In order to encourage activity on 24GHz the UK Microwave Group (UKuG) have undertaken the production and sourcing of some of the key, and harder to make, components. This service will only be available for a limited period. Some inexpensive waveguide switches have been available from the Wellington VHF group but these are in WG22 whereas most equipment for this band uses WG20. The switches will work fine with suitable adapters but many folk will find these are hard to make. So this is the first item we have sourced as (ITEM A). It is supplied ready tuned for a WG20 – WG22 interface and will need only slight re tweaking for best performance in the final configuration.
On the left is a RelCom w/g switch with adapters, on the right ITEM A the adapter.
Perhaps the most challenging component usually required in a 24GHz system is a filter to suppress the image response from the mixer. The G3BNL design is a popular choice but again, its construction can put many folk off. So the UkuG has arranged manufacture of these and in two configurations to suit different approaches to the construction of transverters.
On the left, ITEM C is the filter with SMA sockets at both ends. It is shown without the matching screws fitted. On the Right is ITEM D with SMA at one end and WG20 Flange at the other.
Both are supplied pre tuned to aproximately 24048MHz so they will only need slight tweaking for maximum throughput.
This block diagram shows a system using the DB6NT Transverter along with additional pre amplifier and power amplifier
The above pictures show a system largely configured from DB6NT components.
This block diagram shows a system configured from modules taken from the Alcatel white box. These have been available on the surplus market at a reasonable price. Below are some images of such a system.
These pictures show a system configured from Alcatel modules. Note that the common port of the waveguide switch is coupled to the antenna via a WG22 H plane bend. This particular dish has a WG22 port and so only two WG20 - WG22 adapters are needed.
The above diagram shows a control system using the RelCom switch in conjunction with modules taken from a surplus Alcatel “white box”. In this case the interface is designed to work with an FT817 with an IF of 432MHz. Note that the RX module will require some additional modification to enable this of which more later. Features include an interlock between the –5V supply and the +6V supply to avoid accidental damage to the PA module. The RelCom switch is effectively of the latching variety but provision of pulses to operate it are not necessary as this is taken care of internally. There are two stages of attenuation between the transverter and the FT817. 6dB is common to the transmit and receive circuits and provides some protection against transmitting into the RX module. There is more than enough gain in the RX to overcome this. On TX additional attenuation is provided to reduce the 432MHz to a sensible level. The FT817 should be operated at the lowest power level to avoid overload.The FT817 interface also features polarity protection, buffering of the PTT and a three position switch for PTT OFF, TX and TX + Key Down. (By the way an old mouse plug fits the FT817 DATA socket with the PTT connection.)
Some of the Alcatel modules require some simple modification. Both the transmit and receive modules employ image cancelling mixers some of which are designed to mix high i.e. Local oscillator above the wanted signal and on the RX module these are marked SUP for Supradine on the PCB. To mix low it is necessary to alter the input coupler as shown below - left.
The same goes for the transmit modules so these may also require modification as shown unmodified - centre and modified right above.
In addition, if a 432MHz IF is to be used, it is necessary to change the filter in the RX as its IF was originally ~1.9GHz. The Alcatel TX module incorporates a filter between the mixer / coupler and the IF amplifier. This filter produces quite a notch at 432MHz so it is necessary to modify it for this IF. I simply removed the capacitor as shown left. Others may wish to take a more subtle approach.
Hints and Kinks
Waveguide fixings:- The norm for WG20 / WR42 and WG22 / WR28 is 4-40 UNC. This is very close to M3 which is more readily available in Europe. Also the RelCom waveguide switch has holes tapped for M3. Note that the screws for this should not penetrate more that about 4.5mm into the body of the switch. Socket cap screws are the best type to use and with a ball ended hex key it is possible to work in a confined space, and space will be confined in order to minimise waveguide runs.
Useful Links
There is much useful information on the Alcatel White boxes at this French site.
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