Sunday, 26 June 2011

Flaw discovered in EFHWA matching unit

There has been a very interesting development in my quest to know a bit more about the End Fed Half Wave antenna (EFHWA).

A posting made by Mike ZS6BIM here:

 This got me thinking. I did quite a bit of research and it seems Mike ZS6BIM is correct. The T50-2 toroid seems to be way too small to be effectively used in the EFHWA antenna matching unit as is described by many, including Steve Yates AA5TB on his web site.

The values of peak AC Magnetic Flux (B) appear to be much higher than what would be considered acceptable. This will result in very high core losses.

My calculations reveal the following (hopefully they are not incorrect):
T50-2  -- 7 MHz – 3 turns – 10 watt (31.6 V) è Peak Flux (B) = 303 Gauss and a core loss of 15274 mW/cm^3
The recommended maximum value of (B) is 57 at 7 MHz.

If the toroid was replaced with a T130-2 then the data looks much more acceptable.
T130-2  -- 7 MHz – 3 turns – 10 watt (31.6 V) è Peak Flux (B) =  49 Gauss and a core loss of 278 mW/cm^3

I suspect that we sometimes under estimate our  5 watts QRP and we use components that are not really suited for the task.

73


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