The 49:1 impedance transformer is the heart of any EFHW antenna you will build… most of the time. It doesn’t matter what is the band you will build the antenna for, the 49:1 impedance transformer will always be the same.
The 49:1 impedance transformer will help you bring down the high impedance of your resonant EFHW antenna to almost 50 Ohm. Let’s have a look at a few details on how to build a 49:1 impedance transformer.
You will always have a good antenna with the proper impedance for your TRX. This way you will make sure you have a good SWR reading. Together with an RF choke you should have no issues and end up with a nice and easy to build EFHW antenna for your desired bands.
1. THE 49:1 IMPEDANCE TRANSFORMER
There is not much to the 49:1 impedance transformer. This also the beauty of it, as such a simple accessory will help you bring down the impedance antenna to something close to 50 Ohm that your TRX likes. All you need is a toroid ( usually 43 material ), a connector for your coax cable, and a capacitor. Of course there is the additional hardware as well, like an enclosure, some nuts and bolts etc.
So far from from what I experimented with, the best results I achieved were by using a different number of turns, depending on the size of the toroid used to build the 49:1 impedance transformer. Use the standard 2 turns primary and 14 turns secondary for a toroid like FT 240 – 43. For a smaller toroid like FT 140 – 43, FT 114 – 43 or FT 68 – 43 use 3 turns primary and 21 turns secondary as in the second image ( close turns ). I try to avoid a smaller toroid like an FT 50 – 43 as they seam to heat up if the power of the TRX exceeds over 1W.
Use common sense when it comes to the size of the toroid, and the power you want to put into the EFHW antenna you want to build. For QRP the FT 114 – 43 where your power doesn’t go over 15W it should be more than enough. If you want to use 50W maybe an FT 140 – 43 will do. For 100W use FT 240 – 43.
Lately there has been many articles from fellow amateur radio operators experimenting with different ferrite core types and sizes. A great video to watch about the performance, efficiency and power rating of all sort of ferrite cores was made by Colin ( MM0OPX ). Check his YouTube Video and the XML file he made available.
Also in a few notes my friend Nigel sent me ( ZLNAY ), he specified that instead of using twisted wires for the primary, is better to simply solder a tap. The thickness of the wire used will also influence the impedance transformer. The thicker the wire, the less inductance will result and that should keep the losses to a minimum as well. In my case I did not need a ground or counterpose added. But in some cases, depending on the length and position of the antenna, a ground connection or a counterpoise may help with a lower SWR. Here is an image of the 49:1 impedance transformer I built on a FT140-43 toroid.
2. THE CAPACITOR
There is a lot of debate online if you should use the capacitor or not. The reasons for using the 150pF capacitor are many, but we will not go that deep into the theory part of the impedance transformer. I would recommend you to use the capacitor the first time you are building the impedance transformer. This way you don’t have to do any modifications later on. What the capacitor does, is that it will flatten your SWR on the lower bands, like the 20m and 10m band for example.
So if you build an antenna for the 40m band, you may not need it. But if you decide to use the 20m band as well, the capacitor will do a great job. In some cases I noticed that sometimes by lowering the value to only 100pF gives you better results. Try and see what works best for you. The voltage of the capacitor should be at least 100v for QRP. But I am always using capacitors that are rated up to 1.5KW most of the time. Some good results were obtained with only 56pF as tested by my buddy Vlad ( YO5BDL ).
3. THE ENCLOSURE
Thinking that most of the time your antenna will be out in the rain, you will also need a good enclosure. If you do portable QRP for example, you may not need that if you only plan on going out in the nice sunny days. I honestly prefer to put it in a watertight enclosure to keep it away from the rain. I do the same with the antenna I have at home, just as I do for the 49:1 impedance transformer I use for the portable QRP antenna.
If you build it in a nice enclosure, you will not have to do any maintenance work to it and it will last a long time. Plastic enclosures are good enough. Just make sure is strong enough to hold well the rest of the hardware. Lately I started to use thick PVC pipe and build myself custom enclosures that are 100% watertight. So far I had no issues with that and they seem to hold really well. Use what fits you better. In the image I was using an FT114-43 toroid.
4. THE WIRE THIKNESS
Just as I was saying about the size of the toroid you will use depending on the power you use, common sense also works for the thickness of the wire you will use to make the impedance transformer. For QRP power 5W and under, a 0.50mm wire will do just great. For a power up to 20W maybe a 0.7mm would do well. If your power goes up to 50W then 1mm enameled copper wire will be good enough. If you put more power, also try to increase the size of the wire. For 100W I would use 1.5mm.
As the elderly people usually say, having more than enough is always better than having less. You may not need 1.5mm wire for 100W…. and maybe 1mm would be good enough. But it’s always better to have some extra to “play with” rather than not having enough. When placing the wire on the toroid, it’s a good practice to wrap the toroid in plumber’s tape first ( teflon tape). It will protect the toroid and you also avoid eventual scratches on the enameled copper wire. For safety reasons, I prefer to use wire that has two layers of enamel instead of one. Make sure the wire will be as tight as possible on the ferrite core to avoid losses.
Good luck with the project. With a little patience you will build a great 49:1 impedance transformer that will be very useful for your EFHW antenna. Build it carefully, so you don’t have to re-build it again as I did many times.
73 DE YO6DXE