# RaDAR – The end fed tuner dilemma

The following was quite sufficient for my understanding of end feds and worked well for me, but how well? This tuner supposedly works for 40m right up to 15m. It uses a T50-2 toroid wound with 24 turns on the scondary, link coupled with 3 turns on the primary. An impedance transformer.

Here’s an interesting interpretation. The impedance transformation ratio is important here. Our transformer info lies at the bottom of the list …. an impedance ratio of 1:64. So for 50 ohms on the primary where the rig is connected, we’d expect to see 3200 ohms on the secondary where the tuned circuit resides. Now that sounds more like an end fed’s feedpoint impedance! The circuit on the secondary also looks more like this. The impedance at resonance is high, typically similar to that of the end fed!

So, this was my original understanding of an end fed tuner from the beginning. It made “no difference” what the inductance / capacitance ratios were as long as you have a resonant circuit to which the end fed was connected …..

The impedance transformation ratio is not always a hundred percent predictable which made the Fuchs tuner popular. It was also the first end fed tuner I built. Variable ratios, variable inductance and capacitance for the parallel tuned circuit on the secondary! So why use a simple “L Match” with the complexity and requirements for suitable inductances (and switching) per impedance when you can simply use a link coupled parallel tuned circuit (Impedance transformer) capable of resonance at the required frequencies?

That is the question ……. ?

An interesting comment I found on eham.net written by Dale Hunt, “Notice, however, that the number of turns does NOT depend on the operating frequency!  The capacitor has to be able to tune the secondary to resonance (taking into account any stray reactance).  A variable capacitor with a 9 : 1 capacitance range will cover a 3 : 1 frequency range, so if the coil inductance is chosen carefully a single variable will cover all three bands.  20m near the minimum, 40m near the  maximum, and 30m in the middle somewhere.  For any specific capacitance range, you can calculate the desired inductance to tune the necessary range.  In your case, 10 to 100pf with 10uH will tune 5 to 16 MHz. 10uH requires about 45 turns on a T-50-2 core.  If you use a 5 turn primary that gives a 9 : 1 turns ratio.  You can vary the number of turns to see what gives the better match with the specific wires and capacitor you are using (and any stray circuit capacitance, which will reduce the tuning range.) “.

Ideal for the HB1A!!!

So, then a 10uH inductor wound on a T80-2 toroid will need 43 turns (103 cm of wire) and the following capacitance values / band.

40m – 50 pF

30m – 25 pF

20m – 13 pF

73 de Eddie ZS6BNE

## 2 thoughts on “RaDAR – The end fed tuner dilemma”

1. Pierre |

The spread sheet is ONLY applicable to the LC AMU i.e. in the low pass configuration. The spread sheet was developed to enable the builder to use any frequency and or feed point impedance greater than the input impedance e.g. a EFHWA with a feed point impedance of around 2,800 Ohms. The spread sheet cannot be used for any other configurations…..