The problem is I am confused as to how to convert the voltage reading I am getting out of that diode/capacitor circuit into a real wattage reading using math calculations.
The site says that if I read 22.3 volts DC on my multimeter probing the output of that simple rectification circuit using a 50ohm RL that means the transmitter is putting out 5 watts correct? Well if so then I guess that sounds about right.
Problem is how to I calculate what voltage reading I am getting into wattage? I guess I suck at math or I didn't read far enough into how to calculate this. So if 22.3 volts DC on my meter = 5 watts, then what does ??.?? volts on my meter = how many watts? How do I calculate this without pulling out the hair on my head?
Right about now I wish I had a Bird wattmeter lol.
Actually that helps me out quite a bit! So those are voltages that I would measure using that diode/cap circuit right? As in peak voltage? So if I measured 14.1 volts on my meter (without modulation) using that AC to DC peak rectifying circuit that would mean my transmitter is putting out 4 watts?
I am confused as to if I should be using peak volts or RMS volts to read that chart.
To calculate wattage you take the DC voltage measurement you get from the diode capacitor circuit and convert it to RMS.
RMS = Volts Peak / 1.414
Then take your RMS number squared and then divide that by 50 (ohms). Your calculated value should now show watts if I have this all figured out right.
So I take my DC volt meter with the dummy load connected inline with the capacitor diode circuit. Lets say the meter says 28 volts (peak power).. 28 / 1.414 = 20 (RMS power) 20 ^2 = 400 400 / 50 = 8 Watts carrier power, or 32 watts PEP output
Yippy so now I know my output power and didn't need to waste cash on an expensive wattmeter that can go down to mediumwave frequencies
To calculate wattage you take the DC voltage measurement you get from the diode capacitor circuit and convert it to RMS.
RMS = Volts Peak / 1.414
Then take your RMS number squared and then divide that by 50 (ohms). Your calculated value should now show watts if I have this all figured out right.
So I take my DC volt meter with the dummy load connected inline with the capacitor diode circuit. Lets say the meter says 28 volts (peak power).. 28 / 1.414 = 20 (RMS power) 20 ^2 = 400 400 / 50 = 8 Watts carrier power, or 32 watts PEP output
Yippy so now I know my output power and didn't need to waste cash on an expensive wattmeter that can go down to mediumwave frequencies
Absolutely correct. AM, no matter what band of operation, is always measured for RMS TPO (Total Power Output). PEP output is measured using a constant test tone at 100% unprocessed modulation, which will always be 4 times the power output of RMS.
8=32 16=64 50=200
and so on.
In FM its always measured in RMS TPO as the carrier is not changing in amplitude, but changing frequency deviation from carrier center frequency and notated in percent which indicates the amount of deviation from the center carrier frequency.
This is why it is so important for an AM transmitter to not have any FM artifacts and FM to not have any AM artifacts. In both cases if there are any unwanted artifacts, it will affect the measurements as well as affect the final amplifier performance.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
Best to use a proper power meter but using a 'scope and a 50 Ohm dummy load, MULTIPLY THE PEAK TO PEAK BY 0.35, SQUARE THE RESULT AND DIVIDE BY 50. Simple!
Post by HighMountainRadio on Jan 27, 2018 7:26:03 GMT -6
Very interesting and useful ! I use a Bird Model 43 with appropriate slug for frequency and power level but I know they aren't cheap if used only casually. I use mine quite a bit for testing various antenna performance and VSWR measurements and since I was lucky enough to get a brand new model 43 for free, (found on side of the road in a trash pile waiting to be picked up !!!) ... I was flabbergasted to find this fine instrument, it it's original leather case in a trash pile !!! Obviously the person throwing it out had absolutely no idea just how valuable it actually was and is !! It even came with a few slugs for use in the cellular phone band !! Obviously it was used by a technician working in the 800 ~ 900 MHz band..
That is a very handy method you are utilizing as well, I found it quite interesting ! I am going to set up my oscilloscope later to measure RF with and inductive probe and see how that works out.. I'll post the results and setup at a later time to this forum to assist others.
73... Spooky...
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
Post by ogrevorbis on Jan 29, 2018 19:12:34 GMT -6
Hey, Spooky. I'd like to encourage you to check out radionecks.co.uk. There are a lot of people on there and you will get quick communication. This site is an echo chamber and no one really posts here anymore. I am unsure why, but you'll find a lot of enthusiasts on radionecks.
Maybe if people started posting more it wouldn't be such an echo chamber. Also you have to realize pirate radio isn't as popular in the US compared to the UK.
Anyways using a multimeter has really saved my butt a few times now. Almost feel like this should be wider known because of the simplicity when a watt meter fails which is exactly what happened to me a few weeks back. Was using my cheapo watt/swr meter only to find the internal adjustment potentiometer got dirty making the meter read incorrectly. Had to spray it with some deoxit but of course then my meter was uncalibrated. Used the volt meter to measure wattage as this thread details how to do to calibrate my watt meter against, and now it's back to measuring precisely
BTW as far as forum activity.. it's not just here. I've noticed quite a few forums on pirate and shortwave radio going somewhat inactive as of recently. I think part of it has to do with the solar cycle being so low restricting DX to local only. There's a CB forum I frequent too that is going through the same thing. With this forum it's just a matter of getting more people aware of its existence. A lot of the original members either got out of the hobby or went on to other things. All online forums go through those kind of cycles.
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