a public telephone, a public house (food !) or other useful facility, or indeed the absence of other radio installations, might influence the final choice.
In urban areas, strategic sites are often found at man-made 'pedestals' like the top levels of multistorey car-parks or the parking adjacent to a church (which are frequently sited on relatively high ground). In very dense urban areas, communications teams should not rule out tower-block premises, if standing agreements for access can be negotiated - perhaps via CEPO?
(Obviously, in this last case, the equipment would be removed from the providing vehicles and taken to the top storey; to a corridor or room, or to the roof or the lift-motor room, etc.)
In all cases, the site(s) should be chosen for a good VHF/UHF penetration ('footprint') into the target locality, having due regard for the User-Service HQ sites where Controls might operate. It is useful to consider that strategic sites, for Raynet emergency communications purposes, are not necessarily the sites from which radio DX-ing is done or the local GB3... Repeater is sited - these being often just 'too good'. The test for a realistic relay site might be to consider a pedestrian operator having a hand-held clipped at their waist (to leave their hands free), on its lowest Tx power output (to conserve batteries) and only a small helical aerial, then being in the basement of the most low-lying property in the target area. Frequently, a pair of vantage points on the side of a hill, rather than at the peak, will be more likely to meet this exacting requirement.
* For a full evaluation of Strategic Sites, consider the method and analysis presented by Best and Hazlewood, "Project Prologue", Mobile Radio Systems and Techniques (pp 189-193), IEE, York, 1984.
RI .2 Grab-Bag
Increasingly it may be reasonable to assume that Raynet members, who have transceivers converted to CAIRO-8, will provide themselves with a 'personal' pair of CAIRO-8 TTU modules. These items are inexpensive and simple to construct, and particularly so for anyone who has successfully undertaken the pre-requisite conversion of their transceiver(s) to CAIRO-8 ! Conversely, the CAIRO TTUs are the more complicated modules to construct and will be required by members who only have adaptors for basic CAIRO. Therefore, for maximum versatility, a Raynet Group might invite its Engineering Team to construct paired CAIRO-8 and paired CAIRO TTUs so that any Talk-Through configuration may be implemented with transceivers of either kind.
Once constructed, these paired TTUs would be retained in a suitable 'Grab-Bag' for one or other of the mobiles, being deployed to a Strategic Site, to collect from the Group 'quartermaster'. Good practice would also suggest that the Grab-Bag should contain two sets of some simplified instructions, in waterproof transparent wallets, together with copies of the Site-Plan(s) for the Strategic Site(s) from which Talk-Through relayed operations have been anticipated.
(A condensed set of Installation Set-up Procedures has been provided.)
It is strongly recommended that these should be re-typed, as 4 separate A4 sheets, by selecting the relevant instructions for each configuration, and using Group prefered vocabulary and notations, together with copies of the relevant configuration diagrams, to be found elsewhere in this Manual.)
RI .3 Site-Plan
For the In-Band (IR) Relay, it is essential that the vehicles separate by the prescribed distance to eliminate de-sensitisation during the period of active service for the using outstations. However at first, the operator/drivers should rendezvous at one or other end of the separation distance, (as they would do for a cross-band (XR) relay) to install the TTU hardware together and ensure that each has set (and locked) the agreed frequencies for that Relay. With the aid of hand-helds, which they later use for an 'engineering' link to each other, they may check the talk-through audio quality, though de-sensitisation will be too severe to hear distant outstations.
That done, one vehicle should then depart to its designated end of the separation distance for the two vehicles to remain on site for the duration of the service while the driver/operators stay with their vehicles to maintain the facility, e.g. run their engines occasionally, to ensure the battery supply, and give identification announcements from time to time, if necessary.
If the strategic site is favourable, the vehicles' usual aerials should be adequate for service operation and certainly sufficient to establish the Relay and confirm its initial operation. Later, the driver/operators might erect simple portable mast(s), alongside each vehicle, to raise substitute aerial(s) clear of local site obstructions - particularly for the Rx of an in-band Relay. When using a dual-band T/Rx, at an in-band 'half' station, the typical dual-band colinear aerial may meet the requirement for simultaneous reception and transmission for the Relay and the Link.
Alternatively if separate transceivers are used at each 'half' station of an in-band Relay, it may be expedient to use a pair of semi-directional aerials, e.g HB9CVs, 'facing' each other at each end of the Link. This will ensure that no other signals, from other stations on the Link band, can interfere with the Link, whether inadvertently or otherwise.
RI .4 Frequency Plans
All the modes for Talk-Through operations require the use of separate frequencies, in the 2m and 70cms bands. Each Raynet Group should determine which frequencies it wishes to employ, both as a Primary set for a particular mode, and also a Secondary set for that mode, in case one of the channels of the primary set is in use elsewhere in range at the time of need. It is essential that all group members know which frequencies have been 'adopted' for each mode and are told which mode is in use for a particular duty, during the briefing. As before it is suggested that this matter be agreed and documented, perhaps as an appendix to the Group Manual.
RI .5 Cross-Band Relay (XR) Channels
This mode requires one channel in the 2-metre band and one channel in the 70cms band. Normally, these would be selected from the set of frequencies designated for Raynet use, being;
2m 144.775 144.800 144.825 144.850 145.200 145.225 MHz
70cms 433.700 433.725 433.750 433.775 MHz
For this mode, the Talk-Through relay station vehicle(s) must have transceivers capable of reception and transmission on one and the other frequency of the pair, with the link between them being formed with a pair of CAIRO-8 or CAIRO TTUs, as appropriate.
RI .6 In-Band Relay (IR) Channels
This mode may be operated in the 2m or the 70cms bands with the link in the other band. It requires all outstations, who may operate through the facility, to have radios which allow for split-frequency working. To maximise this, it is usual to adopt the normal repeater split for the band in question and keep to the same convention for the shift between Tx and Rx frequencies. However, in the 2m band, there is a conflict with the satellite service for the only pair of frequencies normally available to a relay mode. Hence the "reverse" split is prescribed and the Relay station is restricted to 25 W erp (14 dBW) and the outstations, 10 W erp (10 dBW).
There are no similar restrictions on 70cms (at the time of writing - 1991).
Outstation-Rx (Relay-Tx) shift Outstation-Tx (Relay-Rx)
2m 'R8R' 145.200 +600 KHz 145.800 MHz
70cms 432.775 +1.6 MHz 434.375 MHz
RI .7 "MEANS" Channels
The above sets of frequencies have been determined by national agreement. Should a major emergency place a requirement for a second and simultaneously operating 2m relay, it would be reasonable to assign this to whichever of the designated Repeater channels (R0-R7) is known to be completely vacant in the emergency locality and surrounding district. This strategy requires each group, who might plan for such a secondary * relay, to determine exactly which it might be according to the localised "gap" in existing repeater allocations in the area. ( * In fairness to other radio amateurs it must be considered as a secondary option for use only in a Major Emergency: MEANS.)
On 70cms, it is permissible to use other shifts, e.g. the American 5 MHz separation.
RI .8 Talk-Through Permits
Talk-Through operations, or the engineering tests prior to such operations, in the UK, require the issue of a Permit. These are only available to registered Raynet Groups, either for single-day activities, at first, or six months duration, later. DO NOT attempt to test a TTU under construction on air * without obtaining a Permit via the National Raynet Committee. Their Permit documents will specify current practice and may well override some of the foregoing generalities. Please check the exact conditions imposed before operating a Talk-Through Relay.
( * Minimum Tx power, dummy-load, 'bench' testing is admissible prior to Permit application.)
RI .9 Engineering Channels
If an emergency or exercise is sufficiently complex to warrant Talk-Through operations, it is wise to allocate a totally unrelated 'simplex' frequency for engineering traffic. The operators manning the Relay, given the separation of their vehicles, may need to use additional radios (e.g. hand-helds) to contact each other and also the engineering member at the User-Service HQ Net Control station, etc. The use of a separate frequency for this and for other background organisation, e.g. mustering more volunteers, avoids disturbance to the main Net(s).
RI .10 Active Augmenting Modules
Experience with operational Talk-Through schemes suggests that outstation operators derive reassurance from certain forms of indicator signalling. Although the Relay is not a repeater (in the GB3... sense and does not have "Logic"), it is useful for it to issue a 'pip' prior to closing. This acts as a prompt, for any outstation operator who may have been distracted by the emergency work nearby, that a message 'over' is now complete and they may make their call if required.
It is beyond the scope of this document to detail the various forms of labelling tones which may augment a Relay nor the full rationale for them or the circuits which provide such facilities. It is sufficient here to state that such circuits are all relatively simple and may be constructed as small in-line modules which may be inserted, between the TTU and the Tx-Rig, when required.
Typically, they take their 'cue' from the incoming PTT line and assert the on-going PTT for a short extension period as they generate a low-level output onto the mike-pair to the Tx-Rig. The microphone interface, in CAIRO (or CAIRO-8) has the appropriate bus properties if the injection signal is coupled via the *XFMR* or *OPTO* techniques described earlier.