Hi, I have two questions about antenna impedance of RF systems:

1. In order to use ASK/FSK Transmitter/Receiver Modules I must know the right impedance that I should choose for the antenna of the transmitter/receiver. But I found that some datasheets of these modules (ask/fsk) doesn't mention anything about antenna impedance. Some people told me that the standard antenna impedance for all modules is 50 Ohms. Is that right? Can I assume the antenna impedance to be 50 Ohms when it is not defined in the datasheet?

2. Since impedance is a complex quantity, I want to know if the "antenna impedance" refers to the real part of the impedance of the antenna or magnitude of it?

## Antenna Impedance in RF Systems

Yes, most of the inexpensive modules offer little documentation, a frustration I share with you. However the 50 ohm figure is not just a number someone picked out of a hat.

The simplest practical antenna is the 1/2 wavelength dipole, which has an impedance of about 50 ohms when fed at the center.

The more common 1/4 wave 'monopole', often called a 'whip', is just a dipole with one element replaced by a 'ground plane'. The ground plane 'mirrors' the whip, creating a 'virtual' second element. Thus the 1/4 wave whip is about equivalent to a 1/2 wave dipole with an impedance of 50 ohms.

Assuming you provide a 'perfect' ground plane.

As I remember the tale, 50 Ohms was a compromise between greatest power handling capacity (30 Ohms) and lowest loss (75 Ohms) (figures for air dielectric). This was settled on sometime before WW2.

I thought the 'impedance of free space' was 377 ohms. They use 300 ohm twin lead from the tv antennas, some times use a balancer/unbalancer transformer. Someone told me if you painted a battleship with 377 ohm paint, it would be invisible to radar.

**kk6gm wrote:**

As I remember the tale, 50 Ohms was a compromise between greatest power handling capacity (30 Ohms) and lowest loss (75 Ohms) (figures for air dielectric). This was settled on sometime before WW2.

You remember correctly :)

**bobgardner wrote:**

I thought the 'impedance of free space' was 377 ohms. They use 300 ohm twin lead from the tv antennas, some times use a balancer/unbalancer transformer. Someone told me if you painted a battleship with 377 ohm paint, it would be invisible to radar.

I would be very surprised if U.S. TV stations used 300 ohm feeders istead of 50 ohm coax. But I am sure that their engineers would know what they were doing.

I like the idea of 377 ohm paint. Is it as good as striped paint ?

My knowledge of aerials is from an old RSGB handbook. The general rule of thumb being that practical dimensions of half-wave dipoles have a characteristic impedance of 50 - 80 ohms. Practical coax is of similar values. Practical twin transmission lines are 150-300 ohm.

So the impedance values of transmission lines are dependent on practical considerations like physical size and cost of copper.

David.

The required antenna impedance for those inexpensive ask/fsk modules will generally be close to 50 ohms.

Dealing with antennas, themselves, is a MUCH harder issue, and more so if you don't have complex and expensive test gear. Lacking that, the best you can do is "close". Every antenna is a compromise (more than almost any other electronic "circuit"). When the antenna is tuned to 50 ohms, it is rarely resistive 50 ohms. What is one to do about all this? Connect the antenna and go. The simple fact is that few of us are capable, and those who are capable rarely succeed, in getting the antenna tuned exactly the way it should be.

You take what you can get and live with it.

Jim

Thanks for the replies.

Dealing with antennas, themselves, is a MUCH harder issue, and more so if you don't have complex and expensive test gear. Lacking that, the best you can do is "close".

You mean measuring or calculating the impedance of an antenna is too difficult? I thought the impedance of antenna was something like resistance of the metal that the antenna is made from. Sorry I don't know much about antenna theory. But suppose I want to make a simple antenna for my ask/fsk module. Can't I do it myself with a piece of wire,etc? Should I be concerned about the impedance of this antenna?

Yes. Of course you should worry about the impedance. However you generally copy a respected design and hope for the best.

RSGB and ARRL used to publish excellent handbooks. They also have tables that show the effects of different geometries. I am sure there are many non-technical plans for Amateur antennas, feeders and matching transformers.

If you are using 2.4GHz then it is as easy to have a printed circuit antenna. Chip manufacturers like Nordic publish reference designs.

If your antenna is distant from the electronics it is very important to match impedances of your feeder cables / waveguides.

David.

Things like Smith charts come into play which usually are published for antennas (plus host of other graphs).

With capacitors and inductors you can then try to match the antenna to your circuit and PCB design. There's software to help.

But it's a very specialists field and not for the casual user. Exactly the reason why IC makers publish reference designs with Gerber files.

You can have a career just doing these things.

I recall an old project with either 315 MHz or 433 MHz inexpensive Ts and Rx modules.

I built several of them and they all responded to a Master Station, tied to a PC.

I can recall trying to calc an optimal antenna.

What I ended up with was rather crudely calculated, but the range matched my expectations, and I was pretty pleased with myself! :)

Then one day one of the Remote Units accidentaly had its antenna broken off.

I clamped a hemostat on it.

It worked just as well as with the carefully calculated antenna. :?

Once again confirming that RF is 1/2 science and 1/2 voodoo.

JC

## Attachment(s):

Dosc, when did the US alter all the latin medical terms like haemostat to hem-o-stat?

Wow, I guess Wiki is Americanized as Wiki Hemostat doesn't even mention haemostats.

But, WikiSurgery Hemostat DOES spell it haemostat.

Go figure.

BTW, I'd never seen WikiSurgery until pursuing this topic. Pretty soon one will be able to get their Medical Degree on line!

JC

Hey Jay,

That looks like a "chrome plated centre-fed twin loop" directional antenna. :lol: :lol:

Agree on the advice with respect to 2.4GHz. There are lots of antenna reference designs out there, from almost every chip manufacturer.

What you need to be very careful about is that the input/output impedance that the chip wants to see is not the same for all those different sources. If you have a module, then it likely wants 50 ohms.

I can provide a little help if you have any idea what sort of antenna you want to use.

Jim

Wiki says Bob Is Right Again. http://en.wikipedia.org/wiki/Imp.... So to match the 50 ohm lo loss cable to the 377 ohm antenna, you need a matching network with some Ls and Cs and a lot of math.

Bob,

Your Wikipedia link goes nowhere !

I am more interested in 377 ohm battleship paint.

Regarding the OP, this article on antenna design is very interesting.

David.

Spacecloth is the generic term. Google that plus impedance to strip the more numerous references to a type of weave.

I got to that page by typing "impedance of free space" then did a ctlc ctlv on the link. Must not have copied right.

A 1/2 wave dipole is about 72 Ohms.

A 1/4 wave vertical with good ground plane is about 39 Ohm.

A folded dipole is about 300 Ohms.

An end fed 1/2 dipole is several 1000 Ohm.

The 50 Ohms is all about transmission lines!

With a transmission lines you get reflections and standing waves when there is a mismatch between the transmission line & the load. Matching is usually done to match 50 ohms, to the various antenna impedances. It is critical especially at higher power levels and higher frequencies where transmission losses are much higher.

If there is no transmission line, impedance can be matched from the output(/input) direct to the antenna,

using L & C, transformers , balun's, etc.

It is advantageous for "systems" that need to be interconnected together using coaxial cables to have nominal input & output impedances which match the characteristic impedance of the transmission lines used. Ie. 50 Ohm is almost universal, but 75 Ohms is used in video & TV. Horses for courses!