Impedence matching: antenna-line-load

You are correct that you must match between the transmitter and the transmission line and between the transmission line and the antenna. However there are some considerations you probably aren't taking into account.

Transmission lines are typically of a standard characteristic impedance and are relatively constant with frequency, this is not the case of an antenna. Some sort of matching circuit is typically required between the transmitter and antenna (such as a transformer at lower frequencies or an LC style match at higher frequencies) however this is typically fairly straightforward because the impedances involved are known. The impedance of the antenna may vary significantly based on geometry, location, and condition of the antenna, not to mention frequency. So matching to the antenna is a bigger issue.

A mismatched antenna will cause the impedance of the transmission line to appear to change and in the case of any quality transmitter there will be a built in antenna tuner which will adjust the drive impedance at the transmitter/feed line interface. A remote antenna tuner, while a cool idea may not be practical for a variety of reasons.

"Dominic-Luc Webb"

** It is important to *impedance match* an antenna to the transmission line supplying it with RF power - otherwise only some of the power flowing down in the line is radiated while the rest is reflected back to the source.

When the *characteristic impedance* of the transmission line is equal to the antenna's impedance at the transmission frequency ALL the energy is transferred to the antenna - without loss.

However, at the source end the game is quite different:

If the transmitter's *source impedance* is matched in value to the transmission line, then exactly half the RF power being produced by the transmitter is lost ( as heat in the transmitter itself).

Often, with low powered RF devices such loss is tolerated, but where hundred or thousands of watts of RF energy are involved it cannot be. So the transmitter's source impedance is made lower than that of the transmission line.

The currents flowing in the transmission line and transmitter output stages are the same, so power sharing is proportional to the relative impedances of each.

...... Phil

I have an opinion that In order for all the transmitted power to leave the antenna, the antenna has to be tuned /trimmed/lengthened to the wavelength of interest. If the antenna is not tuned the power that does not leave the antenna returns back into the radio and can cause damage to the final amplifier. A receiving antenna will receive better if it is tuned to the wavelength but it won't do any damage to the Detector circuitry if it isn't, as no power transmitted to be reflected back into the radio. Just my opinion. Lets hope we will find out. It is normal to use the same antenna for transmitting and receiving, a switch will change from receiver to transmitter. I expect there will be an antenna newsgroup that specialise in this kind of thing?

they call that SWR (standing wave ratio).

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

That is essentially my understanding. I think it is actually called reflection, and this can be measured in order to tune an antenna. I have the Joseph J Carr book "Practical Antenna Handbook" that shows some testing techniques using an oscilloscope. I still have not found a simple way to test for this without a scope.

Dominic

The transmitter and receiver usually has an output/input impedance of 50 or

75 ohms. You would use a transmission line that matches that impedance. So that takes care of the impedance matching at the transmitter/receiver side. So that only leaves matching the impedance at the antenna side. Brian

Also, if you don't match the impedance at the antenna side, it will affect the impedance of the line. Brian

Hi, and nice to to see you dropping in Brian.

Please consider the case of a portable so-called international radio with a broken dipole and/or (a working) jack for an external antenna....

Basically, my question regards the very common "novice" experience of being confronted with a simple battery operated portable radio (a receiver) in which no antenna or transmission line initially exists. There is the common quick fix of "sticking a random stretch of common insulated wire where the antenna should go". In this case, we make no assumption that the antenna or transmission line are matched. The working environment is a typical European garden "collective" situated adjacent to a farm in an essentially flat landscape. Some gardeners with no understanding of electronics sometimes wish to listen to medium or even long wave radio. So of course, their natural inclination is to stick a wire onto the (often broken) dipole or the external antenna connector, which typically yields less than professional results. I plan to improve this situation, starting with antennas of sane length for frequency desired (albeit undecided regarding long wave).

:)

I investigated this problem, looking at the Carr book for instance, and realized that these sources assume readers will use a co-ax cable, in which case impedence is typically specified, as you point out.

And so, I began asking questions of how to pursue this when specs are not known, and neither line nor antenna impedence are likely to conform to the load ratings for the receiver. This is now an intellectual question (I could just buy the correct things). I have an oscilloscope and could probably come up with some tricks, but don't otherwise know a simple test for impedence more than maybe trial and error tuning. The dedicated test devices I have seen are sophisticated and quite specialized (presumably expensive).

Dominic

Hello Dominic,

This company produces add-on amplified AM antennas that could meet your need if you just want to buy a super AM antenna.

Good Luck,

• * * Christopher

Temecula CA.USA

Have you used Google, to try and find the model radio that you have (on the internet)? You might be able to find out what the antenna input impedance is (for your radio), that way. Brian

Take a look at

ACTIVE ANTENNA FOR PORTABLE SW RADIOS Brian