Amateur Radio Antenna Tuners

The antenna that can be tuned electrically to operate across many amateur bands with the greatest RF power output at each frequency..
Unlike commercial services, we no longer want a separate antenna for each HF band, making working the HF bands with one antenna possible.

The first thing we should point out is that when the skip is in on a particular HF band we can work the world using only about 5 watts, or what is now called QRP radio.
Our normal 100 watt radios can be adjusted down in power level to prove this, and in good practice we used to be encouraged to do this.

That said, we now need to explore exactly what is happening on a long wire antenna system and this involves EM resonance at radio frequencies.
We can get a good feel for this using water in a bathtub, and charting both wave height and current flow maximum points.

We get a good feel for this using water in a bathtub, and charting both wave height and current flow maximum points.
We set near the center of the bath tub and start moving back and forth to find the resonance of the tubs longest length.

As the water moves back and forth with a current flow at the center, the waves hit the ends going higher then lower.
On one end the water raises up and then down, on the other end it is doing the opposite.
At the center we have a flow of water that must move back and forth at 90 degrees to the water height on the ends.
The water height at this point remains constant.

The water currents will be greatest at the center point of the tub and they will move up and down on the ends to the greatest and lowest heights.



On a 1/2 wave antenna the water height is the voltage, and the water at the center is the current and these work at 90 degrees to one another.
At the center feed point, that is 1/4 wave length from each end, we have the current node.
At the two ends of the wires we have voltage nodes of opposite charge.
The EM field at each point along the antenna wire slowly change from current at the center to voltage on the ends and this is a gradual change all the way down the wires.
We can do a calculation to show that the impedance [RF resistance] of the wire changes all the way down it where at the end it is infinite and the flow stops then reverses.

In order to radiate the strongest possible signal we must have a voltage node at the exact ends of the wires and a current node 1/4 wave back towards the feed point.
The outer two ends of the dipole are then moving into opposite charges of the EM high voltage simultaneously. Thus it is a 1/2 wave antenna.
When one goes positive the other goes negative then this reverses, so the EM current at the center is changing direction back and forth at the RF frequency.

We can then place an SWR meter at the center of a dipole on the current node, and note that the voltage will remain pretty constant at it's lowest point while the current is changing,
This will work for any frequency that hits the antenna with a current node at that point for a 50 ohm impedance match.

If we move off that location the voltage may climb as the current drops. The power down the antenna wire is the voltage times the current, both are constantly changing along its length.
As the SWR meter may be reading only peak voltage in both directions, it may now give false readings much higher then expected, but the power is not greater.

If the SWR meter were placed on the voltage node location it would probably be destroyed, but still the power is not greater.
On a long wire antenna working on many different HF bands, we have to use an antenna tuner between the antenna and the SWR meter and Transmitter to avoid this.
We must be sure that a voltage node does not get anywhere near our SWR meter or our Transmitters output jack.

Voltage and Current are always found separated by 90 degrees phase of the wave, or 1/4 wavelength of the transmitting frequency in an antenna system.

Now if we make our antenna longer the nodes will not land on the antenna in the correct locations, and coming back to the feed point we see a bad SWR.
To make the wire look longer, we simply place a coil of wire in series with it.
To make the antenna look shorter we place a capacitor in series with it.
The goal is then to make the electrical distance from the transmitter to the end of the wire of the antenna the correct electrical distance. 1/4 wave or some multiple of it.



This is the function of the antenna tuner.
This is a series tuned resonant circuit and can be studied in the Electronics training material.

As we change the frequency of the transmitter, The 50 ohm feed point may now start to move again.
The auto antenna tuner will add the correct capacitor or inductor to compensate so the transmitter gets a pure 50 Ohm impedance, and in this mode we will achieve the greatest RF power out of the transmitter and that antenna on that frequency.
A voltage node will appear on the end of the antenna with a current node at the transmitter as the electrical length is again corrected.

In a modern auto tuner the internal SWR meter is always setting on the radio side of the tuner and this is the side that it will effect.
The side towards the antenna will likely always read bad and never change for the frequency that is being tuned by the tuner.
This is because the length from this point to the end of the antenna is not electrically resonant, and that is why we use the tuner.
Remember the forwards wave and the reflected wave are only in phase at the 1/4 wave points along the antenna as they are moving in opposite directions at different power levels as they cross.

The goal is to get at least 1/4 or 1/2 wave up into the air with a voltage node on the very end of the wire.
I have personally found the End fed antennas are the most versatile. With a good tuner we can work nearly all the HF bands with a good signal output into the air.

The MFJ Tuners have been the best ones I have ever used, as they can tune almost anything automatically.


However my newer Yaesu FTdx10 has a pretty nice tuner inside it that is better then the older radios for its tuning range.

If we want to power from a voltage node point or anywhere away from the center current node  we have to increase the impedance of the feed point we are using
and this will raise the voltage at the new feed point just as happens on a resonant dipole.
We can get good results using a 9 to 1 toroidal transformer or even greater 15 to 1 toroidal transformer, and there are many commercial ones that work up to 1.5 kw or higher.

The impedance rises with the square of the turns ratio as I recall.
Now we feed the system from the low impedance side of this transformer and as we change its tunning we get 4 times the change on the output side at the turns ratio on the antenna feed point.
We can now get the range necessary to match the End Fed Antenna across all the HF bands if we choose our wire length correctly.
The center of the dipole must now land somewhere down the long wire at a 1/4 multiple wavelength from the other end where a voltage node must appear at the point of reflection.
That point can now move with the frequency along the long wire as we change the tuner and the transmitter frequency.

These high impedance toroidal transformers can be used for end fed or off center fed antenna systems that can tune more bands more efficiently then a basic center fed dipole.
They hand the antenna tuner greater flexibility and keep the impedance of the coax transmission line lower, much closer to 50 ohms but amplify the changes as we adjust them with our tuner.



One can consult electronics training materials on-line to understand transformers, and how the power goes through them only changing the voltage and current,
exactly what we see down an antenna wire length.

Turns ratio converts the voltage one way and the current in the opposite direction with a little loss in power as the energy moves through it.
Thus we can now feed the antenna at a different point then its current node position.
We can safely move up towards or even on a voltage node and keep that out of the transmission line to the Transmitter and Tuner.

If we want our antenna to radiate RF energy it must be 1/4 wave or longer, with a voltage node reflection at the end to get out into space well.
The wave must pass through the 377 ohm point on the antenna in order to couple its EM energy into the Space Fabric. This point is very near the current node position but a little further out then the 50 ohm point.
This is how the dis-cone antenna works.

With a long end fed antenna this means we have to get at least one current node on the wires length and one voltage node to the other end of the wire.
If the antenna is longer we can use a tuner to get the voltage node to the end of the antenna.
In theory we can transmit any frequency that is higher with a shorter wavelength if the tuner can deal with it.
Maximum RF will be on the other end and any point between where we hit 377 ohms near any current node point, and a frequency with many wavelengths down the wire will become directional.

In theory if we transmit into a 50 ohm dummy load the signal will not reach very far out into space compared to a well tuned antenna hitting the 377 ohm range..
The dummy load may also get hot as it is sinking the energy into itself. There is a time limit for this based on the watts we are transmitting.