Ham radio-a modification to the end fed matching transformer

Ham radio-a modification to the end fed matching transformer

Since I posted my design thoughts on the end fed transformer arrangement I have been experimenting with various ratios,s of turns on both the secondary and primary windings.

Seeing David,s VK3IL and his project I dropped him a line:-

Ham radio-a modification to the end fed matching transformer.   Thank you for uploading this useful article, I have been experimenting with a short counterpoise on an End Fed Half Wave aerial, using 0.05 wavelength on twenty meters it doesn’t, it seem to make any difference to the SWR or the current distribution along with the length radiator. 

Living in an urban surrounding here in the UK my power is always 100 watts or less, I used a T240-43 ferrite ring with 14 turns on the secondary and two on the primary, to give the 49:1 ratio. My system used in the field on 80 meters needs twenty turns secondary suggesting that the impedance is nearer 5000 ohms than 2500. It’s all good fun. 73 to all John G4YDM

David wrote this reply:- very kind of him to do so

Hi John,
Thanks for your comment. With this type of matching unit, the coax shield essentially plays the role of a counterpoise, so adding a separate counterpoise is unlikely to make much difference as you noted. Assuming you mean an FT240-43 toroid, 2 turns at 3.5 MHz will provide a primary inductance of around 3.7uH which is around 82 ohms reactance. As a rule of thumb, you want at least 200 ohms on the transformer primary winding to avoid the transformer impacting the impedance seen by the transmitter too much. This is why the 3:21 – 3:24 ratio is preferred on the lower bands (which would give about 185 ohms at 3.5 MHz, so still barely enough) whilst 2:14 – 2:16 is preferred on the upper bands (to minimize winding capacitance). Also, it’s worth noting that this type of matching unit is much more complex than a simple transformer in practice. If you want to understand further, see Own Duffy’s analysis of a similar transformer here: https://owenduffy.net/blog/?p=11814

I hope this provides some useful information for you to build your own matching system.  73 de John G4YDM.

Ham radio-a modification to the end fed matching transformer

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