Tuesday, August 18, 2009

FT-897D Transverter interface

The FT-897 has a transverter offset feature, its nice to display the frequency you’re actually working on. But be aware it only adjusts to within 1Khz so you may still have to tweak your transverter to get it exactly on frequency!

My OZ2M transverter for 4M (70Mhz) has a 28Mhz IF so I thought I would have a look at using the FT-897d to drive it.

Initially I wanted to create an internal breakout point between the main board and PA module. These boards are connected via two Taiko Denki leads and work at all frequencies HF to UHF. I had some spare Taiko Denki connectors -

and found a nice coax relay by Omoron which would switch the IF with low loss.

The omron relay was of particular interest. Its capable of 10 Watts and using micro strip line technology can offer low loss up to UHF.

My initial plan was to internally switch out both the RX and TX lines for driving the transverter.

The first issue I encountered was the power level for the PA drive. Unlike most rigs it’s not a few milliwatts but 10 watts! Far to high a level and requiring significant attenuation.

The other issue was space and my willingness to modify my FT-897d.

In the end I decided against the internal board and built a simple external interface.

The advantage of this is not having to modify or change the FT-897d in any way (I bought it new from Martin Lynch & Sons and I decided to maintain the warranty).

On transmit the unit attenuates the 5 watts output by 37db to give the required 1mw drive and on receive I determined a simple 24db attenuator by trial and error.

Building an internal unit would have required the construction of double sided PCB with the incorporated micro strip line to work effectively up to UHF. An external unit only has to switch 28Mhz and in reality a simple change over relay would have been sufficient as I could have compensated for any loss with the attenuators.

But I had the Omoron relays so I thought I would try one out on strip board.
The switch is built into a small Eddistone box with all the RF connectors. Relay switching and power is provided directly from the CAT connector on the rear of the FT-897d. This minimises the number of connections.

As a fail safe the relay switches the antenna line to the transmit attenuator when released or no power is present.
This ensures that should the CAT connector become disconnected RF power won't be applied to the transverter output!

I continued the PTT line to a separate DIN output and this is used for switching the transverter.

Here is the schematic -

Of course a picture paints a thousand words so here is the finished unit.

I started by using an external RX attenuator. Once I knew the correct values I incorporated it onto the switching board. For convenience I built the power attenuator on a separate board. Fine adjustment of the transmit drive level is via the 1K pot.

Be very careful of the 12V supply. This is fed from the FT-897d via a .5 amp surface mount fuse. I did consider an external fuse but in my opinion the greatest risk is a short in the connector. Also SM fuses aren't hard to replace if you have the equipment.

I used the transceiver for transverting to 70Mhz and directly on UHF using the separate coax connector. I leave the switching box connected when its in the shack and for portable operation I simply unplug, switch off the transverter operation and turn up the power.

This is a very simple project and maybe useful to anyone considering transverting from the FT-897d

73 Rex – G8UBJ