Franki,
your 80 meter resonant vertical I assume is about 20 meters tall. You might
want to consider top loading. That allows for two band operation with not
active switching circuits. For example, thee 7 meter top hat wires sloping
downward and loaded with 35 uH provides two band operation.
A base loading coil of 33 uH will get you on 160 meters. A relay can be used
to short it when operating 80 meters. The relay needs to be a high voltage
type if you're running a kW as the voltage across it is over 2500 volts.
Linear loading does not offer an electrical advantage over a simple solenoid
inductor. The 33 uH base loading coil will have a Q of 300 when made with
close wound wire on a 2" PVC form. That gives a loss resistance of 2.4 ohms.
For the simple coil described A KW is a bit much. It will dissipate 45 watts
key down. For this an air wound coil would be better (it can run much
hotter).
Dave WX7G
On Sun, Oct 4, 2009 at 12:28 PM, Franki ON5ZO <on5zo@telenet.be> wrote:
> Gentlemen, please your advice.
>
> If I were to convert my working resonant 80m vertical into a 80/160m
> antenna, I assume I have two basic options according to the literature:
>
> - Use a trap to 'split' the antenna on 80m (resonant at geometric mean of
> 160/80m frequencies), or
> - Use a loading coil that can be switched in/out of the vertical to
> lengthen
> it electrically.
>
> I was leaning towards the coil for simplicity of construction, yet it needs
> to be at the feedpoint for practical reasons, as it seeds a switching
> circuit with control cable.
>
> Does the trap have advantages over the coil, apart from not needing a
> relay?
> Where do I start for practical dimensions to obtain a calculated L value
> for
> the loading coil?
>
> Then I stumbled across the linear loading solution in ON4UN's book. I could
> go up and down once with the verttical wire before going up all the way,
> and
> switch the loading part in or out for 160/80.
> But I didn't really see a practical approach for the wire spacing. How far
> would the loading wires need to be away from each other and the main
> vertical wire. Would 30cm (12") be enough?
>
> Your practical input would be appreciated.
> 73 de Franki ON5ZO
>
>
>
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