Hallo Field Day project-Portable hf antennas

Hi, my name is Artur and my ham radio sign is SQ5NWA. (qrz.com card)
Light, field, effective, multi-band antenna solutions is a project that I called Hallo Field Day. These are my experiences from many years of amateur radio in the field.

Half-wave dipole - assumptions

A field HF antenna should meet at least several conditions. According to my experience, they are:

  • The antenna must be portable
  • Efficiency for 5W and 100W powers
  • Low weight and transport dimensions
  • Easy to assemble in the field
  • Support for many amateur frequencies

The hf antenna emits electromagnetic waves most efficiently when its length is equal to or close to 1/2 or 1/4 of the wavelength.

There are many solutions for HF field antennas. From pocket “miracle” antennas for all bands, ending with simple wire antennas for many or one band. There are antennas horizontal, vertical, open, closed … etc.
Antennas can be discussed for a long time and probably everyone has their favorite and, in their opinion, the most effective. I’m not going to deny it.
I tested a lot of them and … went back to the half-wave basics

Why a half-wave dipole?

My way to an effective solution

  • First, I wanted a small multi-band antenna that could be installed anywhere (park, campground, etc.). I tested many short multi-band antennas.
    It quickly turned out that I am satisfied only when the propagation is very good. When even a slight decrease in propagation conditions occurs, short antennas stop working effectively. I started looking for a better solution.
  • As I like low SSB power (sometimes I like 100W too), and a few amateur frequencies are useful in the field, I was looking for a more effective and multi-band solution. I tried longer, unbalanced, horizontal and vertical wire antennas, which worked better on some bands and worse on others. Then I decided that the antenna working on harmonic frequencies also needed to be improved.

    I made a half-wave dipole for three frequencies, which I manually switched by connecting or disconnecting the connectors on the arms

The final step was to automate the frequency switching of the antenna. I did it and that’s how the Hallo Field Day project was born

automatic frequency switching

The HFD SWITCH is the first component of a multi-band kf antenna. It is a high impedance LC resonant circuit at resonant frequency. Thanks to this, it serves as an automatic multi-range switch of the antenna operating frequency

  • I use mica capacitors in resonance traps. Why?
    The much cheaper ceramic capacitors did not work well during the tests. During attempts to transmit with 100W power, the antenna resonance changed as a result of capacitance instability as a function of temperature and frequency. Mica capacitors have eliminated this problem.

Multiband half-wave dipole. HF antennas

Small size, low weight, effective, 100W

multiband dipole

Antenna impedance

Antenna signal loss

The half-wave dipole is a symmetrical antenna. Therefore, the antenna should be fed symmetrically or asymmetrically through a 1: 1 current balun.
 
The asymmetric antenna feed line is very popular. A 50 Ohm coaxial cable is enough. You must remember that if you connect a symmetrical antenna to the radio with an unbalanced cable, unbalance currents and losses in the antenna will arise. You can reduce the loss common current using BalUn 1: 1

I designed two solutions

  1. PCB, from which you can make BalUn 1: 1 and UnUn 49: 1. You can put this PCB in a waterproof case, which you can also buy in our store.
  2. The second solution is a PCB with a similar use, with the difference that you do not install it in a waterproof ceasing. It is designed for field activity. An additional advantage is air cooling of the ferrite core.

Both PCB boards are matched to a 35.55 mm core (FT 140-3). I have been using this core for 100W SSB, CW work for years

Both solutions ensure stable and uniform ferrite core winding. By using the f PCBs, everything becomes orderly and standardized.

A bit of information to start with

The End Fed antenna is an unbalanced structure based on harmonic frequencies.Its unquestionable advantage is the simplicity of installation in the field.
If you want to refine the effectiveness of this antenna, there are a few things you should know

  • radial pattern (this is important to know) – at higher frequencies the radiation beam tends to shift its direction of radiation parallel to the antenna wire. At lower frequencies, the antenna radiates like a half-wave dipole, i.e. perpendicular to the antenna wire. Therefore, the antenna on some frequencies may be more effective in one direction and on others in the other. The antenna has a better radiation pattern when you hang it as a straight line than as an inverted V, for example. If you decide to hang the End Fed in the shape of an inverted V, try to have a refraction angle of 120 ° -130 °.
  • impedance transformer (this is worth considering) – there are many different opinions on the end fed antenna transformer winding and its effectiveness for various ferrite core variants. The proportions and the method of winding play a major role here. It is difficult to clearly indicate which solution is the best. There are many studies and experiments. Everyone can judge it individually.
  • asymmetry (it’s worth doing)
    despite the fact that the antenna is made up of one arm, it should be remembered that there are no single-arm antenna systems. An antenna always has two arms, although they can take different forms.
     
    In the End Fed antenna, the second arm often becomes the outer part of the coaxial cable. Therefore, there are problems of asymmetry – sometimes visible as a pinch when you touch the radio housing, and sometimes they are invisible. However, they always cause antenna losses. The phenomenon of asymmetry of this antenna can be eliminated in two ways:
    1. Use a counterweight or ground the antenna
    The most effective counterweight length for the End Fed 80-10m antenna is 0.05 λ for the lowest bandwidth of the antenna.
    2. Use a common current inductor.
    In the latter case, it should be remembered that the choke has the best attenuation at 50 Ohm impedance. Therefore, it should not be installed close to the power point because the impedance is much higher there. The filter should be located approximately 1/4 wavelength from the feed point. Take the average for multi-band antennas.

 

YOU CAN CHOOSE THE VERSION OF THE IMPEDANCE TRANSFORMER

Two versions are available: a light, varnished version or a version with a very good quality waterproof housing.

Antenna wire

You can build an antenna from practically any wire. In multi-band antennas, however, one of its properties is important – it should not stretch, because there will be problems with tuning the antenna

Hallo Field Day multiband dipoles are made of wire reinforced with Kevlar.

  • The antenna wire must be light
  • The antenna wire must be UV resistant
  • The antenna wire must not stretch

 

Other accessories I use

I tested various antenna accessories and selected for you those that I found practical and very well made. Thanks to these accessories, your antenna will be comfortable to use.

HFD antenna isolator

I designed a convenient antenna isolator. It is different from those you often see in stores.
This insulator has clamps that make it easy to tune your antenna. Too short an antenna is no longer a problem

Hanging Portable Antennas

Out of the many ways to hang portable antennas, I chose the ones that I like best.

A mast or an arborist dart. These two solutions are different but can complement each other. I discovered the dart recently. I don’t know how I have been able to live without it so far