On Thu, 16 Dec 2010 03:34:56 -0800, Wimpie wrote:

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.

OK.

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.

Then I shall use a dipole...

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

Not at this time. This is planned.

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

Sorry... Yes, your design seems quite simple. But as a beginer, I think i
get some problems to understand how things are connected to the variable
capacitor, I mean I think the way the variable capacitor works is missing
in my information pool...

Based on your photo let say we get :
On the upper side : two connectors
On the lower side : two other connectors

But the upper side connectors are connected to the lower side
connectors... via the capacitor itself. Am I right?

The tap coil preselectors seem quite easy to carry out. You use different
connectors on the tap to select the band you want to use.

You get a switch on the upper side that seems to avoid using one gang of
the capacitor... But I cannot see of it is connected to the ground or not.

The air capacitor ground is connected to the ground.
On the right side, the tap coil preselectors are connected to the ground
too.
There is a tiny capacitor that connects one gang to the ground again.

The central board should be used to avoid both tapped coil preselectors
to get an inductive action on the other?

Is this correct? Would it be possible to get the schematic so that I
could try to carry it out?

How did you compute the place to put the connectors (number of turns) for
the tapped coil connectors?

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.

How do you use it? Quite a blind test, no?

This can be of use when you have strong signals in your neighborhood.
Â* Because of portability, I frequently use an AOR AR8200. Without
preselection, such a wide band receiver is useless with an external
antenna.

I have a big QRM (well, I don't know what it is) at 1500 MHz and other
smaller ones elsewhere that I cannot switch off )

Sebastien.