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.
Here is how this translates to an RF dipole Antenna system:

Antenna Theory for Amateur Radio Stations

As we play with making the wire longer we get new patterns down the wire at the same frequency as the 1/2 wave antenna and nodal points that will work the same.
With this design we will now have a full wave in the air, with  two current nodes and 3 voltage nodes. We can feed this from either current node location.


The disadvantage now is when we change the frequency the current nodes will move so the feed point cannot feed  just any frequency no matter how we tune it.

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By moving the feed point towards one end of the antenna wire, we can now feed the system with other frequencies and the current nodes are free to move.

This is the basic end fed configuration.  Now what we shoot for is to get a voltage node on the far end of the wire, and then match the impedance on the other end to the phase that lands on it.
The current nodes can now move about anywhere as long as they are not too close to the Balun Transformer at the feed point. The toroidal transformer is inside the box and makes the voltage jump,the current drop,and the impedance raise also on the active antenna. The Antenna tuner can now reach this much higher impedance, from a much lower one and we can get all the amateur bands operating on this system configuration.We can now make the antenna long enough for a low frequency HF band and still fit the higher frequencies on this same antenna with more nodes along it, and we only have to feed it on one end.

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 correct 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 current nodes will now start to move.
The auto antenna tuner will add the correct capacitor or inductor to compensate so the transmitter gets a pure 50 Ohm impedance, a current node,
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 corrected.

The antenna system now includes all the components from the SWR meter outwards and acts as a resonant antenna system.
With correct impedance matching both the forwards and the reflected wave will move through all the components in both directions.

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 as they cross in real time.

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 will appear at the point of reflection.
That current node 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.