End Fed Half Wave (EFHW) Antenna Upgrade Part 1 - The Matching Transformer

For several years the EF-10/20/40 MKII “EndFedz “ has been my main station antenna for HF. It is basically an end fed half wave dipole on 20m/40m and an end fed full wave dipole on 10m. The antenna is effective and the materials and construction are of a very high quality. However, a key restriction is its 25W power rating, due to the small matching transformer. A lot of radio amateurs have made similar antennas to this with larger transformers for higher power handling. Dutch radio amateurs in particular have been active in this area:
John, PD7MAA's blog -
Jos van den Helm's (PA1ZP) article published in Radcom:
"A 3- or 5-Band End Fed Antenna", February 2016, Radcom, published by the RSGB in the UK.

So I built myself an improved replacement matching transformer. In fact, this was the second one I had built, as I had also made one for a 30m/15m end fed antenna.

The finished transformer is shown below. The transformer consists of:
Primary winding - two turns of 0.94mm (approximately 20 SWG) enamelled copper wire.
Secondary winding - 16 turns of 22 SWG enamelled copper wire, bifilar* wound with the primary winding.
Core - FT140-43 toroid.

* "Bifilar wound" means that the primary and secondary wire are twisted together before winding onto the toroid. This reduces the leakage reactance and improves coupling.

Before winding the coil I wrapped the toroid with PTFE tape. This was a (probably unnecessary) precaution to improve the insulation, since the transformer may be operating at fairly high voltages.

Most constructors seem to use the reverse winding technique as illustrated here:
This practice seems to originate from designs for common-mode chokes (baluns). The original idea was that this would reduce the interwinding capacitance by physically seperating the start and the finish of the coil. I am not convinced that this has any real benefit, since it all depends on the density of windings around the core. In fact, I wonder if the method may increase the leakage reactance, hence I did not bother using this technique for my transformer.

The ground end of the primary and secondary winding were joined together and connected to the shell of the SO239 connector via a solder tag. The 'hot' end of the primary winding was soldered to the centre pin of the connector. A 150pF 500V silver mica capacitor was connected across the primary winding. This improves the SWR of the transformer at 28MHz (I believe it cancels out the transformers leakage reactance).

The 'hot' end of the secondary winding connects to the end of the antenna via M5 stainless steel hardware.

I made the backplate from 3mm plastic. The box lid and all holes were sealed with RTV silicone sealant. It is important to use a sealant intended for electronics applications. Standard silicone bathroom sealant contains acetic acid (you can smell this) which reduces the cure time, but can corrode electrical connections. As an added precaution I put a small bag of silicone gel crystals in the box to absorb any moisture that might still get in there. Commercially-made matching transformers and baluns are usually encapsulated with some kind of resin, presumable to make them waterproof. I am not convinced this is a good idea. Since potting compounds will have a much higher dielectric constant (permittivity) than air, I would imagine this would increase the interwinding capacitance, thus lowering the self-resonant frequency.

The purpose of the transformer is to transform the high impedance at the end of the dipole (around four kilohms) to the 50 Ω of the transceiver. I tested the transformer using a 3.94 kΩ dummy load made from (non-inductive) carbon composition resistors. It gave the following VSWR readings on the different amateur bands:
1.4 @ 3.5 MHz
1.0 @ 7 MHz
1.2 @ 10.1 MHz
1.4 @ 14 MHz
1.6 @ 21 MHz
1.2 @ 28 MHz.

I will be covering the upgrade of the EFHW antenna wire and loading coil in a future post.

Disclaimer: This is my personal blog. Views expressed in my posts are my own and not of my employer. The information provided comes with no warranty. I cannot be held responsible for the content of external websites. Any practical work you undertake is done at your own risk. Please make health and safety your number one priority.