Posted by
Bob Nielsen on
Jan 05, 2006; 7:39pm
URL: http://elecraft.85.s1.nabble.com/T1-Antenna-Considerations-tp385235p385245.html
On Jan 5, 2006, at 10:43 AM, Martin Gillen wrote:
> Hi, Peter.
>
> Please excuse my ignorance I'm fairly new
> to amateur radio ;)
>
> But I have a question:
>
>> The only reason that the "use the nearby bushes"
>> construct now can be tuned by the T1 is that the
>> impedance goes low because a part of the antenna is
>> lying "on the floor" now, giving high capacitance
>> against ground which allways lowers the impedance.
>
> Yes, I understand that; any radiation from the
> counterpoise is effectively lost into the ground.
>
> However another antenna I have used is a 66ft length
> of wire at 15feet, fed in the centre with 300 ohm
> twinlead.
>
> In this case, the driven half of the wire is still
> 33ft long - but I get really low SWR om 20m and have
> worked DX with it,
>
> So why does that work? Is the feedline doing
> something to help me out here? Or is a dipole just
> a completely different scenario altogether?
>
In the case of a dipole, both halves of the wire are driven and
contribute to the radiation pattern. Ideally the feedline will have
equal but opposite currents in each conductor and will not radiate.
This antenna is about a half-wavelength long and has a feedpoint
impedance of approx. 73 ohms. The 300 ohm feedline will transform
this to a different value (depending on the length), but this will
certainly be within the range of a T1 (which should be used with a
balun like the Elecraft BL-1, since you have a balanced antenna and
the T1 is designed for an unbalanced case).
73,
Bob N7XY
K2 #3273
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