jjimenez101

Hello,

I need someone to clear something up for me:

For example, if there is a signal source connected to a transmission line ( assuming a loss-less line ), and at the end of the transmission line we have a transmitting antenna ( assuming an antenna with 100% efficiency ). Ok, so in order to supply the antenna with the total or maximum amount of power from the source, since the antenna acts as a load impedance, the antenna impedance needs to match the transmission line impedance, this way the coefficient of reflection is 0.

Now, other references that I have read state that the load impedance needs to match the source impedance in order for the load to recieve the maximum amount of power from the source. If they do not match, we can put a transformer ( with appropriate turns ratio ) in between the source and the load to "fool" the source into thinking that both impedances match.

So what is the most important impedance matching to consider when dealing with these applications? Is is the matching of the source to load, or load to transmission line?

Thank you

stevez

I think this is one of those "it all depends" answers. If your transmission line is really lossless the it would seem that all power goes to the antenna where it is either radiated or turned in to heat - but you said it was 100% efficient so it must all be radiated. Given that set of circumstances it would seem that as long as the transmitter/source matched whatever appears at the transmission line then you've got the ideal conditions. Thing is - there are no lossless lines or 100% efficient antennas. Bear with me - I am far from an expert - just providing some thoughts based on what I've learned.

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stevez

Papabravo

Impedance is frequency dependant. No matter how hard you try to match everything everywhere there is usually only a narrow band of frequencies where the match between the transmitter and the feedline, and the feedline and the antenna is such that the reflection coefficient approaches zero.

ronsimpson

I think the question is; What to match. Source/Line/Load (antenna)
The answer is YES.

Transmission lines are more or less the same impedance at all frequencies and not adjustable. A 50 ohm transmission line for AM broadcast (1mhz) is 50 ohm at FM (100mhz).

Antennas are another story. For high efficiency it is nice to have a resonant antenna. If I moved form 98.6 to 98.9Mhz then I might not retune the antenna but much farther and I go up here and change the antenna. There are broad band antennas.

If we don’t talk about linear amplifiers; All transmitters I have used/built have a resonant output stage and are frequency dependent.

If my power output stage has an impedance of 1000 ohms then I adjust the final transformer/filter to match the 50 ohm line. Turn ratio squared = impedance ration. For broadcast I change the antenna to match the line. For armature radio where the antenna is of unknown impedance I adjust another matching transformer/filter at the antenna.

Typical FM antenna; The antenna is a LC that oscillates at my frequency. The C can be adjusted to change the frequency. The L has a tap at some point. Lets say at 25%. That makes a 4:1 turn ratio transformer (by turns), or (16:1 by impedance). Power is inserted into the coil/transformer at a point that is 50 ohms.

For best results everything needs to be matched.

For my cable TV system; The amplifier is 75 ohm, the cable is 75 ohm, all splitters are 75 ohm, most TVs are 75 ohm and one TV is 300 ohm but it has a 75 to 300 ohm transformer. The impedance of all match pretty good from 2mhz to 500mhz. With in range there is on frequency worry.

Confused?

RadioRon

To reiterate what the others have said, the answer is that both the source-to-line interface and the line-to-antenna interface must be matched for maximum power transfer of source to antenna.

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RadioRon