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Building the EFHW with Common Mode Choke

Updated: 7 days ago

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

Also, you are about to build a relatively new concept in antenna design.  Not that an End Fed Half Wave (EFHW) is unique or a Common Mode Choke (CMC).  But the combination of both on a single PCB is.

The idea behind this PCB is the have your antenna isolated from your coax.  Unlike most other EFHW designs this is not the case, they use your coax as a counterpoise. Which quite often means you really don't know whats happening on the coax. With this kit your coax is isolated from the antenna and if you have a CMC at the tranceiver the coax is also isolated there as well.

In theory, this means that all common mode current and noise are eliminated from the tranceiver and you will know via measurements how the antenna will perform.

The flip side is you must have a counterpoise on the antenna itself.  We will get into that a little later.


So lets start.



This is not an absolute beginner kit because adding a T240-43 to the board can be fiddly, this is the only part of the build which is not as easy as our other kits to get right. 

Also take special care when drilling the case, I suggest the counterpoise hole is drilled at least halfway down the box towards the SO239 connector.  This is to keep the counterpoise clear of the antenna wire. 

Also, it helps with pigtail tuning on higher bands.  I’ll discuss pigtail tuning in a later post.

Other than the two points above this is no harder to build than our other EFHW kits.  The build time should take about 1-2 hours, if you are taking it easy and drinking tea or coffee.



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 to a post or hand from something.

You may want to add an eye and thimble to ensure the antenna is very secure, this will be in the kit.  However, the antenna EFHW-CMC I made in these images is used on a 12.5m fibreglass post with traps for 20m, 15m and 10m so I did not to use them.

If you are going to use the eye and thimble you will need to drill one hole in the top for the eye and two on the side for the antenna wire and counterpoise.  If you not going to use the eye, you do not need to drill the additional.


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.  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 is you are using the eye or 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 screws 12mm in the top two holes, it makes the final build easier.

Thats the case drilled.



Building the transformer

It’s entirely up to you how you want to wind the transformer. 

In this example it’s a 56:1 EFHW that’s 15 primary and 2 secondary windings.  This works very well with a 1002 core.  If you want to create an autotransformer or other type of winding your build will differ from mine. You will see a small chip in the toroid below, I dropped it. Be careful.


When you have would the transformer put it on top of the PCB.  You will need to trim the wires going through the PCB, do this now.  You need about 2 to 3 mm of clearly visible enamelled wire on the reverse side of the PCB.

It should look something like this when trimmed.

This is a good time to clean off the last part of the wire you are going to solder to the PCB.  I find a file, knife, lighter or soldering iron is ok to do this job.


Building the choke

It’s also entirely up to you how you want to wind the CMC.  I normally do 12 turns of coax through the 240-43 toroid or 8 through the 2631101902 toroid.  The one thing I would say is ensure you have enough coax on each side of the toroid to be to solder to the PCB pads and SO239.

To wind a 240-43 thisis what you need todo. First get the parts you need.

Then attach one end.

Wind 5 turns, then across the midde that's 6, then 6 more.

Just ensure you end up starting on one side and ending on the other. Or its going to be a nightmare for you to make all the solder joints.


Final assembly

Next step. I would then suggest you solder the transformer and capacitor in place. Then secure with the transformer with the provided cable ties.

Then you can attach one side of the Common Mode Choke to the PCB.

You’re going to strip some of coax outer insulation and trim to length the braid, and centre to create a Y in the coax.  The inner centre conductor going left into the Centre of Coax pad and the outer going onto the Coax shield pad.  Do not short out the centre and braid.  If you do your antenna will not work.  You will see on the picture I have then soldered the coax in place.

Also when soldering ensure the soder flows to both sides of the PCB. The tracks are on both sides to ensure it will handle 400w SSB. I would suggest soldering from the bottom of the PCB. Then if needed the top. However a good soldering iron and flux solder should do the job in one go.

Now this is the tricky part if you have selected the T240 and RG58 option. 

When you wound your choke (providing you have done this the way I have).  You will have one end of the coax on one side of the toroid and the other end on the other side.

It’s good to have it this way, it means you have a natural layout for attaching the coax to the PCB and SO239. In the image above you will see I have used the lower part of the Coax for the PCB.


Now cable tie down the toroid, then place the PCB in the enclosure with the transformer and toroid in situ.


With what is left of the coax I suggest you cut it just after the enclosure, mark back to about 5mm past the outer diameter of the toroid.

You’re now going to strip the outer insulation from coax and make a T with the shield going left right and the centre going forward. You’re going to need to eyeball how far back you strip the centre insulator.  Strip it back and check it all fits.  You are looking for a fit where the coax centre goes into the SO239 connector.

 Now you will need to solder the centre of the coax into the SO239.  You may find it easier to tin the centre of the coax and pre-solder the SO239 and just re-heat and push the centre in place.

Either way you should not have the centre in place.  Now offer up the shield T to the 12mm screws.  Trim the shield and solder on the solder tabs.  Now you can bolt the tabs into place.


The last thing you need to do is solder on the antenna and counterpoise solder tabs in place and put the supplied M5x16mm bolts through the solder tab out through the enclosure holed you drilled and then tighten then up with the supplied washers and nuts.

Also screw the PCB in place with the provided M3 screws.

Your done!

If you want to secure the nust and bolts from the weather and also the SO239 you could use a glue gun. Also, for the coax at the capacitor side you could insulate that with glue or nail varnish.

This is what I have done on my build.


It’s at this point you may have realised you drilled the 5mm holes in the wrong place.  So hopefully you read these instructions 1st.




This is simple to test now if you have a VNA of Antenna Analyser.  You can place a 2.4kohm transistor from the antenna to the counterpoise and scan from 1mhz to 30mhz.  You will see the sweep lower at around 3mhz trhey go up a bit around 28mhz.

I have not included the sweep of the antenna here yet.

Please not because you have a combined transformer and common mode choke the SWR on the sweep will be slightly higher than a transformer on its own. 



To tune is easy.  Cut antenna wire and a counterpoise to the suggested lengths. Here is a brief table of some suggested wire lengths.

Frequency MHZ

Wire Velocity Factor

Half Wave

Quarter Wave

Full wave
































The key here is the counterpoise, this MUST be added to this antenna for it to work.  Your coax will be isolated from the EFHW transformer, therefore your coax will not be a counterpoise.


Put your antenna analyser on the EFHW-CMC and the wire up in the air where its going to be and tune for a low SWR.  Or low SWR and Resonance.  The second being more difficult and sometimes impossible.  But a low SWR is a must at the bottom of the lowest band of use.  Like 7.000 mhz.

If you want to see what happens to an EFHW without a counterpoise remove it after you have tuned your antenna and seep it again.

We will put some notes on alternate tuning methods on the blog soon.

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