Discussion in 'Electronic Basics' started by Dominic-Luc Webb, Jun 28, 2006.

1. ### Dominic-Luc WebbGuest

There is something not clear from info online and books on
antenna design. Impedence matching is understood as necessary
to optimize power transfer. However, it seems that the designs
typically have a single point of impedence matching, but two
interfaces at which power must be transfered: antenna to
transmission line and transmission line to receiver. The designs I
have seen add impedence matching only at the antenna to transmission
line interface. Maybe someone can clarify how this works?

Dominic

2. ### blahGuest

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.

3. ### Phil AllisonGuest

"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

4. ### GuestGuest

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?

5. ### JamieGuest

they call that SWR (standing wave ratio).

6. ### Dominic-Luc WebbGuest

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

7. ### BrianGuest

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

8. ### BrianGuest

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

9. ### Dominic-Luc WebbGuest

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

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

10. ### ChristopherGuest

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.

http://www.ccrane.com/antennas/am-antennas/index.aspx

Good Luck,

* * *
Christopher

Temecula CA.USA
http://www.oldtemecula.com

11. ### BrianGuest

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