Difference between revisions of "Power amplifiers"

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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 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!
+
  The average FM or SSB amplifier MUST be backed off by over 50% or 6 - 10dB by reducing the drive from the transmitter.  This means a 50 watt amplifier will only deliver a maximum 15 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!
+
[[File:146 333ks 30watts.JPG|400px]] <<<< 30 watts (ugh) ----------------        7 watts >>>>> [[File:146 333ks +10dB 7watts.JPG|400px]] 
 +
 
 +
For example, these pictures show a RA60H1317M Mitsubshi 60 watt brick on 146 MHz driven by a Portsdown transmitter. 
 +
 
 +
The first picture is driven direct and delivering 30 watts but occupies the whole of the band - the second has a 10 dB attenuator between transmitter and amplifier and delivers 7 watts (almost 10dB down on the rated amplifier output) and needs 5 amps at 12v to achieve this!
  
 
===Transmitter output levels===
 
===Transmitter output levels===
Line 15: Line 19:
 
===Practical designs===
 
===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.
+
Dave G8ADM has written an article for CQ-TV which describes how to use amplifiers from DG0VE when driven by the DTX1 or Portsdown.  (The full article is available CQ-TV 257)
 +
 
 +
Note this article was written for the DTX1 and when used with a '''Portsdown''' you DO NOT need the additional driver stage on 146 or 437MHz and in addition  a 10 dB attenuator MUST be put between the Portsdown and final amplifier.
  
 
[[File:ADM PA.jpg|400px]]
 
[[File:ADM PA.jpg|400px]]
  
 +
===Multiband PGA103 amplifier===
  
===146 MHz amplifiers===
+
The PGA103 is a very versatile device that can be used as both a transmit driver and a low noise reciever pre-amp.
 
 
* Sam Jewell G4DDK, 8W amplifier kits, pcb, manual etc.
 
http://www.g4ddk.com/144MHz8WPA.pdf
 
 
 
* Anglia Live, suppliers of Mitsubishi power modules
 
http://www.anglia-live.com/products/rf-products/rf-discretes-modules/rf-mosfets/list/perpage/21/page/1
 
  
* 60W power amplifier pcb (available from G4DDK above)
+
Tests with a Portsdown transmitter showed:
http://www.ifwtech.co.uk/g3sek/vhfdx/gm3sek-pa-module-board
 
  
* Rob M0DTS published an easy to construct design using the RD series of FETs in CQ-TV 247 and 249 - a good way to get a clean 5 watts on 146 MHz.  (The full article is available in high resolution in the magazine)
+
* 23cms - no attenuator needed = +15dBm (40 milliwatts) (without amp = 4 dBm)
  
[[File:rd70.JPG|400px]]
+
The output from the '''Portsdown''' on 437 MHz and below is too high and MUST NOT be used to drive a PGA103 as it becomes very spurious and may damage the device.
  
===437 MHz===
+
NOTE - you MUST use a filter after the PGA103 in both applications!  Sam G4DDK described the amplifier design in CQ-TV 242 and sells kits of parts on his [http://www.g4ddk.com/Products.html web page].
  
 +
[[File:pga103.JPG|400px]]
  
 +
Details of amplifiers for the DATV bands can be found on the following pages.
  
===23cms===
+
[[146 MHz amplifiers]]
  
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)
+
[[437 MHz]]
  
[[File:23cms pa G3GJA.JPG|400px]]
+
[[23cms amplifiers]]
  
===13cms===
+
[[13cms]]

Latest revision as of 17:28, 14 December 2017

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 be backed off by over 50% or 6 - 10dB by reducing the drive from the transmitter.  This means a 50 watt amplifier will only deliver a maximum 15 watts of clean DATV power!

146 333ks 30watts.JPG <<<< 30 watts (ugh) ---------------- 7 watts >>>>> 146 333ks +10dB 7watts.JPG

For example, these pictures show a RA60H1317M Mitsubshi 60 watt brick on 146 MHz driven by a Portsdown transmitter.

The first picture is driven direct and delivering 30 watts but occupies the whole of the band - the second has a 10 dB attenuator between transmitter and amplifier and delivers 7 watts (almost 10dB down on the rated amplifier output) and needs 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. (The full article is available CQ-TV 257)

Note this article was written for the DTX1 and when used with a Portsdown you DO NOT need the additional driver stage on 146 or 437MHz and in addition  a 10 dB attenuator MUST be put between the Portsdown and final amplifier.  

ADM PA.jpg

Multiband PGA103 amplifier

The PGA103 is a very versatile device that can be used as both a transmit driver and a low noise reciever pre-amp.

Tests with a Portsdown transmitter showed:

  • 23cms - no attenuator needed = +15dBm (40 milliwatts) (without amp = 4 dBm)
The output from the Portsdown on 437 MHz and below is too high and MUST NOT be used to drive a PGA103 as it becomes very spurious and may damage the device.

NOTE - you MUST use a filter after the PGA103 in both applications! Sam G4DDK described the amplifier design in CQ-TV 242 and sells kits of parts on his web page.

Pga103.JPG

Details of amplifiers for the DATV bands can be found on the following pages.

146 MHz amplifiers

437 MHz

23cms amplifiers

13cms