Once I had upgraded the matching transformer and loading coil for my EF-10/20/40 MKII "EndFedz" (see Parts 1 and 2), I thought about replacing the conductors as well. Once I got this idea in my head I couldn’t help thinking about a classic episode of “Only Fools and Horses” (BBC TV):
https://www.youtube.com/watch?v=BUl6PooveJE
By replacing all of the components, the original antenna could be redeployed for portable use.
The original conductors on the EF-10/20/40 have a cross-sectional area of 0.75 sq.mm. I had some 1 sq.mm (32/0.2) PVC coated wire available, so I used this. This is a bit thicker than the original conductor, but that is not a problem for a fixed installation. The replacement wire has a lower resistance than the original, so ought to be more efficient, although not significantly.
I cut the wires a bit longer than the originals and trimmed them for best VSWR. The 14/28 MHz radiator was adjusted first, and then the 7MHz radiator.
The finished antenna performs well, and the increased power rating means I can run it comfortably at 50W, getting better signal reports.
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| Nominal Dimensions |
Unless you are running very low power, say below 5W, I strongly recommend using the antenna with a common-mode choke. To some extent the feeder will act as a counterpoise and therefore there will be common mode RF on the coax. This means that stray RF may find its way back into the ‘shack’ and may be picked up by the microphone cable or ancillary equipment. Putting a common-mode choke inline will prevent RF on the outside of the coax getting back into the shack. According to some sources (ref.1), an end-fed half-wave antenna needs a 0.05 lambda counterpoise to work correctly. In the case of the "EndFedz" EF-10/20/40 and its clones, where we are effectively using the coax as a counterpoise, we can meet this requirement by ensuring the feeder is at least 0.05 wavelengths long at the lowest operating frequency. Therefore, for an antenna operating on a maximum wavelength of 40m, there should be at least 2m of coax between the matching transformer and the common-mode choke.
I made my common-mode choke by winding nine turns of RG316 coax on an FT140-43 ferrite toroid (see photo). A good common-mode choke will have a high common-mode impedance over the range of frequencies being used. The aim is for the impedance to be mainly resistive (rather than reactive). Please refer to the excellent work carried out by Steve Hunt (G3TXQ) on this subject (ref.2). There is no need to encapsulate the choke, especially if it is going to be installed indoors. Potting compound has a higher permittivity than air and would therefore increase the inter-winding capacitance.
Ref.1: http://www.aa5tb.com/efha.html
Ref.2: http://www.karinya.net/g3txq/chokes
Disclaimer: This is my personal blog. Views expressed in my posts are my own and not of my employer. The information provided comes with no warranty. I cannot be held responsible for the content of external websites. Any practical work you undertake is done at your own risk. Please make health and safety your number one priority.
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