An End-Fed Half Wave Antenna – with no impedance transformer?

One of the great benefits of being a ham radio blogger is that it opens up a new door to learning. Through that door comes valuable input from readers of Ham Radio Outside the Box. Let me state one thing very clearly: I am not an expert; the intent of this blog is to report experiments I have performed in the hobby. When I err I humbly accept corrections proferred by other hams. In that vein I am going to give a shout out to Dale WB6BYU who commented on my post “An Off-Center Fed Sleeve Dipole” to correct my misconception regarding the relationship between impedance and SWR. Dale is the owner of the excellent website at practicalantennas.com.

Another contributor, via email, is Edwin DK2TY. Edwin contacted me to tell me about his project involving an L-match to tune an end-fed wire. The project sounded very interesting and will be discussed in a future post here on Ham Radio Outside the Box. Edwin also pointed me to a very interesting article written by Martin Blustine K1FQL published on the website of the Nashua Area Radio Society in 2022.

K1FQL’s article provides a very detailed description of how to build an L-network to match the very high impedance at the feed end of a half-wave antenna. End-Fed Half-Wave antennas are usually matched to 50 ohm feedlines by way of an impedance transformer – typically 49:1 – but, as we have previously discussed here on Ham Radio Outside the Box, despite the enormous popularity of these “49:1 ununs”, many dispute their efficiency. By contrast, an L-network is heralded as much more efficient. If any reader can expand on why impedance transformers are inefficient while L-networks are the opposite, your input would be most welcome.

Where’s the Gotcha?

An impedance transformer is a broadband device. As Ham Radio Outside the Box has discussed in the past, it could possibly be used on multiple bands – with some reservations. An L-network, on the other hand, is a single band device. If you like to band hop this may not be for you.

I decided to pursue the idea of building my own L-network following the guidelines in K1FQL’s article. Here is the basic simple schematic:

There is a lot more detail of the construction technique in the original article which is recommended reading. My own build of the L-network is based closely on K1FQL’s design. Rather than repeating what has already been written in that article I will focus on the details of my own build and how it performs.

A little wire and a bit of coax

I chose to build an L-network for the 20-meter band since that is where I spend most of my operating time. The first task was to establish component values for the inductance and capacitance required. The equations are provided in K1FQL’s article so I built a LibreOffice spreadsheet to experiment with different transformation ratios and frequencies. It is important to note that the only essentials required to build this L-network are a length of wire and a short piece of coax. To make it pretty and more practical you can also add a project box and some connectors.

Let the computer do the math

The spreadsheet showed an inductance of 3.36 microhenries and a capacitance of 37.36 picofarads is needed to transform the impedance of my Coil-Loaded End-Fed Half-Wave (CLEFHW) from 1800 ohms down to 50 ohms. Out of curiosity the component values for a more standard 2500 ohms EFHW impedance were also calculated; they turned out to be 3.98 microhenries and 31.83 picofarads – very little difference! Of course these are only guideline values because there are differences between computer models and the real world. In the real world there is stray capacitance and inductance that may vary based on construction techniques.

Aha, that’s why that didn’t work!

For my test L-network, the component values from the spreadsheet did indeed differ slightly from the computed values. It was interesting to understand why my previous attempts to tune an EFHW with an L-match I had built a long time ago was not successful. It was never able to get an SWR below about 5:1 despite having a variable inductor and variable capacitor. The problem arose because the switched variable inductor had increments of around 1 microhenry – too coarse – and the variable capacitor had a fully-meshed capacitance of 350pF which meant it would be very difficult to get the fine adjustments needed.

Winding the coil

Winding the air-core coil is not as easy as one might think. The required inductance is very small so it is advisable to choose a small diameter coil former so that more turns are required. In that way it is easier to tap the coil at the precise inductance values required. My own technique involved several tap points along the coil because – in the real world – surprises are inevitable.

Trimming the capacitor

How can you get a precise value of capacitance that can also withstand the high voltages involved at RF? The solution is very simple: use coax. Coaxial cable has a characteristic capacitance by virtue of its construction and it’s designed to carry high RF voltages. You can look up the value online for any common type of coax. I chose RG-174 because I had a few odd lengths of it in my junque box. The capacitance of this type of coax is around 30pF per foot so it was easy to calculate the length required. The center conductor is one side of the capacitor and the braid is the other side. K1FQL used the center conductor and braid at the same end of the coax for his capacitor. I am not sure that is important; it might be more convenient to connect the center conductor and braid at opposite ends depending on the construction technique. To adjust the capacitance simply snip very small pieces of coax off one end until a capacitance meter shows the required value.

So the concept works – thank you Edwin and Martin

Since I had already established that the feed point impedance of my CLEFHW is 1800 ohms my test setup used an 1800 ohm resistor to simulate the antenna. I hooked up my RigExpert antenna analyzer and was slightly disappointed to find the SWR was higher than expected. However, remembering that I had taken the precaution of using taps along the inductor, a quick adjustment brought the RigExpert’s SWR measurement down to 1.22:1. The resistor was replaced with the CLEFHW’s loading coil and 18.5 feet of wire and that reproduced the result inside my home. Now it only remains to tidy up the construction and take it to the field.

An update on winter chez Ham Radio Outside the Box

Spring may be on the way and the great thaw has begun. That is both good news and bad news. Our heavy snowfall this winter still lies deep and crisp and even on the trails and in the parks. As snow falls it compresses the snow already on the ground and turns it to heavy ice which takes longer to thaw. My driveway became temporarily clear of snow as a few warm days and a bit of sunshine gave my snowblower a welcome break from duty. Then the warmer air caused the snow load on my steel roof to loosen. The weight of the compressed snow/ice mix finally overcame the snow guards on the edge of the roof and, with an accompanying sound akin to artillery shells exploding, a huge quantity of the dreaded white stuff crashed onto the driveway blocking our exit from the house and garage.

I could have carried on operating throughout the winter from inside my truck; some call that PLOTA (Parking Lots On The Air). It’s perfectly acceptable for POTA, but not for SOTA. I prefer to get outside to operate anyway and, it’s hard to play with experimental antenna ideas when you are freezing your butt off. An update will be posted when the CLEFHW with an L-network has successfully passed the fresh air test out in the Big Blue Sky Shack. Until then 73 de John VA3KOT.

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