[AMRadio] antenna tuners transmission lines and more


Gary Schafer garyschafer at comcast.net
Sun Apr 23 22:51:01 EDT 2006


Hi John,

Boy you can tell you are an AM'r with the long post! :>)

In some of the following I am saying the same thing as John with a little
different explanation. 
Some is a bit of a correction.
Lots of good stuff from John!

I don't consider myself an expert either.

This was going to be rather short but...

SWR: 
The swr on a line can be found by measuring voltage peaks or current peaks
on the line as you described. To truly measure swr the line must be at least
a quarter wavelength long.

What we normally measure with our swr meters or watt meters is an impedance
ratio, which can be done on any length of line. The impedance ratios are
representative of the standing wave ratios but we are not directly measuring
standing waves. The impedances are compared to a resistor in the Swr Bridge.

LINE LOSS:
High swr "can" be an indication of wasted power but in the form of feed line
loss if the feed line is a low impedance line such as coax.
High swr can produce very currents on the line which result in I squared R
loss.

High swr on a higher impedance line is not usually much of a problem because
the I squared R loss is much lower due to the current being less just
because the line is higher impedance.

TUNERS WASTING POWER:
A tuner can dissipate substantial amounts of power depending on the load it
is trying to match and if it is not adjusted properly. Even one with high
quality components. For example the most common T type tuner can be
misadjusted with the improper L /C ratios causing very circulating tank
currents which heat the coil substantially. 
But if adjusted properly this is not usually a problem.

REFLECTED POWER TO THE FINALS:
As you noted high swr is an indication of reflected power on the feed line
but that reflected power does not make it back to the finals in the
transmitter.

The reason it does not is because any reflected power that comes back down
the line is re-reflected back to the antenna and eventually gets radiated.
It gets re-reflected by what is called a conjugate match at the antenna
tuner or the final tuning. 

A conjugate match presents an equal and opposite match to the line at the
antenna tuner end of the line as what the mismatch to the line is at the
antenna. 

If the line presents an inductive reactance at the tuner then the tuner must
present an equal amount of capacitive reactance to the line. That is what
some people call "resonance". That gives a flat, no swr, between the radio
and the tuner. All the reflected power that came back to the tuner will be
reflected back to the antenna at that point.

RADIATION RESISTANCE:
The definition of radiation resistance is, 
The total EM power radiated in all directions divided by the square of net
current causing the radiation.
In other words radiation resistance is equal to, a resistor if substituted
for the antenna, that would absorb the same amount of power that the antenna
radiates.

Radiation resistance is not the feed point resistance of an antenna. The
feed point resistance of an antenna also includes resistive losses in the
wire. Power dissipated in that resistance is wasted in heat.

FOLDED DIPOLE:
The radiation resistance of a folded dipole is the same as that of a regular
dipole.
Even though the "feed point resistance" is 4 times as high for a folded
dipole its radiation resistance is the same as a regular dipole.
The folded element in the folded dipole only acts as an impedance
transformer just like a 4:1 balun would do.

The same holds true for a vertical monopole with a folded element to raise
the feed point resistance. The radiation resistance is still the same as if
the monopole were fed in the normal manor at the bottom against ground.

SHORT ANTENNAS:
A short antenna will radiate just as well as a full length antenna. As a
matter of fact an infinitely small antenna will radiate just as well as a
1/4 wave or 1/2 wave length antenna.
The problem is getting the power into the short antenna.

A loaded mobile antennas radiation resistance is usually very low, in the
order of a few ohms. Adding a loading coil to raise the feed point
resistance to 50 ohms still leaves the radiation resistance of the antenna
itself at those low few ohms to work against ground in getting power into
the antenna for it to radiate. The coil dissipates most of the power applied
in heat.

If only 1 watt actually is radiated by a short mobile antenna it will
produce the same signal strength as a full quarter wave length antenna with
the same amount of power radiated.


RADIATION RESISTANCE OF A FULL WAVE DIPOLE:
I believe the radiation resistance of a 1/2 wave dipole is in the
neighborhood of 2000 to 4000 ohms. I think it is the same as what the
impedance at the end of a 1/2 wave antenna would be.
I saw once how to calculate it. I will have to dig around again.

Again I believe that an infinitely long dipole will have a similar radiation
resistance to the full wave dipole.

RHOMBIC ANTENNA:
A rhombic is a different antenna than just a long dipole. The two elements
interact with one another.
An unterminated rhombic is a "standing wave" antenna. It has reflections
back from the end of it just like a regular dipole antenna does.
A terminated rhombic is a "traveling wave" antenna. There are no reflections
back from the far end of it. I don't know offhand how the radiation
resistance is found on either.

73
Gary  K4FMX







More information about the AMRadio mailing list

This page last updated 21 Oct 2017.