70CM 500W Amplifier

70CM 500 W Power amplifer

As I built and installed my new 70 cm EME antenna system, the need for more power also came as an added bonus :) The antenna system uses 4 times 23 element yagi's from Antennas&Amplifiers, and 500W seems to be an reasonable number to start out with.

In order to "get the most" out of the amplifier and at the same time, save some space in the lab/shack, I decided from the beginning that the PA should be mounted on the small tower that holds the 70cm EME antenna system.

This triggered a number of new projects that needed to be done in order to operate the PA remotely. 


So far, the system consists of the following sub modules, some designed by myself, and some bought:


  1. Cooling system with two fans. I designed a "generic" system that can be used in other PA systems.
  2. Overcurrent detection and on/off switch. Described and documented here also.
  3. Dual RF Head (logarithmic amplifier) that samples the fwd/ref signals from a directional coupler.
  4. Remote PA Monitor (REPAM), allows control and data collection/security of a remote PA.
  5. W6PQL 500W module
  6. W6PQL LPF (low pass filter)
  7. Power supply from RECOM RACM1200-48SAV/ENC: Datasheet for power supply
  8. Box for tower mounting of the PA from Fibox, link to product at Fibox. You need a mounting plate also.
  9. Various stuff for mounting, a relay for remote power/on off, a 12VDC PSU (for bias etc)



A few videos of the REPAM module and the testing of the PA.

Block schematic of the interconnections between the various components.

I started with the design and construction of the cooling system. I did some research and calculation on this bit as I wanted a design that was easy to replicate for other PA systems that I will need. I documented the process on my webpage also.

I then acquired (with the help of a good Ham friend) a finished W6PQL 70cm 500W module, a finished low pass filter and a kit of the PA module (as a spare) all directly from Jim W6PQL

The picture below shows both the PA module and the LPF in the box/heatsink assembly.
The box used is a "Bud industries" type number AC-424.

As I wanted to use my Dual RF Head to measure the forward and reflected power, I chose to modify the LPF from W6PQL. On the LPF, there is a dual directional coupler. This coupler is able to deliver a DC voltage for the forward and the reflected ports. As the Dual RF Head measures RF signals, I had to bypass some of the circuit in the directional coupler. Basically I just removed the capacitor that passes the sampled RF to the diode/rectifier. I then took a coax/SMA and connected that on the attenuator on the PCB and sampled the RF from that.

I then measured the coupling on both the forward and the reflected ports, found that the coupling was around -64 dB for the forward port and around -48 dB for the reflected port.

I also measured the low pass filter using my VNA, just to check that there were no surprises on the insertion loss etc. It turned out very positive, the filter behaves very well, and well within specs.
The insertion loss was measured to be -0.1 dB, attenuation on 2nd harmonic and upwards was -48 dB.

Next up was an seemingly endless amount of holes that needed to be drilled! All signals coming out and going in from/to the PA are feed thru feedthrough capacitors (4mm, 1nF).

The DC power supply enters thru a XT90 connector.

As the LDMOS is a sensitive device, we need some protection for it. There are a few things that can kill it, overdrive on its input, too high current thru the drain, high temperatures, bad SWR and too high supply voltage (and probably a lot of other situations). In order to limit the maximum current drawn (adjustable), being able to switch the power on/off to the module and monitor the current drawn, I designed the "PA Overcurrent Protection" module.


This module acts as a on/off switch, it has two inputs, on and off, if you bring the on input to ground, the switch will close enabling the 50+ VDC to reach the LDMOS. If you bring the off input to ground, the switch will open and disable the 50+ VDC to the LDMOS. 

You can adjust a 10 turn trimmer on the board, this allows you to set the maximum current the switch will allow before it switches itself off.

The protection board has an output that delivers a voltage that is proportional to the current the LDMOS draws. The Iout output delivers 50 mV for each Ampere the LDMOS draws.
The on and off inputs are driven by two relay outputs from the REPAM device, the Iout is connected to one of the analog inputs on the REPAM device. This allows the REPAM device to monitor the current drawn by the LDMOS.

The REPAM module also monitors the voltage from the power supply and the voltage that is fed to the LDMOS. As the analog inputs on the REPAM module only allows 0 to 5V and the power supply delivers 50+ VDC  I have mounted two voltage dividers made of a couple of resistors. This allows the voltage to be at a level the REPAM analog input can handle. Both the voltage from the power supply and the voltage reaching the LDMOS are measured this way. It is then easy using the "AI scaling" in the REPAM module to convert these readings to sensible values (the PSU voltages will be shown as 52000 mV in the REPAM module and in the PAMonitor application. More on this on the REPAM product page.

The power supply for the PA is a RECAM switching power supply. It is default set at 48 VDC but can be adjusted to a higher voltage using two small pushbuttons on the power supply. I set the PSU to 52 VDC (the LDMOS ART700 I use are rated for 55VDC).


Link to the power supply at Mouser.

As the REPAM Module I use supports 1Wire sensors, I made a sensor for the LDMOS temperature sensing. I used a "cable lug" and inserted a DS18B20 sensor in it, glued it with heat transferring epoxy glue. I could then attach the lug to one of the screws that keeps the copper heat spreader attached to the large heatsink. This means that I measure the LDMOS temperature pretty close to the LDMOS housing. The other temperature sensor is mounted at the exhaust port of the box, this measures the temperature of the air that exists the PA compartment, this air is delivered by a small fan at the inlet.

As mentioned, the PA is tower mounted. I have one single coax cable going from my transceiver (ICOM IC-9700) to the PA. At the input of the PA, I have a coax relay that will either connect the transceiver to the input of the PA module, or pass it thru to a connector in the other end of the PA enclosure. This means that I have a "TX" and a "RX" coax coming out of the PA. The preamp that is mounted at the antenna feedpoint, has similarly a "TX" and a "RX" port. The RX on the PA and the RX on the preamp are connected, and the TX on the PA and the TX on the preamp are connected together. At the other end of the preamp is a built-in relay that either connects the antenna to the preamp input or to the "TX" port on the preamp.
Later in the process, I changed some of the RG-402 semi rigid cable (to RG-401) as it became pretty hot during TX!

Next up was all the connections between the components. I used A LOT of ferrites around the cables, this was to avoid getting (too much) RF out of the box where it potentially could interfere with the controlling electronics, power supply etc. I could probably have saved big time on the number of ferrites used, but I did not wan to take any chances (and the ferrites were cheap).


The Bias to the PA module is switched by my sequencer in the shack. Together with the bias switching, the PA also receives a "RX relay" signal. This signal makes the relay at the input of the PA switch to RX when energized (this follows the power to the LNA at the antennas).

The bias signal from the sequencer is used to switch a relay inside the PA box, I did not want to have the actual 12V bias going the 25 meters from the shack to the PA module, so I instead uses it to switch a "local" relay in the box, that then feeds the 12VDC (from the small 12V PSU) to the bias input on the PA module.


Because of the PA being tower mounted (outside in the elements), I built everything so it fits within a water tight enclosure from the company "Fibox". Everything is mounted on a steel backplate that just screws into place in the box. The box also connects the rotor cables etc. to a 16 wire multi cable that goes into the shack.

The PA is being tower mounted very close to the antennas (EME array). This serves two purposes:

  1. Reduces losses. I would need 25 meter coax if PA was in shack, and even with heavy coax, there would still be loss.
  2. Reduces clutter in the shack. As my shack is also my lab, space is precious. Mounting the PA at the tower makes more room.


I chose to use a box from the company Fibox. Fibox makes a lot of boxes for outdoor use, I have been using these for more than 30 years for various (antenna) projects. The box used for the PA is their "ARCA 504021", this is a 50 x 40 x 21 cm large box.


When mounting the PA assembly inside this box, there is a need for ventilation to the outside. The PA assembly has 3 fans, two large and one small. I made some square holes in the bottom of the box (where cables etc exits) and mounted some aluminum/stainless steel filters. These are located right where the two large fans are mounted.


To help me visualize the box and all the major components, I used Autodesk Inventor to lay out all components, check for collisions etc. It is infinite easier to move components around on the screen instead of cutting holes the wrong places and buying new boxes etc!



As the PA assembly needs air in and out from the box, I needed some kind of exhaust ports. I searched for some ready made ones, but did not manage to find "the right one". I then simply designed some myself. The size was chosen so I could get some standard "foam filters", I modelled the foam filters in Inventor and derived from this shape. This gave me some "filter assemblies". A cover was the designed for these as I need to keep rain out of the box, while at the same time a lot of air needs to be able to escape. As I already have used the bottom of the box for air inlet and all the cables exiting, the exhaust ports needed to be located on the two sides of the box.


The exhaust ports are all 3D printed, I used "ASA" filament as this is able to withstand UV and general outdoor conditions (I am using ASA printed parts, sleeves etc, for my 15 meter HF tower also, has been holding for years)

A couple of pictures of the box for the PA. As it also functions as a junction box for the signals to the rotor (see description on the "home made" rotor here. The rotorcontroller I designed for it is shown here.


I have already a 16 wire multicable running from the shack to the tower where the PA will be installed, this cable carries encoder signals to to the Az and El rotors as well as control signals to the PA and signals from the sequencer in the shack.

Below are a list of signals and wire numbers, this is mostly for my own documentation.

So, how does it work?

Luckily, everything seems to work perfectly! The PA module is supplied with 52 VDC, the LDMOS is actually rated for 55 VDC, but I choose to keep it at 52 VDC. The current drawn at full output is around 16.5 Ampere, at this current the PA delivers very close to 500W. This gives a very nice efficiency of around 58% (power input is 858 Watt, 500 Watt  output so around 360 Watt needs to be dissipated as heat in the cooling system.


Using the REPAM Module and the "PAMonitor" PC application I logged some data from the PA.

More info to follow..