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EFHW with built in Common Mode Choke

Updated: Apr 24, 2023

I wanted to answer the question of where to put a common mode choke on an EFHW and do I need a counterpoise all at the same time. Kinda relating to this blog post.


Normally an EFHW uses the outside of the coax as the counterpoinse and the coax isdeally needs to be at least 5% of the lowest band you operate. In reality it tends to be the length of the coax to tranceiver. For example at my QTH it's 8.4m from the EFHW transformer to my tranceiver, with a CMC at the tranceiver end.


The build principle is pretty simple, put two T140-43 in series, one setup as a basic CMC with a DB loss of approx. 30db+ from 3mhz to 30mhz (I'm ignoring reactance in the choke for now). Then follow it with an earth/counterpoise post and the 49:1 EFHW transformer. This arrangement is good for about 100W.


The design


After the design came the build. I drew a case for the CMC/EFHW and printed it out (I might make the design files public for this design). Installed the hardware as per the schematic.


The build


Quite simple to buid, but does need some hot glue to keep everything in place. Also some liquid electrical tape as well.


Built now a scan with my AA54 antenna analyser with a 3.2kohm resister in place.



As expected the scan looked pretty similar to other EFHW I have built.




The testing


I did test the antenna as a verticle pole in my back garden with about 10.1m antenna wire and .15m counterpoise. It worked, with the counterpoise and did't work at all well without it. I forgot to take images of the scans so my apologies.


The advantage of this design is you are isolating your feed line from the antenna. The disadvantage is the outside of the feed line to tranceiver can still pickup local noise and ideally you should have a second CMC at the tranceiver end. However, if you are on top of a hill somwhere it's unlikely you will have local noise to contend with.


I have sent the transformer to Carl M0SZT, he has a nice youtube channel I would recommend and will be testing the antenna soon.


I'm not sure I will put this into production yet, let me know how you feel about the design.


73's

M0VUE

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