The
challenging UNUN Transformer for End-Fed Half-Wave (EFHW) antenna
An end-fed half-wave antenna looks like a "dream" antenna for
HF. Theoretically
it is a multi-band
antenna but there
are several trade-offs. One challenge is the feeding UNUN transformer. Typically an UNUN
is made for 1:49
impedance ratio. An UNUN looks simple to construct but it is not easy
to achieve wide band performance. Because of the huge impedance transformation
ratio a compensation capacitor is needed to achieve at least a reasonable SWR on all bands.
Additionally an end-fed antenna needs always a counterpoise which can be
radials, tower, counterpoise
wire or
even the feeding coaxial cable shield.
Typical end-fed UNUN transformer schematics Optional high Q compensation RF capacitor 47pF - 100pF/2kV But the most challenging thing is the power handling
capability of an UNUN transformer.
The ferrite core may easily heat up to Curie-temperature
due to the extreme flux density in a ferrite.
At Curie-temperature
the ferrite
permeability drops to zero and an UNUN represents a short circuit which may break
your transceiver RF power amplifier or break your linear amplifier.
Typical end-fed UNUN transformer construction Primary 2-turns (twisted on secondary), secondary 14-turns PTFE insulated Amidon FT-240-43 = FairRite 2643803802, cross-section 1.57cm2, Curie-T = 130ºC
Flux density equation [gauss] You can also use this VK1SV flux density calculator The highest flux density occurs on the lowest frequency, thus 1.8 MHz is the most demanding. To reduce the flux density, few toroids can be stacked to increase the cross-section area.
============================= CONCLUSION ============================= Do not trust to the power handling specs of commercially available end-fed antennas! A 100W end-fed UNUN transformer can be made rather easily but a QRO UNUN needs caution! ====================================================================== Read more in Steven's K1RF end-fed PDF presentation (pages 8-18) Update 2021-12-18 OH7SV Back to OH7SV ham projects |