I was browsing the antenna section of an old ham radio book this morning and the LC tuner configurations were quite prominent. In all cases the tuners consisted of a tapped inductor and a capacitor to ground on the antenna side which is a well known configuration for tuning long wire antennas. A good earth requirement was often mentioned.
The past couple of years the end fed has become popular with me specifically because a good earth is NOT a requirement and fits in well with RaDAR which operates far away from anything close to a perfect world! However, delving deeper and deeper into the complex world of end fed antennas a strive for near perfection appears to be valid.
In that old book I read this morning, mention was made of using a neon tube to indicate maximum brightness when tuning the tuner. This is nothing other than ensuring MAXIMUM POWER TRANSFER through proper matching of the rig’s 50 ohm impedance to the variable high impedances of the long wire antenna and this will be the topic of discussion here.
My goal is to build an antenna tuner similar to that designed by the late Colin Dickman, ZS6U and is well known as the “ZS6U Minishack special”. His tuner uses a coil of specific dimensions and I want to do much the same but using a toroid. The antenna is a 21.03m length of wire which can be used for 40m, 20m, 15m and 10m.
Colin’s calculated optimum values of inductance and capacitance values for the LC tuner for the various frequencies and feedpoint impedances of the 21.03m length of wire.
The toroid I have in my parts box and the same one used for my recent 20m end fed experiment appears to be a T130-2. It’s dimensions are as follows.
In order to achieve the same inductances calculated by Colin, I used an online calculator to determine the windings required for a certain inductance on this specific toroid. The windings can be tapped and taken to a suitable selector switch.
I’ll be using my modified MFJ945e tuner keeping only the SWR meter circuitry but removing all the other coils and replacing them with this configuration.
Well now, proof of the pudding is in the eating. I’ll build the tuner tonight and hopefully I should have a tuner that can be used for these bands using a single, relatively short, wire – An end fed!!!
To double check without a guess I measured the size of my unknown toroid and it was larger than I thought and appears to be a T157-2 (Red)
The site I was using for turns calulations was somehow no longer operational so I used http://toroids.info/T157-2.php
Last night I wound the coil on the “T157-2” toroid according to the above online calculations using a hand drawn diagram as a guideline.
and wired it into the old MFJ945e tuner housing. Tonight may see an on air test using the FT-817ND, my rig of choice.
73 de Eddie ZS6BNE