The most popular way of operating multimode on 4m is by using a 'transverter'. A transverter is simply a device which allows a transceiver designed for one amateur band to be used on another. Unfortunately most of the latest HF transceivers don't offer transverter facilities unless you are prepared to modify them yourself.
Experience has found that some of the older 'Trio/Kenwood' transceivers such as the TS830S, TS430S, TS930S and TS940S are excellent as 'prime movers' for transverters. These provide all the interconnections on one 8 pin DIN connector and make for a convenient and professional installation. The TS430S in particular is particularly good in that when changing mode from CW to USB the transceiver changes frequency slightly so that a readable CW tone is received and not zero beat like most transceivers. This is useful on 4m as there are often either cross mode or changing of modes mid-QSO for the long haul contacts where QSB is often present.
The Icom IC735 is also particularly good for transverting, as it offers a low-level transverter drive output.
The Kenwood TS2000 has a transverter facility which allows display of the transmitted frequency. Minimum power output of 5W is automatically switched in when using this feature. There is also a separate receive input which can be used to avoid a relay to switch between receive and transmit chains.
The Yaesu FT817 has a maximum power of 5 Watts, which can be set to even lower levels. This set can provide outputs at 28 or 144MHz.
Microwave Modules of Liverpool were well-known in the 1980s for their amplifiers and transverters. Many of these are still working, and appear on the second-hand market - check the dealers, or RadCom members' ads. These were constructed inside a compact Eddystone diecast box with either BNC or SO239 sockets for RF interconnections and a 5-pin locking DIN connector for DC and switching connections. Models were produced for use with 144MHz transceivers as well as HF transceivers using a 28MHz IF. They have been proved to be quite robust units over the years with many proving useful for both portable and home station use. They produce around 10W output and are suitable for all-mode operation.
The input power required to drive these units is around 0.3 Watts: they were originally supplied with an external 15dB power attenuator (as shown in the photo), which dropped the output power from a 10W transceiver to a suitable level.
The same company produced some new models in the mid-1990s, which are reputed to have a much better performance than the originals.
Dave G4FRE has set up an archive of MM circuit information
and Steve M0BPQ has provided a scan of the schematic.
RN Electronics also produced transverters for the 4m band although these too are no longer available new. They were very similar in size and concept to the Microwave Modules devices and were also produced with IF's suitable for both 144MHz and HF transceivers. They were generally better than the Microwave Modules types in that some models gave 25W output and the receive sections were a little quieter. They are very rare on the second hand market however as very few were manufactured but again RadCom members' ads may turn one up.
Mutek produced a nice dual-band transverter for the 50 and 70MHz bands with 25W output. They produced versions for 28MHz and 144MHz IFs but were quite expensive when new, but had a reputation for superb RF performance.