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If you want to use your ATLAS 2210 at its  top level you will need sooner or later a wattmeter to finely adjust its various circuits .When you can buy a vintage ATLAS210 from 150 US $ up to 250 $ depending on its state there is no reason to  put several time more in buying a fine  but very expansive equipment like a BIRD wattmeter .You can  easily build your own appartus as you will see .
The first thing is to make a dummy load using non inductive resistors .The maximum output of ATLAS being in the range of 80 watts under a 50 ohm impedance  you will have to solder in parallel the needed number of resistors so to have 50 ohms .In my own design i decided to use 17 resistors each having on average a value of 1000 ohm .The total resistance is 60 ohms not far from the theoritical value of 50 ohms . Each resistor is able to dissipate around 3 watts of heat and therefore the dummy load is able to run at the 50 watts output level  on a continuous manner and at 100 watts for a few minutes .By just putting your fingers on the resistor you can know when it is time to stop tuning your rig so as to not overheat the resistors .
ATLAS210 WATTMETER wattmeter probe
To visualize the HF current flowing into the dummy load the simplest way is to put in series with one of the resistor a small light bulb .You choose a model being able to run at a few volts with a 0.1 ampere   current .Just by looking at the light bulb you are able to have a rough idea of the output power .

If you want to have better precision  the best way is to measure the HF voltage found across the dummy load .The schematic is shown .In parallel with the resistor you put a voltage divider so as to divide by 10 the voltage (combination  of a 18 k + 2 k resistor ) .This voltage is rectified with a schottky diode (BAT 41) , the continuous voltage obtained being the peak voltage of the HF current .This voltage is measured using any voltmeter schematic
TO know what is the output power you applied the well known formula  : P=U*U/R  (as you measure not U but U max the value that you measure has to be divided by 1.414 to get the the U used in the formula ) .To prevent the capacitve coupling between the voltage divider and the dummy load it is a good idea to solder a metal plate in between .
You can also put the voltage divider and the schottky diode  directly on a BNC socket  and insert the device using a T coupling in the line going from your ATLAS210 to the dummy load  .The set up is quite simple as shown hereunder .The voltmeter used to measure the continuous voltage should have an internal resistance high enough .Compare to professional wattmeter the result is within the 10% range atlas210 wattmeter setup

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