On 15 dic, 22:32, Sébastien MEDARD wrote:
On Tue, 14 Dec 2010 09:17:40 -0800, Wimpie wrote:
Regarding tuning, loops and (long) wires.

You probably locate your antenna far away from noise sources. *Mostly
this is not where your receiver is.

That's a problem, indeed.

In case of tuned loops, you have to go outside for each significant
frequency change. *This is one of the reasons that I want my
preselection / tuning adjacent to the receiver.

It seems there is another solution. The varactor diodes seem to act as a
variable capacitor driven by their voltage (the capacitance of this kind
of diode decreases as the potential is increased).

It seems there are some loop designs with varactor diodes as tuners.

But I don't know what could be the price of a such solution.

That is an option, as they are frequently used in VHF/UHF systems. The
varactor is a non-linear device, so you may get intermodulation in a
large signal environment. You may use the varactor as fine tuning in
combination with fixed capacitors.


Loops aren't magic things. A wire from your balcony to a tree or other
tie point may give better S/N ratio then the best (expensive) loop on
your balcony.

I think so, but there is nothing I can change for that today.

Best matching isn't required for reception at HF, just sufficient
signal. *In case of a PI tuning / preselector device, you need to adjust
two capacitors in case of major frequency change. *In my opinion a
single capacitor device is more pleasant to use.

I will try different configurations when I get them.

If you want to experiment with some long wire (unbalanced) antennas,
make sure to have a ground (counterpoise) provision outside; otherwise
your coaxial cable will be part of the antenna. This may result in more
interference from indoor sources.

Yes, but the place I plan to experiment should be far away from this kind
of trouble... I won't be able to get a good ground there.

Did you try the balcony fence or other large metallic structure as
floating ground (counterpoise)?


If you use a tapped coil preselector (like the one in my link),

Sorry, where is it in your web site?

It is the simple thing I referenced earlier
www.tetech.nl/divers/SimplePreselector2.jpg


and have
sufficient noise from the antenna, you may even change the taps. The
preselector will give more insertion loss, but it will be more selective
also.

How is called this design?

I don't know the name for it, it is just a parallel resonant LC
circuit where the input and output goes via an inductive tap. Moving
the tap changes the coupling between the source/load and the resonant
circuit, hance changing the loaded Q-factor.


This can be of use when you have strong signals in your
neighborhood. * Because of portability, I frequently use an AOR AR8200.