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Building the EFHW Antenna Kits (EFHW broad band transformer).

Updated: Jan 6

We have written this article as a guide to building a Radio-Stuff.com End Fed Half Wave antenna kit.


All our EFHW kits are built the same way, just with different PCB and Core sizes. The all come with either BNC or SO239 connectors. Some of the kits will also include wire, please check the product, and option you have ordered.


Before moving onto the build, I would like just to like to say there is an awful lot of information out there about how to wind toroid’s and how efficient EFHW are. There is some great work out there, also there are a lot of misleading work. Some YouTube videos are excellent some misleading.


However, our instructions will give you a working EFHW, maybe not the most efficient on any given single band, but one that works as a multiband antenna on the band ranges.


Also, do not get stressed about the build. Layout the parts, take you time and enjoy it.


There is nothing like completing a radio amateur project and contacting your first contact using it. Lastly, if you get stuck drop us an email with pictures and we will help you. We are licence holders and we are trying to help the community.


Using our MK1 Ultra End Feed kit as an example it comprised of;


The PCB
Single Fair-Rite T50-43 core
M3 fittings including wing nut and solder tab
100pf capacitor
Approx. 1m enamelled copper wire
BNC connector
Approx. 10.25m green SOTA wire
Kite winder and 30m nylon string




Firstly check everything is there, if its not contact us.


The first this we would suggest you do is wind the toroid. This is done but creating a bifilar wind, this is done by bending a section of the wire back on itself and then twisting it.

If you have some cotton lying about or thin wire use these to test the build first. When you are comfortable with how much wire you need use the wire supplied.  See below.



You will then need to twist the wire to give you bifilar windings. The twists below were done using a small vice. If you do not have a vice, use your fingers. It's not too important how tight the twists are.



To make a 49:1 transformer pass the bifilar windings through the core twice, then five more turns of the single wire giving you seven turns on the toroid. You then pass the wire across through the toroid centre (8th turn), then six more windings to give you a total of 14 windings with 2 bifilar giving a 49:1 transformer, see below.




Cut the excess of the wire giving yourself enough to poke each wire through the solder points. Remove the enamelling at the end of each wire. I use a soldering iron to burn the enamelling off with a little solder, a file or sharp knife will also work (at your own risk, trust me I know). The wire is solderable, so you can just poke it thorugh the holes and solder it.




Push the end of each through the holes. It is important to push the bifilar windings through marked "Bifilar".


Then solder all the remaining components in place, our PCBs now have words for each component on the PCB.




Above, you can see I have connected the antenna wire to the PCB by making a knot through one of the strain relief holes.  Then soldering the wire to a solder tab and securing using the supplied wing nut.


This is the best thing to do to ensure mechanical reliability of the antenna when you’re operating.




When the transformer is built you can test it.


Impedance reduction test


With a VNA or Antenna Analyser by putting a resister between the values of 2.5-3.2kohm antenna connection and outer connector of the BNC connector (or GND connection if present). You should see some results like the ones in this article.


Continuity test


If you do not have a VNA or Antenna Analyser don’t worry.  You can do a continuity test using a multi meter set to Ohm’s.  Check the BCN inner and outer, these should be zero ohms.  BCN inner and outer to the antenna connector, again zero ohms.

 

If you do not have a VNA, Antenna Analyser or Multi meter, don’t worry if your soldering is good, you should be fine.


Additional comment for our white SO239 and BNC board, you will need to solder a small piece of wire from the center of the SO239 or BNC to the hole just before the capacitor, see below.





Morten LB0FI has also done a build video for the the board above here.


Tuning your antenna


If we have supplied the antenna wire it is going to be to long.  This is because we do not know how you will setup the antenna up in the field.


So, some tips on tuning the antenna.  Firstly, this is the approximate length of wire you will need for a 40m-10m multiband antenna is 19.41 meters.  This number is calculated as 143/frequency * velocity factor of the wire (.95) in free space.


However, as an example the fibreglass pole uses is about 7m high.  I use it as an inverted V with about a 140-degree angle and its apex.  Due to the centre of the antenna being low to the ground it is electrically shorter than expected. 


My antenna is about 18.7m long?


Therefore, we would suggest you start with about 20.5m of wire, set it up how you are going to use it and trim it until you reach the bottom of the lowest frequency you want to use.  For example, tuned to 7.000mhz should also work well on 20m, 15m and 10m.


If you have an Antenna Analyser or VNA this is going to be easy to do.  Or a transceiver with an SWR graph is going to be quite easy.


If you do not have either and your transceiver has inbuilt SWR meter, then start at 20.5m on the 40m band a check the SWR on the bottom of the band and then the top (7.000mhz then 7.200mhz).  If you see an SWR low at 7mhz and higher at 7.2mhz you are about there.


Never transmit (unless you are happy to do so) with an SWR higher than 1:5, you may damage the finals in your transceiver.  Also, get your friends or amateur radio club members to help you if your stuck.


When the antenna is trimmed it will work well if you are using similar conditions in the future. 


Mine was tuned in my back garden years ago and has been consistently great out in the field.  You can see a good example of my antenna working in this video at the time of 5 miutes form the start. Yes that's M0VUE in the video.

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