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VK5ZVS |
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| Making a tuner for an end fed half wave antenna for 160m. |
The time had come to try a half wave on 160m.
No ground radials required. Minimum ground losses = more radiated rf. That's the theory.
I found a program on the internet that helped provide a starting point for the tuner.
TUNEHALF.exe by the late R.J.Edwards, G4FGQ, usually available as a free download
from the internet.
A screenshot of the opening page of the program, is shown below:
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When you press E, you are taken to the data entry screen, shown below.
This is where we enter our initial data.
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I've entered the data and then modified the variables using the hot keys,
to give the desired values that I think should work.
i.e. change the antenna height so the length is electrically closer to being a half wave, for example.
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From the data displayed, we can see that the coil Q is
reasonably high (495) but the overall circuit Q is low (8.3). This will mean that very little
tuning, if any, will be required to cover the band.
So, let's confirm the coil winding details and inductance using the mini-ring core calculator, as used in a previous
project. Let's use 43 turns, 91mm diameter by 265mm long. This can also be used to find out how
long the coil wire needs to be, as a minimum.
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Collect the bits we need and arrange placement.
The plastic coil former was made from the bottom of a computer keyboard.
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The load impedance is about 4700 ohms,
so let's use a 4k7 ohm resistor as the load and see where it tunes up.
Best swr can be determined by using an alligator lead and a scalpel blade initially
(to pierce the coil wire insulation), to find the best matching point, after
the variable capacitor is tuned for the desired frequency.
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| Reasonable vswr reading for a resistive load. What will a true antenna test show? |
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Let's go and test it out! Where else could you have
80 metres of wire in the air?!!!
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| The radio, tuner and kite wire restraints. |
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| 80m of wire rising steadily into the air.
Pretty strong wind this day and no one on the beach. |
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