EFHW Antennas


With the advent of Parks On The Air (POTA) and Summits On The Air (SOTA), the interest and popularity of portable radio operations has soared in recent years. And with that popularity has come the quest to find portable gear and antennas that are convenient, easy to pack and work great. One such antenna that has risen to the top of the portable choices is the End Fed Half Wave (EFHW). These antennas have a resonant radiating element that is 1/2 electrical wave length long. Feeding this antenna at the end of the wire (rather than in the middle as with a typical dipole) presents an impedance mismatch that must be dealt with. A center fed dipole has around 72Ω impedance. An End Fed can have an impedance of 2000-4000Ω. Thus it requires a high impedance matching device or transformer.

EFHWs are great portable choices because they only require one end to be elevated using a mast or a tree. The feed point can be near the ground or it can be up high. You can use many configurations for the wire too from vertical to sloping to inverted Ls and Vs. Also, you do not need any additional ground/counterpoise wire as the EFHW will use the coax shield for this. And, these antennas are efficient because they are resonant and do not require a tuner. They are multi-banded as well in that they will be resonant on band harmonics. You can easily ground them too to dissipate static build up and maintain a low DC potential.

The transformer is the key to making this antenna successful. They typically use a Type 43 toroidal core wrapped with enamel coated wire. Recent experimental work has been done with other core types too, including 52 and 61. Also, new core geometry has been found to provide incredible efficiencies of over 90%. The other component needed for this antenna matching circuit is a 100-120pF capacitor. The cores and capacitors mentioned in this article have been tried first hand and have proven to work favourably. Be sure to check out our guide to transformers and chokes.

Any sized Type 43 toroid will work for a transformer (but some are better than others). We are recommending the 2643251002 Type 43 core with outside diameter of 1.540" (39.12mm). It has incredible efficiency and its "chunky" geometry makes it less likely to heat up when being hit with full duty cycle modes like FT8.

Colin, MM0OPX, has done a tremendous job of experimenting with EFHWs and sharing his findings on his YouTube channel. We recommend his autotransformer build as seen in this video:

Here are a few tips for winding the toroid tightly:

  1. Cut a notch in an old tooth brush. You can use this as a tool to push the enamel wire through the middle of the core. This is way better than feeding it through like a needle and thread. It keeps the wire very tight to the core.
  2. Continue to push the wire through using the tooth brush. The plastic won't scratch the enamel wire.
  3. You can then pull the loop through the center with the tooth brush.
  4. Your wires will be kept very neat and tight to the sides of the core.

You can then mount your toroid core into a project box or container of your choosing and solder the connections to the coax connector, along with your capacitor. You can connect an additional mounting point to ground if you wish to deploy a dedicated counterpoise wire and not use your coax.

A word about windings and ratios. The traditional, multiband EFHW formula is 49:1. This works great and gives you 40m, 20m, 15m and 10m on a ~66' wire. However, through experimentation we have found that a 56:1 ratio works exceptionally well for monoband EFHWs like 20m on a ~33' wire. This higher ratio seems to help peformance when the antenna is deployed at a lower than optimal height. As an example of this, we built a 40m EFHW for use as an emergency/NVIS antenna. This antenna is meant to be deployed very low to the ground. Using an 81:1 ratio worked very well to keep the SWR happy across the entire band. The working theory is that the extra inductance helps compensate for the greater capacitance being closer to ground.

How do the ratios work?

Wire Length: The wire is literally a half wave length for the desired band. There are loads of apps and websites that will calculate this all for you now. Or, you can use the magic formula of 468/frequency which will give you the wire length in feet. Then you can tune and trim to suit.

Thanks for checking out this build!

VE5REV 73!