Antenna/feedline impedance

[zguy1243]

Can one explain in a fairly simple manner the basics of antenna and feedline impedance? I understand that some are 75ohm and others 50 that are good for transmitting. I would like to understand the science behind what impedance really is. I understand that impedances in speaker systems are the resistance to current. But I am sure that impedance of antennas is something different.

TIA
Jody

 

[ReceiverBeaver]

Hi Jody,

You're "sure that impedance of antennas is something different"

Well you'd be wrong. It's the same thing.

Impedance is the resistance of current flow in an AC circuit. Radio frequency work, as well as audible sound, (your stereo speakers), are both AC systems.

[Resistance] is the resistance of current flow in DC circuits.

 

[zguy1243]

So impedance is a measurement of resistance in a AC circuit?

 

[nd5y]

Impedance is a combination of resistance, capacitive reactance and inductive reactance in an AC circuit.

 

[N_Jay]

Can one explain in a fairly simple manner the basics of antenna and feedline impedance? I understand that some are 75ohm and others 50 that are good for transmitting. I would like to understand the science behind what impedance really is. I understand that impedances in speaker systems are the resistance to current. But I am sure that impedance of antennas is something different.

TIA
Jody

Let me understand?

You want us to summarize a one or two semester 300 level course (or set of courses) that requires 2 years of solid math as a prerequisite so you don't have to do a few Google searches and ask one or two well thought out questions?

 

[zguy1243]

Let me understand?

You want us to summarize a one or two semester 300 level course (or set of courses) that requires 2 years of solid math as a prerequisite so you don't have to do a few Google searches and ask one or two well thought out questions?

Excuse me for asking questions! it seems everything in this damn forum turns into some damn smartass **** that never helps, so F*CK it. Ill go somewhere else with my questions where there is no smartases like you.

 

[ReceiverBeaver]

Yes N Jay, yes. That's what he wants. You are one sharp cookie there good buddy.

 

[TooLate]

It was only a matter of time!

 

[N_Jay]

Excuse me for asking questions! it seems everything in this damn forum turns into some damn smartass **** that never helps, so F*CK it. Ill go somewhere else with my questions where there is no smartases like you.

Sorry, but Feel free to ask away.

I just think it is only polite to put at least as much thought into your question as you expect in an answer.

 

[zguy1243]

So now you are calling me a idiot. They should ban you from the forums, You are counterproductive. Im sorry for being off topic but I get tired of seeing all the threads ruined by people like this. Moderators need to delete these smarta** post. People come here to learn, you Mr Njay will just run people off with your attitude. Some users of these forums are just kids. You should help not hinder.

 

[pjmccourt]

ZGuy,
I have been reading up to take my ham radio general class test and it has a lot of info in it on impedance. The ARRL also puts out the "Radio Amateur Handbook" and " The Antenna" book, both are excellent books to read. Both will teach you more than you more than you need to know about impedence.

You may even find that you will want to go out and get a Ham License and make your scanning more fun. Join in the conversations that you can only monitor now. Because of studying for my ham license I have improved my scanner antennas.

Don't ever be afraid to ask a question!

Good luck and don't let others get to you.

 

[pjmccourt]

Ok lets see if I can help answer your question.

Take 300 ohm TV twin lead wire, it gets its 300 ohms impedance by the spacing of the 2 wires from each other and the size of the conductors. It is considered a balanced feedline to an antenna. Coax comes in many different impendance's, the popular ones are 50 ohm and 75 ohm. Since your scanner antenna input requires 50 ohms you need to feed it with a 50 ohm line in order to get the max signal to your radio.

Your antenna also has an impedance, you must also match it to the feed line. A simple ground plane antenna has an impedance of around 50 ohms, so it is a simple matter to match everything for max signal. A 50 ohm input on your radio, 50 ohm coax (RG58), and a antenna with a 50 ohm impedance.

If your antenna is not 50 ohms then you must use some kind of device to match it. Just like you use a 75ohm/300ohm matching device on TV's.

I hope this helps a little. Like I said in my other post doing some reading will really clear things up for you.

 

[kb2vxa]

Hi Zguy and all,

N_Jay has a reputation for being a smart ass know it all pain in the butt dummy load so you can either outsmart him at his own game or just ignore him.

"Impedance is a combination of resistance, capacitive reactance and inductive reactance in an AC circuit."

Thanks for the simple and accurate answer Tom, there is no such thing as pure "AC resistance" in the real world but we won't confuse the lad with a bunch of complex equations.

 

[slicerwizard]

Since your scanner antenna input requires 50 ohms you need to feed it with a 50 ohm line in order to get the max signal to your radio.

With their wide band coverage, scanners will rarely exhibit a 50 ohm input impedance. The actual impedance will jump around with the RX frequency. Same applies to any wideband antenna. That makes cable losses far more important than impedance. A low loss 75 ohm cable will easily outperform a mediocre 50 ohm cable.

And while not defending N_Jay, zguy, you need to grow a thicker skin. You're coming across as a bit of a drama queen. Just my opinion, no offence intended.

 

[pjmccourt]

With their wide band coverage, scanners will rarely exhibit a 50 ohm input impedance. The actual impedance will jump around with the RX frequency. Same applies to any wideband antenna. That makes cable losses far more important than impedance. A low loss 75 ohm cable will easily outperform a mediocre 50 ohm cable.

You are correct about the coax for scanner use, I use LMR400 for my scanner antenna. A little on the expensive side but it works good. Thanks for pointing this out!

 

[NAVCAN]

As stated a few times already, the impedance of an AC circuit depends on the capacitive reactance, inductive reactance, and DC resistance.

The two major contributing factors are the capacitive reactance, and the DC resistance. The capacitive reactance is a direct function of the DISTANCE between the center conductor, and the outer shield. (a capacitor is 2 metal plates seperated by a dielectric (space) ). And well, resistance, is well, resistance. So if you look at an RG58 cable (50 ohm), and RG59 cable (75 Ohm) the distance between the center conductor and shield is greater in the RG59.

Inductive reactance does play a role in the overall impedance, but it's not as significant in a coax system.

 

[N_Jay]

So now you are calling me a idiot.

1) I did not call you an idiot. (but if the shoe fits . . .)

They should ban you from the forums, You are counterproductive. Im sorry for being off topic but I get tired of seeing all the threads ruined by people like this. Moderators need to delete these smarta** post. People come here to learn, you Mr Njay will just run people off with your attitude.

2) you are the one to get all pissed off, when all you needed to do was form a better question. (Maybe state what you do know, so the person answering has a point of reference to start from.)

Some users of these forums are just kids. You should help not hinder.

3) and even kids can learn to ask well thought out questions.

 

[N_Jay]

Hi Zguy and all,

N_Jay has a reputation for being a smart ass know it all pain in the butt dummy load so you can either outsmart him at his own game or just ignore him.

Or you can not use the quote feature like Warren. :twisted: :twisted:

 

[N_Jay]

As stated a few times already, the impedance of an AC circuit depends on the capacitive reactance, inductive reactance, and DC resistance.

The two major contributing factors are the capacitive reactance, and the DC resistance. The capacitive reactance is a direct function of the DISTANCE between the center conductor, and the outer shield. (a capacitor is 2 metal plates seperated by a dielectric (space) ). And well, resistance, is well, resistance. So if you look at an RG58 cable (50 ohm), and RG59 cable (75 Ohm) the distance between the center conductor and shield is greater in the RG59.

Inductive reactance does play a role in the overall impedance, but it's not as significant in a coax system.

Answers like this are why you need better questions.

 

[N_Jay]

Start here:
http://en.wikipedia.org/wiki/Electrical_impedance
http://en.wikipedia.org/wiki/Characteristic_impedance
http://en.wikipedia.org/wiki/Impedance_matching

Then ask a question to help you understand the pieces.