CommunicationsAudioInterface for Remote Operations

CAIRO-8 Talk-Through Relays

Variable Network Planning

No two emergencies or major exercises are exactly alike nor do all communications teams establish similar networks under equivalent circumstances. A range of fixed and variable factors will determine which one, of several alternative plans, is most appropriate in a specific instance. The detailed nature of the local terrain, the spread of the emergency (or exercise) ground, the number, type and duties of the outstations being deployed and the band(s) on which they must operate, together suggest that a single, inflexible plan could not be viable on every occasion.

However, network plans fall broadly into four Modes, each with merits and detractions:

1) Single Simplex - SS - Network : all stations operate on one simplex frequency.

The predominant message traffic passes between the outstations and one Control station, there being only occasional instances when traffic may need to pass directly between outstations.

2) Multiple Simplex - MS - Networks : outstations are divided, for geographical reasons or organisational purposes, into sectors. Each sector has a separate but simultaneously operating simplex Net and a Control station. All Controls would be co-sited so that the activities and information on the separate networks can be co-ordinated. The outstations, on a particular network, might exchange messages directly with each other to a limited extent.

3) Cross-band Relayed - XR - Network : a single Talk-Through Relay station links together two separate simplex nets so that message traffic may pass directly from one to the other. Typically, the two nets would be on separate bands with the main body of outstations being on one of these, e.g. 2m (VHF), and the Control and other special-duty stations being on the other, e.g. 70cms (UHF). This mode is most suited to operations which are controlled from a User-Service HQ at some considerable distance from the emergency-ground. However, the mode might also be employed in 'cross-service' relays where, for example, radio traffic may occasionally pass between a (RAYNET) network and a User-Service radio network, in support of an Emergency. (Not yet permitted in UK.)

4) In-band Relayed - IR - Network : a pair of Talk-Through Relay stations provide a repeater-like coverage for all outstations which then operate with split or shifted frequencies, which for Raynet may either be within the 2m or 70cms bands. The coverage of the Relay is normally such that it gives a very good penetration into the emergency-ground to enable stations, in very poor locations or using very low-power transceivers, to maintain service. In particular, all stations hear each other's traffic which may pass directly between outstations when required.

Major Emergency Augmented NetworkS : "MEANS" : the service supporting a recovery operation following a major emergency might well require multiple versions of these Relay modes.

Fig. 21

Four Modes
Radio Networking

Oops- 'U', 'Y' and 'D' are supposed to be icons representing buildings and trees - must get that fixed !

TT .1 Temporary Relays

The underlying assumption in the CAIRO approach to Talk-Through Relay operations, is that the Relay must be capable of being brought into service "at a moment's notice" and must function reliably for the duration of the Emergency or duty. It must be an 'instant' facility.

For full portability and flexibility, the instant Relay may be configured, at a pre-determined Strategic Site, from the resources of two vehicular mobiles - usually the first two to become available with appropriate transceivers for the frequency bands of the operation. Their existing CAIRO-adapted transceivers (into existing aerials) will be augmented with previously engineered and tested CAIRO units to form a working Talk-Through facility, without any on-site engineering beyond that of making simple, pre-arranged connections with these units. Furthermore, these simple units must be capable of supporting any Talk-Through configuration.

TT .2 Cross-Band Relays (Mode-3)

The cross-band relay (XR) requires all relevant signals received on a single frequency in one (FM) band, e.g. 2m, to be retransmitted on a single frequency in another band, e.g. 70cms, and then vice-versa. If the two mobiles are chosen for this relay, so that one has a 2m transceiver and the other a 70cms transceiver, then the formation of the relay simply involves a 'back-to-back' connection of the baseband audio, i.e. CAIRO signals, between them via Talk-Through Units: TTUs. The only RF constraint on this mode is that the two frequencies should not be harmonically related.

Normally, the transceivers would remain in their original vehicles, on their existing aerials and power supplies, with the two vehicles being parked in reasonably close proximity so that the links between them may be implemented with two CAIRO Lines, e.g. 2 Orange Reels.

Alternatively, a cross-band relay may be configured within any single vehicle which is routinely equipped with separate transceivers for the two bands of this mode.

Fig. 22

Cross-Band Relay (XR)

based on the rigs of two vehicles parked together

TT .3 In-Band Relays (Mode-4)

For the in-band talk-through mode (IR) it is usual to adopt the normal "repeater" offset, -600 KHz on 2m, or +1.6 MHz on 70cms, to ensure the maximum contribution by the volunteer outstations whose transceivers, it is reasonable to assume, will be capable of these standard shifts. At the baseband level, this relay is less complex even than the cross-band version since one transceiver acts throughout as the Rx-Rig and its audio output passes to the second transceiver which always, when keyed, acts as the Tx-Rig. Thus only one signal path is required from Rx-Rig to Tx-Rig, being 'half' the requirement of the cross-band relay.

However, the frequency 'split' brings the relay's Tx-Rig into close spectral proximity to the Rx-Rig (~0.4%) such that 'de-sensitisation' will occur unless preventative steps are taken. Because it is intended that a mode-4 relay should be configured from the first suitable mobiles to become available, the significant factors which affect de-sensitisation, Tx ERP and the levels and spread of wide-band noise, or Rx sensitivity to such noise and its threshold to blocking by the Tx, can not be determined beforehand. Instead, a reliable strategy must be imposed on the installers.

TT .4 Isolation by Separation

De-sensitisation is encountered, to a greater or lesser extent, whenever a transmitter and receiver radio pair operate simultaneously in close physical and spectral proximity to each other. If the frequency 'split' is large (3% or more) the required isolation is small (30 dB's) and may be achieved (or almost so) by disposing the two aerials above each other at a vertical separation of about 2 wavelengths. But, with the significantly narrower in-band 'split' of the amateur service, the minimum isolation increases (55 - 60 dB's) to require a vertical separation of some 5 to 6 wavelengths; about 36ft for a 2m, or 12ft for a 70cms facility. Nevertheless, vertical separation is a viable strategy for a permanent installation (e.g. the GB3.. Repeaters) when time allows the installers to give due consideration to the type of aerials and to design (or share) a mast which is sufficiently substantial to permit the required separation and also allow the lower aerial to be reasonably elevated too. Even then, the physical dimensions for full isolation may be unrealistic so the deficit can be achieved, at the precise frequencies of operation, by inserting high-Q filters ("Cavities") into the aerial feeders and selecting radios which meet quite exacting specifications.

TT .5 Instant Relays

For the instant relay, isolation by vertical separation is not a viable strategy because the assembly of mast and aerials, even if prepared in "kit form", will be too cumbersome to transport to the site and erect quickly and safely. But if a reduced-separation assembly is considered as a compromise, then cavities or similar filters become essential components and these are not portable items either because they require on-site tuning. This is a critical and time-consuming adjustment which calls for prior experience and some specialist test-gear, so it would defeat the objectives for haste and simplicity, where emergency radio is concerned.

So, for a realistic, fully portable instant scheme CAIRO assumes horizontal separation.

All other things being equal, horizontal isolation requires distances some 15 times greater than the vertical case, since the aerials now 'see' each other along their axes of greatest radiation. For a minimum 60 dB's isolation this requires 80 wavelengths, horizontally.

However, the temporary relay is to be based on 'unknown' transceivers which may well employ frequency synthesis and, as transmitters, may thus generate greater 'noise-floors' than the single-frequency, often crystal-based, transmitter of the fixed Repeater. Thus, to ensure sufficient isolation in the worst-case, separations equivalent to about 80 dB's should be achieved. Translated into distances for a reliable strategy this requires the two vehicles, of a 2m Relay, to separate by about 1 mile and the vehicles, of a 70cms Relay, to separate by about 1/3rd mile.

TT .6 Wireless Links

Because there can be no significant compromise in these isolation distances (+/-10% is OK), it is impractical to form the link by means of remote line connections. Instead a wireless link is used and the in-band talk-through relay comprises of two cross-band 'half' relays. The INput mobile receives the signals from the outstations on the primary band, e.g. 2m, and relays them at low power, e.g. 1 W, on a secondary band, e.g. 70cms. The OUTput mobile receives the link signals and re-transmits them on the 'split' frequency in the primary band, at 'high' power, e.g. ~ 10 W. It may be noted that two TTUs are used in this chain, so the fidelity of each must be close to 'ideal'.

Fig. 23

In-Band Relay (IR)

based on two well-separated cross-band 'half' relays

TT .7 Dual-Band Transceivers

Figure 23 shows that the path, for all signals relayed for the using outstations, is in one direction only. Therefore each transceiver, of the pair in one vehicle, acts either as the Rx-Rig or as the Tx-Rig of that station while a single TTU provides the audio link between them. Increasingly, it is reasonable to plan such a Relay configuration on the basis of CAIRO-adapted dual-band transceivers (2m/70cms), almost all of which can be switched to 'cross-band duplex' ("dual") working. As the transmitter and receiver sections operate simultaneously on one and the other bands, it is feasible to use such a transceiver as a complete 'half' of a talk-through scheme.

Already some dual-band hand-helds can be programmed for this function, without external TTU couplers, and would be ideally suited as the complete INput 'half' relay station in the above scheme, while some dual-band mobile transceivers, having a higher Tx output power level, can be similarly programmed and would immediately satisfy the OUTput 'half' relay. However, the typical installation is most likely to be formed from individual transceivers, or dual-band rigs which cannot be so programmed, hence the need for the TTU as an external enabling module.


Circuits for reliable TTU coupling have already been described, *XFMR* (see C8 .10) and *OPTO* (see C8 .11), and in subsequent figures the entire circuits of Figs. 14 or 15 are used. Either form of coupling is equally applicable since both require the Rx-Rig of a 'half' relay to be a full CAIRO-8 host, though the Tx-Rig of each 'half' relay may be just a CAIRO-adapted transceiver. Two separate TTUs are required for the in-band configuration (one in each vehicle). With the following wiring plan, the two TTUs may be used either for in-band or cross-band configurations.

The single CAIRO-8 TTU module has one CAIRO-8 tail, one CAIRO tail and a CAIRO socket (DIN-7) as shown, Fig. 24 A. The CAIRO-8 cores for Audio-Line and Squelch (and Power in the *OPTO* version) pass to the isolator PCB to become the outgoing CAIRO line mike and PTT signals. The other CAIRO-8 cores; mike, speaker and PTT, pass to their respective pins of the DIN socket. In particular, the CAIRO-8 speaker core is at pin-1 so that the CAIRO speaker core is at pin-6.

Fig. 24 A

Fig. 24 B

Single CAIRO-8 TTU

Cross-Band with 2 x C8-TTUs

For the In-Band (IR) configuration, this socket is a convenient point for the manning operator to plug binaural headphones and hear through traffic, in the 'primary' earpiece, and the Link channel, in the 'secondary' earpiece, whenever there is no through traffic and the Tx-Rig is receiving. Listening levels are set at the respective rig's volume controls which have no effect on the talk-through path. Should it be necessary to do so, the manning operator may also plug in a full termination accessory set, to include a mike and PTT, and may transmit on the Rx-Rig, to announce the use of that frequency to distant stations. The full IR set-up will require two identical TTUs, one in each vehicle, between appropriate rigs.

Conversely, for cross-band relays (XR), these two separate TTUs may be plugged together 'back-to-back' between two CAIRO-8 host transceivers, as shown in Fig. 24 B, either directly if the rigs are close together, or via CAIRO Lines (Orange Reels) if two vehicles are used (Fig. 22).

Finally, for the In-Band 'half' relay (IR) which might use a CAIRO-8 converted dual-band transceiver with a "duplex" mode but no internal audio coupling path, a single C8-TTU module is used with its CAIRO tail plugged back into its own CAIRO socket; a self-loop termination.

Clearly, these TTUs are intended to be very versatile and to serve any one of the several plausible configurations for Talk-Through Relays, equally well. This optimises the investment in the TTU hardware items which, in reality, may only be used infrequently.

It is useful to note that these TTUs need not be furnished with a switch in the Squelch-PTT coupling because the act of plugging/removing the CAIRO tail at the Tx-Rig achieves this control. Instead, it is appropriate to equip the module with an LED, from the CAIRO-8 power supply line, as a simple indicator that the connection to the CAIRO-8 host transceiver has 'activated' the unit.

It is also useful to note that a C8-TTU (*XFMR* version), when not required as part of a Relay, may be used into a portable Tape-Recorder to log incoming traffic, say at a Control station. The CAIRO tail feeds the recorder; "PTT" being applied to the start/stop control, and the remote line to the operators is plugged into the TTU's socket which has all the main signals of the host rig.