The challenging UNUN Transformer for End-Fed Half-Wave 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 2020-10-28 OH7SV
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