Re: Off Topic -- Warning re surplus Nokia Tetra base station Power amps on 70cms


Russ Stewart <g4pbp@...>
 

Christopher Bartram wrote:
Having designed TETRA PAs, I assume that these surplus amplifiers are designed to deliver 50 - 60W pep, and will produce >200W simply because they were run 'backed-off' in order to get good large signal linearity with a relatively poorly-performing lineariser. I should stress that I don't know these amplifiers, but the problems which Mark and Rob describe aren't black magic!

The issues which will arise from running the amplifiers at higher powers will (probably) be due to the choice of substrate for the microstripline output matching elements. If, as I suspect, the amplifiers are built on 0.62 or 0.31mm FR4 to save money, the major issue will be resistive heating due to dielectric losses, not dielectric breakdown. While you can't do a lot about the latter, the heating due to resistive losses in the substrate can be dealt with in the same way as you'd deal with losses in a resistor: heatsinking!
The power dissipated in a microstripline can be calculated, but as a rule of thumb, those in a 200W PA will probably only dissipate a few watts.
The characteristic impedance of a microstripline is relatively uneffected by heatsinking structures placed on top of the conductor. A U-shaped heatsink of, say, 0.25mm copper sheet soldered to the conductor will radiate the heat dissipated.

One tool which anyone designing, building or modifying a solid state power amplifier should possess is an infra-red thermometer. These are cheap and being non-contact, can be used to look at a the temperatures of PA components while it is running. Thermal design is often neglected in amateur designs.

Vy 73

Chris
GW4DGU


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Hi Chris and I guess you are right on all counts.

I am actually using one these amps but envoked radical surgery to stop it burning up. It currently gives 280W o/p cold and drops to 240W when hot. It easily does 250W PEP !

The surgery involved firstly, cutting out the PA section from the main board and remounting it on the original heatsink. I left the bit of board which was the o/p line of the driver stage, (for matching purposes). I connected a piece of UT141-50 to this line and put a 10pf ceramic trimmer across the the coax / stripline transition to the gnd plane; this gave a 1:1 match when the o/p peaked (1st time ever :-)

At the output end I cut off the SWR monitor line part and stripped off the 50ohm track feeding it, right back to the Sageline combiner, just leaving a small pad which the Sageline is connected to. The used a piece of UT141-50 to bring out the RF. Without the line removal I could only get 230W cold.

It was interesting to note that when I stripped off the stripline, the bit of board under where the o/p N-type spigot was soldered, was already black + burned even under the amps normal operating condition !

The o/p Sageline combiner is at or just over it's power limit at 280W and seems to be the primary heat source; you can't hold your finger on it after 5mins of carrier !!! So I blow the combiner and am currently testing use of a 20mm fin (copper strip) soldered to it's length in the air flow. I will know how well this works shortly .... need an IR camera for Christmas for all this PA work Hi. It would be easy to replace the Sageline with an external Wilkinson in UT141-75 as an alternative.

Finally and most importantly, as Mark found in his original mods, you must 'beef-up' the DC feed to each device and not rely on the via's and pc tracks than feed the DC pwr; this will result in a 'burn-up' and general mess.

If anyone needs it, I have photographs of the finished PA, these may already be on Mark's website.

All I need now is someone to work on 70cms :-(


regards de Russ G4PBP

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