The End-Fed Half Wave Antenna a bit Differently

Istvan Agg, HA5CLF / N9EU

My favorite SOTA antenna is the End-Fed Half Wave Antenna. There are many good descriptions on the net about this type of antenna. I recommend a good reading on this subject by Steve Yates, AA5TB at: www.aa5tb.com/efha.html. However, let me write about it from a slightly different perspective that was not very obvious from the articles I read. This article will describe a 40/30/20m band end-fed half wave antenna, focusing on its matching unit. It will present its parameters, so anybody with basic home-brew skills will be able to build it. Nevertheless, I will also describe, how it is designed, if one decides to embark on building it for different bands.

Based on a 2014 survey, the most popular antennas for SOTA (Summit On The Air) activation are the following (see sotabeams.co.uk/blog/5-most-popular-hf-portable-antennas for details):

#

Antenna type

% Of respondents surveyed

1

End-Fed Half Wave

18%

2

Linked Dipole

16%

3

Single Band dipole

12%

4

Random Length End Fed wire

12%

5

Ground Plane Antenna

11%

Let us not confuse the End-Fed Half Wave antenna with the Random Length End Fed wire. That is an end-fed, but very different then the half wave construction. This article will not address that antenna type.

What are the advantages and disadvantages of the end-fed half wave antenna? I would mention first its ease of installation. Throw one end of the antenna wire, extended with a rope, as high as you can on a three, and walk away with the other end and attach it to the tuner. One end may even be close to the ground, not yielding the best radiation efficiency, but much activation proved it still operational. Of course, if both ends are raised higher, it will produce a more efficiently radiating antenna, but the portable and temporary installation is always compromised by conditions given by a location and its restrictions. Therefore, installation is quite easy, and its tuner circuit is rather simple as well. It does not need any special components; in fact, most of us may have them in our shack drawers already.

What are its disadvantages? I tried to operate from a hotel, and learned the hard way that this type of antenna is not suitable for such location. It requires a set distance to hang both ends of the antenna, and that is not always given. In an urban area we are much more restricted, especially if we need to be stealth about it.

So, how to construct an end-fed half wave antenna? Let us touch on some necessary theory to begin with.

Tuner01j

In an ideal setting, the center of the antenna will have the lowest voltage, consequently impedance, which is why it is possible to feed it by 50 or 75Ω coaxial cable. We also know, that both ends of the half wave antenna will have the highest voltage, thus impedance, which may reach up to several kilo-ohms. This is why we need the impedance transformer from the 50Ω coaxial feed-line to the several kilo-ohm antenna connections. The impedance matching circuit is essentially a simple parallel resonant LC circuit. Select components with good insolating materials and properly chosen, relatively high voltage rated parts.

003

Essentially, the matching is done with a tank circuit, which represents the highest impedance when it is tuned to resonance. It matches the impedance fairly well to the half-wave antenna’s high impedance at one of its ends. There are two aspects, however that we need to consider:

  1. The impedance ratio of the transformer
  2. The tank circuit’s resonant frequency

The impedance ratio of the transformer

Let’s begin with the transformer. We do not know what the actual impedance will be at the end of the antenna radiator. It may be one or even five kilo-ohms. It is best to make the impedance matching adjustable, using switched transformation ratio.

Because we are considering this tuner for SOTA activations or casual portable operation, we do not need to design it for more then 100W. Most of the time 5 to 20W will be adequate for activation anyway. For the transformer, a T130-2 toroid will be sufficient, on which we wind the seconder with multiple taps, as shown in the below table.

 

Tap number

Primer turns

Seconder turns

Impedance output in kΩ

Measured inductivity in μH

1

2

10

1250

2.0

2

2

12

1800

2.3

3

2

14

2450

2.6

4

2

16

3200

3.0

5

2

18

4050

3.3

6

2

20

5000

3.6


Note: If we change the primary number of turns, the impedance ratios will considerable change also. If you need to change it, is necessary to recalculate the secondary turns, and the resonance frequency needs to be adjusted also. To calculate the impedance ratio, use the following equation:

1

Where:

Z ki / Output impedance

Z be / Input impedance, in this case 50Ω

N sec / Seconder number of turns

N prim / Primary number of turns

A completed tuner is shown here, which depicts the inductance assembly. The toroid with its many taps may be soldered directly on a multi-position switch.

004

The tank circuit’s resonant frequency

The next important parameter is the parallel resonant circuit’s resonant frequency. It depends on the selection of the inductance and the capacitor values. It is important that resonance may be achieved at each tap of the inductance. We need to measure the inductances of the toroid with the tapped coil. The previous table already contains these measurements. These inductances are required in determining the right capacitance values, of which two values are important: the minimal and the maximum capacitance values.

We also need to know what are the frequencies or bands for which we plant to use this antenna. Assuming, most of us use the 40/30/20m band for HF SOTA activation, so the following table shows the SOTA frequencies, and the calculated half wave radiator wire of the antenna.

 

Band

SOTA FRQ

Mode

λ/2 [m]

40m

7.032

CW

20.22

30m

10.118

CW

14.05

20m

14.285

SSB

9.95

 

Consequently, let’s pick the lowest frequency 7,000MHz that we need to be able to tune the inductance to, for each of the taps on the inductance. The highest frequency could be14.350MHz.

Consequently, this is how the capacitance values are determined. Calculate the capacity for the resonance at the lowest frequency to be covered with the lowest inductivity taken in consideration. This will yield the maximum capacity of the variable capacitor.

Accordingly, calculate the capacity for the resonance at the highest frequency to be covered with the highest inductivity taken in consideration. This will yield the minimal capacitance of the variable capacitor.

Selecting the variable capacitor that can be varied from this minimum to maximum capacity, will allow tuning to resonance for 40, 30, and 20m bands at each of the switch position of the tuner. This will allow good matching of the half-wave radiating antenna wire to the 50Ω coaxial cable at virtually any location. It is not necessary to calculate the above; the results are sown in the following table.

 

Lowest frequency = 7.000 MHz

Highest frequency = 14.350 MHz

Lowest measured inductivity = 2.0 μH

Highest measured inductivity = 3.6 μH

Calculation result = 258 pF

Calculation result = 34 pF

 

In case you need to perform calculations, use the formula:

2

Where the frequency “f” is in MHz and the inductance “L” is in μH.

Finally the tank circuit’s specification is complete, which is sown below. The “Counterpoise” is a short, typically one twentieth (0.05 * λ) of the wavelength. From experience, I found that with this tuner my SWR was better by simply disconnecting the counterpoise at lower bands. It is worth experimenting with its length also.

002

 

It is important to consider the voltages on this tank circuit. At 5W input the connector at the end of the antenna wire may be up to 160V, and at 15W it may be 270V. Be cautious, such voltages may cause a quite painful skin scorch.

It is quite possible that designing more than three or four bands into a tuner will lead to a failure. It is quite difficult to satisfy both requirements of achieving proper impedance ratio and resonance of the tank circuit at each tap of the inductance. It is better to build one for 160 and 80m, perhaps add 17m to the design described in this article, and for higher bands it is easier to use a linked dipole.

I installed a QRP SWR meter into the tuner that makes tuning very handy. During portable operations, I noticed that I forgot the toggle switch in the tuning position, just to discover that no one calls me back to my relentless CQ-ing. Therefore, I picked a momentary push button to switch the operational mode to tuning mode while tuning up. With such a switch, it is impossible to forget to switch back to operational mode. See the second completed tuner depicted below. The disadvantage of this switch is that one needs one hand to hold it, another to key down, and yet another to tune the capacitor.

How does this SWR meter works? Tuning the antenna to its resonance will result in 50Ω seen at the output of the SWR circuit. That will be the missing “resistor” in the resistor bridge of the below circuit. The bridge than will measure zero voltage across the bridge, and the LED will turn off. If the antenna is not tuned properly, and it represents other than 50Ω, the diode will rectify the current flowing across the bridge and the LED will light up. It is recommended to use at least 2W non-inductive resistors for this purpose.

001

Another completed tuner, which is more compact then the pervious one shown, also it is lighter, more suitable for portable operations. Note that the primary winding on the toroid is using the inner lead of an RG-58 coaxial cable. This provides proper insulation from the relatively high voltages on the seconder winding.

ew

I found it very useful to label the tuner with switch and capacitance positions. It is likely that next time the tuning may be set according to the already found positions, and it will not be far off, requiring only slight adjustment. Of course, you will need to find your tuning positions and apply your own position scale.

 

Band

L

C2

Counterpoise

40m

5

45

I did not need it

30m

5

75

I did not need it

20m

6

90

1m

 

From this article one may build a 40, 30, 20m end fed half wave antenna, with no special parts needed, or no special skills required. The computation method is also touched on. There is much more that can be written about this antenna and its matching, but that was not my aim. My goal was to describe the parameters of an antenna that can be easily build, yet show enough theory for someone to design his or her own antenna and antenna tuner. I wish you lots of productive experimentations and successful portable operations.

 

Best 73!

 

HA5CLF / N9EU István

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