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Forward countersign radio

[Music] hello auggies worldwide i'm dave casler amateur radio call sign ke0ge here with another episode of ask dave our question today comes from jim ke0nre and he is asking about the concept of forward power reflected power on a coax transmission line in situations where antenna impedance and feed light impedance don't match what i don't understand when i insert a watt meter between my transmitter and antenna feed line is the difference between p forward and p ref readings the power reaching my antenna how does a common mode current on the outside of the coax shield affect p forward and p reflected readings interesting questions let's take a look at what he might be using this is an mfj849 okay it's a digital swr and want meter and measures the forward power the reflected power and gives you the computed swr which is easy to compute from the forward and reflected power and it's got two ranges one is hf and one is vhf okay it needs 12 volts to operate so we're just using this little battery here now let's look at a diagram here we'll take our coax okay and we're gonna put a so the coax has a center conductor and of course this you have grounded in your in your scheme now for something like this you can just simply loosen one of these bolts right here and attach a ground to that and that's what i would suggest doing okay now forward power if you've got a transmitter over here let's go ahead and put in the transmitter it's going to be feeding out now you've got an antenna that has resistance this is called the radiation resistance it has resistance which is the ohmic resistance of the wires it has capacitance and it has inductance okay so it's going to be slightly reactive either capacitively or inductively when you get those two to exactly cancel out then the antenna is resonant okay meaning there's no complex part being reflected but these resistors may not add up to 50 ohms in which case you will have a non the swr will be something other than one be higher than one if you can keep it under about one point six to one you're doing really well okay so the forward power comes here goes through and hits this it's 50 ohms and it is purely resistive so it has a phase angle of zero okay now the reflected power comes back the other way because this is not a mismatch we store energy in the reactive components and that energy is transmitted back and this is the reflected power so this is you can see it as a voltage usually or as power either way you want it if you're going to do it as power it'll be referenced 50 ohms so that's the reflected and this is the the forward now if you don't have a tuner the situation you have...