Power amplifiers
The choice of power amplifiers for DATV is very critical and due to a characteristic called spectral re-growth it is vital that amplifiers are not overdriven when used on DATV.
The average FM or SSB amplifier MUST backed off by over 50% or 6 - 10dB by reducing the drive from the transmitter. This means a 50 watt amplifier will only deliver 20 watts of clean DATV power!
For example, on 146 MHz we are aiming to achieve shoulders which are 60 dB down on the main signal and the RA60H1317M Mitsubshi 60 watt brick can only deliver a clean 7 watts out and draws 5 amps at 12v to achieve this!
Transmitter output levels
It is very important to know the power output level of your DATV transmitter so you can decide how much amplification you need.
- DTX1 is very low power at around - 5dBm and requires an additional driver stage compared to the Portsdown or DATV express
- Portsdown output is frequency dependent and varies from +4dB (4 milliwatts) on 23cms to +12 dBm on 71 MHz. Output level at 146 and 437 is approximately 10 dBm or 10 milliwatts.
- DATVexpress is approximately 10 milliwatts on all bands.
Practical designs
Dave G8ADM has written an article for CQ-TV which describes how to use amplifiers from DG0VE when driven by the DTX1 or Portsdown - note this written for the DTX1 and you may not need the additional driver stage for the Portsdown. (The full article will be in available CQ-TV 257)
437 MHz
23cms
In CQ-TV 242 John, G4BAO, published a design for a 2.5 watt LDMOS driver amplifier. This is a very useful driver circuit which John sells in kit form on his website which when driven by a Portsdown / DATVexpress transmitter will deliver around .5 watts (to be confirmed.) of clean DATV signals.
In CQ-TV 249, Clive G3GJA published a review / designs notes on 2 watt amplifiers available from ebay. (The full article is available in high resolution in the magazine)
