You are about to build a portable 9:1 End Fed Random Wire (EFRW) antenna! The idea behind this PCB is to make it as easy as possible for you to build the antenna and eliminate any issues around the wiring of the transformer.
A key point with this design is that you do not want to use a resonant antenna length for the antenna to work. You need to use a length of antenna wire that will give you an antenna impedance close to 450 ohms. Let me explain.
The way a 9:1 UNUN works is by transforming the impedance at the end of the antenna wire from 450 ohms to 50 ohms, 50 ohms being the required impedance your transceiver requires. This can only be done by picking an antenna length that presents itself as 450 ohms when connected to the 9:1 UNUN.
So just bear in mind you will need a specific length of antenna wire to get a 450-ohm feed point. Good choices of wire length are:
Length in Meters | Length in feet |
8.84 | 29 |
10.82 | 35.5 |
12.5 | 41 |
17.68 | 58 |
21.64 | 71 |
25.6 | 84 |
If you’re going to use this kit for 80m, then you should ideally have a random wire length of 12.5m or, better, 17.68m.
In addition to the length of the antenna, you will need a counterpoise or ground radials for the antenna to work well.
We would recommend one raised/elevated counterpoise at least 4.57m long; this needs to be facing away from the antenna. Or, a minimum of eight random-length radials; they don’t need to be very long, but the longer, the better.
You will find in your kit one 18m and one 5m length of our ultra-light antenna wire. You will need to trim both to the desired size, but these are close and will give you a great start with the antenna. Just trim them to the antenna length of 17.68m and counterpoise of 4.57m. We are assuming the counterpoise will be raised.
Please google radials; there is a lot of information on them out there. The basic rule is to try to use as many ground radials as possible, or an elevated radial shorter than the antenna but non-resonant in length.
An ATU is also preferable for this type of antenna. Lastly, please use a common mode choke. We sell them, and our portable one will work fine with this antenna.
Building the transformer
With a 9:1 transformer, there is a very specific way to wind the transformer. See the circuit below. However, with this kit, we have made the connections for you on the PCB, so you only need to worry about the trifilar windings. They just get soldered into the correct holes.
We have supplied three colored wires to help you match your winding to the diagrams. Unfortunately, we could not find Blue, Red, and Yellow. Your kit will have Blue, Red, and Gold. So, to start with, you will need to wind nine or ten bifilar turns with the wire.
Each turn through the center of the transformer counts as one turn. You can see in the picture below there are nine turns through the core. You will have enough wire for ten turns with a little spare. I made mine with nine turns; the more turns you can get in, the better. Anywhere above eight turns is okay, just try to make them as even as you can around the toroid.
So, let's wind; we're going to do three windings, giving us trifilar windings. You can do this one of two ways. I tend to wind one wire around the toroid first to get the spacing right. Then I wind the second, then the third wire.
Or, you can hold all the wires together and wind them in one go. Either way, you're looking to end up with your winding looking like the image below.
It's important to keep the wires parallel to each other as above. You now need to strip the insulation from the wires. I use a soldering iron, but a lighter or file will work. Your wires should look like this.
The next part of the build is to push each of the wires through the holes in the PCB. The holes look like this.
You just keep the wires in the same order they are wound in. Put them through the holes and solder them in place. This is the only fiddly bit of the build; I pushed each wire through one at a time, then soldered them all from the back of the PCB with the toroid sitting on the desk. You may want to cable tie the toroid in place first to get everything aligned. It's easier, but if you make a mistake (I did the first time), you may have to cut the cable ties to fix any issues.
When you have soldered the toroid in place, it should look like this.
You can see the gold, red, and black wires are in the same order at the top and bottom of the PCB. Also, I'm using very runny solder; apologies for the mess.
When you have wound the transformer soldered on top of the PCB, you're nearly there, woohoo progress.
All that remains now is to fit the SO230 or BNC (depending on the kit) and solder the center pin to the "Connect to XXXXX center" label on the PCB. Use an offcut of the copper wire for this part of the build.
Lastly, fit your M5 bolts, washers, nuts and wing nuts and your done.
Thats it, now to test.
If you have an antenna analyzer or VNA, you can perform a test on the kit.
All you need to do is get a good quality low watt-rated resistor of 470 ohms. Connect one end to the antenna connection and one to the ground/counterpoise connection, as shown below.
Now sweep the transformer from 1 MHz to 50 MHz; you should see something like this.
This is a 200-2 core, which is ideal for the 40m to 10m band. If you picked the T140-43 option, you will see a lower SWR on the lower bands.
If you do, then well done, you have built the kit! If not, then there may be something wrong. Just go back and check your work, but try a different resistor first.
All you need to do now is connect your antenna wire and trim it to get a low SWR on the bands you want to use.
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