top of page
Search
M0VUE

Building the 9:1 End Fed Random Wire antenna with Common Mode Choke

Before we start the build instructions, we would like to say a big thank you for supporting our shop.


You are about to build a relatively new concept in antenna design.  Not that an 9:1 End Fed Random Wire (EFRW) is unique or a Common Mode Choke (CMC) is, but the combination of both on a single PCB has not been released commercially to date.


The idea behind this PCB is to have your antenna isolated from your coax.  Unlike most other EFRW designs where this is not the case. Requiring you to purchase an addition CMC in addition to the 9:1 UNUN.


To help you understand the build a little more and the technical details of a 9:1 we strongly recommend you spend a little time looking at the following YouTube videos and the VK6YSF web page.



So let’s start.

 

Difficulty

This is not an absolute beginner kit because the winding of the toroid for the 9:1 is not as easy as an End Fed Half Wave transformer.


One key point with this design is you do not want to use a resonant antenna length. You need to be very specific about the length of antenna wire you use so the impedance of the antenna at its feed point is close to 450ohms. Let me explain.


The way a 9:1 UNUN works is by transforming the impedance at the end of the antenna wire 450ohms to a 50ohms', 50 ohms’ being the required impedance your transceiver requires.


The simple maths behind this matching is that the impedance of the antenna wire at its feed point is around 450ohms’ and the UNUN needs to transform the impedance at a ratio of 9:1. So if we divide 450ohms' by 9 we get 50ohms’.


So just bear in mind you will need a specific length of antenna wire to get a 450ohm 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.  Please google radials there is a lot of information on them out there. 


We would recommend one raised counterpoise at least 2m long, or a minimum of eight random length radials, they don’t need to be very long, but you do need some.

 

Drilling

We are using the same pictures from our EFHW with CMC kit for the drilling of the enclosure. So, the first thing to do is to drill the case.  You will need to drill 5mm holes for the antenna and counterpoise connections.  The box supplied has flanges to attach the case to a post or suitable support.


For drill bits, 3mm, 4mm, 5mm, 12mm and step drill are what you really need to make things super simple.


If you have a 3d printer and would like to download the drill template drop us a note and we will provide you with the STL file or use this Thingiverse template.  It will help you out a lot.


If you do not have a 3d printer mark where you want to put the components with a sharpie.  To do this I would drill the larger hole 1st put the SO239 in the hole, then mark out where the smaller bolt holes go, then drill them.

 

Using a 3mm drill and the template place the template onto the case and drill as per the image below.

 




Template in place drill all 6 holes.  This is one pilot hole for the SO239, four holes for the SO239 screws and a breather hole for the enclosure (if you want one).




 

Turn the enclosure around.  Use the template to drill one hole putting the antenna connection on the top.




 

Put the template on its side and drill, along one side of the enclosure (take note of where the counterpoise solder pad is on the PCB) and drill holes in the middle of the case where you want the counterpoise.






Attach the S0239 using the screws and nuts provided.  Put the longer screw 12mm in one f the top holes, you will be aligning a 3mm solder tab to this screw.




Thats the case drilled.

 

 

Building the transformer

With a 9:1 transformer there is a very specific way to wind the transformer.  See the circuit below. It's important you look at where each colour goes on the diagram, this will become apparent later in the build.



 

We have supplied three coloured 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 centre of the transformer counts as one turn.  You can see on 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 ok, just try to make them as even as you can around the toroid.


So, lets wind, we're going to do three windings giving us trifilar windings.


Winding 1



Winding 2



Winding 3


I find it easier to wind one wire at a time, this way I can get good spacing with one wire and then just add the others in.


When you have would the transformer put it on top of the PCB, woohoo progress.



You can see I have trimmed of the wires a little and positioned the toroid where it will go when complete.


Now the tricky part with this build. Look at the diagramabove and look at wires you have wound. You will need to wire the transformer as per the wiring diagram. In my example we have the following.





So in my case the wiring is as follows;


The blue is the antenna 450ohm long wire.

The Red top connects to the Blue bottom.

The Gold top connects to the Red bottom.


If you look at the diagram you will see the Gold goes to the Ground/counterpoise hole on the PCB. This hole tracks to the coax braid on the PCB.


The Blue goes to the 450ohm unbalanced hole on the PCB.


Please check how you have wound the toroid, if your colours are in a different location you will need to work out the wiring, what I have said above will not work.  Just be careful at this step.




This is my build, I used my soldering iron to remove the enamelfrom the ends of the wires. After they were trimmed to length. For the Gold to Red, I scrapped the enamel of with a sharp knife, then I soldered the wires. You will also see the placing of the 450ohm Blue. The gold to ground and the Red to Cenre of Coax.


I did solder the board in situ with the top left and right cable ties in place.

 

Building the choke

The choke you have is of the barrel type torroid. To make the choke up you simply pass the provided RG316 through the core 8 or 9 times. Do not pull the choke tight.


Each pass through the middle counts as one turn.



The finished choke.



 

Final assembly


You should attach one side of the Common Mode Choke to the PCB.



Be careful not to short the braid to the centre of the coax.


When this is done, I recommend you pop the PCB into the case to check alignment. This will allow you to work out how much wire you need for the antenna wire bolt and ground/counterpoise bolt.



This is a good time to also trim the coax at the SO239 side and add the 3mm solder tag.






In your kit you will have one 3mm screw longer than the others, this is the one you will attach the 3mm solder tag to on the SO239.


Make up the Pigtail cables you’re going to use for the Ground and Antenna bolts using the 5mm solder tags and some of the leftover wire. Solder the Pigtails to the board (see completed picture below).


If you have not already done so, ensure the SO239 is fitted in the case with the longest screw 12mm aligned with the 3mm solder tag on the coax. You will use four of the five 3mm nuts for the installation of the SO239.


Put the completed PCB into the case at an angle so you can put the bolts for the Ground and Antenna through the 5mm solder tag then push the bolts into the pre-drilled holes in the case.


Then add a washer then a nut to the bolt outside the case. Hold the back of the bolt with a 5mm Allen key or grip them with pliers, then tighten up the nuts on the other side of the case.


At this point I normally slide the PCB in and screw it to the case.


You will need to solder the centre of the coax into the centre of the SO239.  You may find it easier to tin the centre of the coax first. Then attach the 3mm solder tag to the SO239 long screw with the remaining 3mm nut.


You’re done!


This is what the build should look like. You will see I have added some hot glue where I removed the insulation from the 9:1 UNUN.



Thats it, now to test.

If you have an antenna analyser or VNA you can perform a test on the kit.


All you need to do is get a good quality low watt rated resister of 470ohm. Connect one end to the antenna connection and one to the ground/counterpoise connection, see below.



Now sweep the transformer from 1mhz to 50mhz, you should see something like this.



If you do, then well done you have built the kit! If not, then there may may be something wrong. Just go back and check your work but try a different resistor 1st.




 


 

110 views0 comments

Commenti


bottom of page