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70cm In-band T-T

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General

Most ad hoc talk-through stations used by RAYNET are cross-band.  Depending on circumstances, the radio network is probably engineered to allow the control station to hear all outstations.  That could mean that many outstations can't hear each other.  In many circumstances, there are none or few outstation to outstation messages.

An in-band talk-through station allows all stations to hear each other.  One method is to use two cross-band talk-through stations to form a single in-band talk-through system.  The two locations must be in easy range of each other.  Each must provide the required coverage range.  As two locations are used which may be half a mile or a few miles apart, the transmit and receive ranges are likely to be different, but it may not matter.

The above method of providing an in-band talk-through system has no technical problems.  It does require two vehicles and two RAYNET operators.  A single station can use the same dual band aerial for both transmit and receive, if filtering is available to prevent the transmit signals getting into the receiver at any noticeable strength.

2m In-band Talk-Through

To use a standard frequency split of 600 kHz between the transmit and receive frequencies requires the duplexer to have enormous isolation losses and high Q factors for a single aerial to be used.  That usually equates to the cavity duplexer having long and large diameter cylinders, either 6 or 8 of them.  Using two aerials separated horizontally gives little isolation.

70cm In-band Talk-Through

To use a standard frequency split of 1.6 MHz between the transmit and receive frequencies is a challenge, but using a standard 7.6 MHz split is very feasible.  Fairly small 6 cavity UHF duplexers are available for about 62 (in 2012).  There is no necessity to have repeater logic or cross-band talk-through radios.

Equipment Required

For standardisation and ease of use, it is best to use mobile rigs which have a standard 6-way mini-DIN data port connector.  The impedances and voltage levels will be about the same on any such mobile rig.  The advantage is that the talk-through station doesn't require a special repeater-style radio.  Many different kinds of 70cm rig can be used.  The equipment required to provide a 70cm in-band talk-through station in one car is:-

Two 70cm mobile rigs, each with a standard 6 way mini-DIN data socket.  One will be used only as a transmitter, the other as a receiver.

One 70cm aerial, either car roof-mounted or on a portable mast.

One UHF cavity duplexer.

One cable to connect the two 70cm rigs.

UHF Cavity Duplexer

These usually come pre-tuned by the retailer to the two frequencies specified by the buyer.  It is possible for a RAYNET operator with enough knowledge, equipment and experience to tune them.  They have a comparatively low Q factor and may well work not only on the designed channel, but also on adjacent channels.

Searching eBay for "uhf duplexer" (in 2012) shows 20W devices for 53 including delivery, 35W for 55 and 50W for 62.  An example 50W version is 3.5cm tall and 19cm by 19.5cm.

The Connection Cable

This cable has a 6 way mini-DIN plug at each end.  Its circuit isolates the connections between one 70cm rig and the other.  No power supply or battery is required.   The cable does two things:-

The relatively loud receive audio from the receive rig is connected to the transmit audio of the transmit rig via a volume control.

The open or closed squelch indication voltage from the receive rig is converted to an earth (zero Volts) to operate the PTT of the transmit rig.

 
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Page updated on 09 January 2017

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