Hams impoverished for space to erect a ‘full size' antenna for HF pursue a range of options and end fed wires figure highly.
New hams who heard the maxim that ‘any antenna is better than no antenna at all' will try to use bed frames, window frames, balcony railings, anything metallic to get on air. This cry for mediocrity is to prevail over a quest for understanding, ham radio as it has evolved.
The ‘net abounds with conflicting advice:
- (Yates 2010) recommends a T50-2 powdered iron core;
- (VK2AVR 2014) states
do not use a powdered iron toroid.. they won't work.
They cannot both be correct.
(PA3HHO nd) gives a design for an antenna system that incorporates a matching transformer using a T140-43 core for 40-10m bands.
Above is PA3HHO's transformer, it is an autotransformer with a two turn primary, and the concept is to adjust the antenna length for a ‘perfect match'. Some other designs use a variable capacitor in parallel with the secondary to ‘tune' the system for best match.
Either way, if the system delivers a 50Ω resistive load to the coax connector, then a good approximation of core losses can be obtained by measuring or estimating the complex impedance or admittance of the two turn primary winding.
Above is a calculation of the impedance and admittance of the primary winding with no other connection to the transformer. The R and G components are due to core losses and can be used to estimate efficiency when loaded for 50Ω total input Z. Efficiency is the power out of the transformer divided by the power input to the transformer, and ignoring copper losses:
From elementary circuit theory, we can calculate:
just over one third of the input power is converted to heat in the ferrite core. Not an attractive proposition if using QRP (as is almost always the case with these things).
Low efficiency of the matching system is a feature of many of these designs, more so those using ferrite transformers.
Do not read this short article to mean that ferrite cores are generally unsuited to this application, or that powdered iron cores a better suited… it comes down to transformer design and a feature of articles describing these things is that (almost) no one lays out quantitative design rationale.
- PA3HHO. nd. Multi/any band end fed. http://pa3hho.wordpress.com/antennes/multiany-band-end-fed-english/ (accessed 08/09/2014).
- VK2AVR. Sep 2014. Posting on VKLOGGER.COM. http://www.vklogger.com/forum/viewtopic.php?f=41&t=12537&p=49320#p49320 (accessed 08/09/2014).
- Yates, S. Jan 2010. The end fed half wave antenna. http://www.aa5tb.com/efha.html (accessed 08/09/14).