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

Frequency
1.8 MHz
3.5 MHz
7 MHz
14 MHz
21 MHz
28 MHz
Max Flux den.
115 gauss
85 gauss
57 gauss
42 gauss
36 gauss
30 gauss
Amidon recommends these maximum flux densities to avoid excessive heating


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.

Toroid FT-240-43 (1.8MHz)
Continuous carrier
50% duty cycle
25% approx SSB
Single toroid
30W
60W
120W
2 pcs stacked toroids
120W
240W
480W
3 pcs stacked toroids
270W
540W
1080W
Conservative power handling capability on 160 meter band

Toroid FT-240-43 (3.5MHz)
Continuous carrier
50% duty cycle
25% approx SSB
Single toroid
50W
100W
200W
2 pcs stacked toroids
200W
400W
800W
3 pcs stacked toroids
450W
900W
1800W
Conservative power handling capability on 80 meter band

============================= 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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