Mast-Mounted EXTRA-2 144 MHz Contest Preamplifier
Published in the 3/2012 of DUBUS Magazine
The EXTRA-2 144MHz preamp [1, 2] has lived up to expectations in practice, though there is need for the possible placing of the preamp in the feed point of the antenna, while containing the high frequency relays, as well. It would be an elegant solution if two professional coaxial relays were used for switching, but thus the preamp would be unaffordable. Now Im presenting a solution that provides good isolation with cheap relays, and can be used with hundreds of Watts, as well.
144-146 MHz (-1 dB)
- The Mast-Mounted Preamp includes a HF VOX, but I do not recommend to use it. (It is not safe).
- The HF VOX works from even some mWs.
- The usage of the sequencer is highly recommended while using the PA.
- Maximal power is 750 W (Legal Limit)
- I have no version of higher power, and I do not plan to produce it.
- The feed of the DC Voltage can only be made through DIN connector.
- The PTT also works through the DIN connector.
- The 12V must be always switched on (in the RX and TX period).
- The PTT must be switched to ground in TX period. The relays are switched this way.
- Without 12V DC the preamp works in feed-trough mode.
I chose an RX114012C Schrack relay from the cheap ones. Its extremely short pins ensure excellent SWR at 144 MHz and its contacts of 12 A can switch even 1 kW power at 50 Ω. The only problem is that the big surfaces of the contacts result in very little isolation. This problem can be solved by connecting the high current relay with a microwave relay in series, at both input and output. In TX period the applied high frequency relays switch both the input and output of the FET to GND, thus protecting the FET from the power of some Watts passing through the high current card relays. In the case of 1 KW TX power this saturated power could reach even 7-8 W without microwave relays, which could cause the breakdown of the FET.
The Part list (PDF)
The circuit diagram of the preamp can be seen in Figure 1 (PDF). The circuit diagram of the surroundings of the FET is the same as the one of the original EXTRA-2 REV 6 version . In favor of higher protection BAS70-04 processing diodes have been changed to microwave PIN diodes at the input and output, as well. This solution together with the relay composition presented above ensures the best protection for the FET. The DC power supply line is equipped with reverse polarity and overload diodes, and this latter can protect the preamp from close but not direct thunderbolts.
The preamp has been built on a 1,5 mm thick FR-4 type PCB. The connection between the two power relays has not been designed on the PCB, because it can easily burn through if applying Legal Limit power. To avoid this, semi-rigid cable of .141 has been used for the connection between the two relays.
The upgraded and modified preamp has been built in a waterproof and sealed cast aluminum box (Hammond 1550Z108). The connectors are also sealed, so the complete preamp can be mounted on a mast. The preamp includes a VOX circuit, but using a PTT control is more preferable. The DC power and the PTT line can be connected to the preamp through a 3-pin DIN connector. At this version DC cannot be fed through coaxial cable!
The layout of the Mast-Mounted Preamplifier PCB can be seen in Figure 2. Figure 3 shows a photo of the ready PCB. The semi-rigid connection between the two power relays can be observed in Figure 4. Photos of the boxed preamplifier can be found in Figure 5, Figure 6 and Figure 7. The parameters of the preamp are the same as the ones presented in . Figure 8 shows the through SWR of the preamp in TX mode, closed with a good quality dummy load.
The preamp can be used directly with all 50/100 W VHF radios that switch GND to PTT line in TX period. If applying a medium power PA, a sequencer circuit is essential to be used to avoid breakdowns.
1. Dipl. Ing. Gyula Nagy, HA8ET: EXTRA-2 144 MHz Contest Preamplifier. DUBUS 4/2010 p.78
2. Gyula Nagy, HA8ET: EXTRA-2 144 MHz Contest Preamplifier (REV6). Dubus 4/2011 p. 70
The status: Out of stock, no longer available