Voltage & current after amp.

[Crusher]

I was wondering, if I was using an amplifier on 12 volts DC and seeing say 400 Watts RMS output. How would I figure out what kinda voltage and current I would see thru coax and Antenna? Would it be as simple as Ohm's law or with the voltage being AC (and having to figure phase angles and all the fun stuff) how would I go about figuring this out. Any comments or even suggestions on books to read up on would be great. Thanks in advance.

Crusher

 

[Captain Kilowatt]

You need to know the resistance component of the antenna impedance and then use basic Ohm's law to find what you are looking for. Use chart below to find the formula to use.The component you are looking for is the letter in the centre and there are three formulas to find it depending on what other values you know. For example V=I (current) times R (resistance) or P (power) = I squared times R etc.

400 watts would be 2.828 amps into 50 ohms and have 141.421 volts.You must be able to determine the R of the antenna.

 

[Crusher]

OK, thanks. I have a MFJ 259B that should get me in the ballpark of what the actually resistance of load is. Back in the day when I went to school for electronics, they never discussed this kinda stuff in the textbooks. As I am sure you know or have an idea of, they just teach you the basics and let you go from there. They don't teach you alot about RF, actually Very little except RF is AC just at a higher frequency.

 

[Captain Kilowatt]

OK, thanks. I have a MFJ 259B that should get me in the ballpark of what the actually resistance of load is. Back in the day when I went to school for electronics, they never discussed this kinda stuff in the textbooks. As I am sure you know or have an idea of, they just teach you the basics and let you go from there. They don't teach you alot about RF, actually Very little except RF is AC just at a higher frequency.

I did this kind of stuff for 22 years as a broadcast engineer. I had to be able to determine the power delivered to each tower in a multi-tower array using RF current meters and an operating impedance bridge that would show what the actuall impedance was of a tower with full RF power applied to it.Sometimes a meter calibration would be bad and an oscilloscope was needed to measure the RF voltage and then it could be calculated what the current meter should be reading. Fun stuff. :roll:

 

[Crusher]

With that same formula, it should give a idea idea then of the proper WV needed for a capacitor that is used in the output tuning stage. Thanks QRN.

Crusher