This article continues on from several articles that discussed the ARRL EFHW kit transformer, apparently made by hfkits.com.
This article presents a redesign of the transformer to address many of the issues that give rise to poor performance, and bench measurement of the prototype. Keep in mind that the end objective is an antenna SYSTEM and this is but a component of the system, a first step in understanding the system, particularly losses.
This is simply an experimental prototype, it is not presented as an optimal design, but rather an indication of what might be achieved if one approaches the problem with an open mind instead of simply copying a popular design.
- This prototype uses a Fair-rite 5943003801 core, equivalent in size to a FT240-43, it does not use NMG-H material (“Amidon 43”).
- A 3t primary is used, barely sufficient but a substantial improvement on the original 2t primary.
- The winding configuration is a primary of 3t and secondary of 21t, the secondary is close wound with 0.7mm ECW and the primary is wound using CAT5 cable wire wound over the middle of the secondary winding. This is done for two main reasons:
- to permit some level of isolation of the ‘radiator’ and ‘counterpoise’ from the coax feed line with a view to reducing common mode current (most effective at the lower frequencies); and
- to reduce leakage inductance with a view to improving broadband InsertionVSWR.
Leakage inductance
Leakage inductance is the enemy of broadband performance.
Total leakage inductance was assessed by measuring input impedance at 5MHz with the secondary shorted. Total leakage inductance is about 190nH (this slightly overestimates leakage inductance due to resonance effects). This is two thirds that of the hfkits.com / ARRL original winding layout.
Through measurement with nominal load
The transformer is loaded with the nominal load comprising several 1% SMD resistors and VNA Port 2.
Above is a SimNEC design model, calibrated against measured input impedance with the nominal load.
Above is a pic of the measurement setup, a NanoVNA-H4 is to the right of the equipment shown. Note that the NanoVNA does not correct Port 2 mismatch error.
The transformers are described at Conversion of NOELEC style balun board to 1:1. The setup was SOLIT calibrated, the reference plane was the output side of the grey terminal blocks.
Above is a close up view of the prototype transformer with compensation capacitor. The loads are two 1% 2400Ω in parallel making 1200Ω, one in each secondary leg.
Measurement of the transformer was saved as a .s2p file.
Above is a SimNEC model importing the s2p file and adjusting for the voltage division of the 2400Ω and Port 2 impedance (assumed 50Ω, but uncorrected). The indicated Loss is just a little higher than predicted by the model, keep in mind that ferrites have quite wide tolerances.
Above is a plot of measured ReturnLoss and InsertionVSWR from the .s2p file.
Above is a plot of measured InsertionLoss from the .s2p file, and its components Loss and MismatchLoss. See Measurement of various loss quantities with a VNA for discussion of loss terms.
Common mode impedance
Mention was made that the common mode impedance may help to reduce common mode current at lower frequencies. Zcm will look like a small capacitance below 10MHz, of the order of 10pF for the layout shown. For that reason, I would not deploy an inductive common mode choke near to the transformer, put it at the other end of the feed line.
Conclusions
This is simply an experimental prototype, it is not presented as an optimal design, but rather an indication of what might be achieved if one approaches the problem with an open mind instead of simply copying a popular design.
The revised transformer has substantially better performance than the original: ARRL EFHW (hfkits.com) antenna kit transformer – measurement .
The modifications are mainly about:
- sufficient magnetising impedance; and
- reducing leakage inductance.
Loss remains a little higher than I would, an opportunity for the reader to find further improvement, a learning opportunity!