End Fed Half Wave (EFHW) antenna kit with build in Common Mode Choke SO239
 
kit is designed for the enthusiast of the End Fed Half Wave antenna. Where it differs from other kits on the market is in its inclusion of Common Mode Choke on the PCB, just before the EFHW Transformer. The benefit of this design is where the transformer is isolated from the feed line. This takes the randomness out of tuning. 
 
With a Common Mode Choke on the PCB, you MUST have a counterpoise installed on the completed antenna.
 
The kit offers exception performance on bands from 80m to 10m we have picked very efficient Fair-Rite cores for the HF bands. 
 
Supplied in 100w and 400w SSB versions. That’s 25w and 100w CW/Data respectively. We have tested both core sets over these limits so we are quite confident that when built and tuned you will be able to comfortably handle the power levels stated above.
 
PCB is designed in the UK, finished in white. It has wide 3mm tracks on both sides of the PCB connecting all the components. Creating these twin tracks allows us to be confident of the power handling requirements for 400w SSB. The PCB is held in place using screws. The CMC and EFHW transformer are held in place using cable ties.
 
The Kit contains.
The PCB designed in the UK
IP rated ABS case
Two Fair-Rite cores on for the Transformer one for the choke, see option details below
M5 & M3 fittings including bolts, nuts, wing nuts, solder tabs and wire crimp connectors
Cable ties
100pf 3kv TDK capacitor
Approx. 1.5m enamelled copper wire
1m coax for the common mode choke
SO239 connector
 
The cores selected for each version of the kit are.
 
400w kit, Fair-Rite 240-43 for the CMC and Fair-Rite Mix 43 2643251002 for the transformer.  With 1m RG58 coax.
 
100w kit, Fair-Rite 2631101902 mix 31 round core for the CMC and Fair-Rite 2643101902 mix 43 for the transformer. With 1m RG316 coax.
 
We have chosen the choke core and EFHW Transformer core based on their exception performance and efficiency across the 80 meter to 10 meter bands.
 
Instruction for the build can be found by scanning the QR code on the back of the PCB.
 
Please read the notes below.
 
Please check your SWR before using the antenna to ensure you have a good match with your transceiver before use.
 
To calculate the length of antenna wire to use start with one half wavelength of wire.  However, here is a brief table of wire lengths.
 
Frequency MHZ
Wire Velocity Factor
Half Wave
Quarter Wave 
Full wave
Counterpoise
3.5
1
40.86
20.43
81.71
4.25
7
1
20.43
10.21
40.86
2.12
14
1
10.21
5.11
20.43
1.06
21.35
1
6.70
3.35
13.40
0.70
28.5
1
5.02
2.51
10.04
0.52
 
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.
 
To tune trim the antenna wire, not the counterpoise.  The values in the table above are likely to be longer that you need.
 
An antenna analyser or VNA is a very handy device to have when building this kit.  You can however manage without on if your tranceiver has an SWR meter built in.
 
Lastly, we still recommended you have a common mode choke at the transceiver side of your shack to eliminate and stray RFI.