Re: BCB sensitivity

Posted by Chuck - AE4CW on Jul 19, 2008; 4:03pm
URL: http://elecraft.85.s1.nabble.com/BCB-sensitivity-tp548048p573185.html

Vic and Drew,

The ATU in the K3 is a low-pass "L" design which has a series inductance and *either* an input (Ct) *or* output (Ca) shunting capacitance.  There is only one capacitance in this desigh, not two.  The single capacitance is switched between the input (Ct side - transceiver) or output (Ca side - antenna) of the inductance.  So, when you are in CONFIG:KAT3 --> LCSET --> ATU TUNE, tapping ANT switches the capacitance between the input and output of the inductance.  You can hear the relay switch when tapping ANT and sometimes hear a distinct difference in the received signal.

In general the capacitance will need to be on the input (Ct) side when the antenna impedance is below 50 ohms and on the output side when the antenna impedance is greater than 50 ohm.

You can look at the circuit design in the K3 schematics (documentation page on the web) if you would  like to get a more detailed understanding.  Or, you could also look at a program like TLW (transmission line wizard) that will calculate and show you a picture of the correct values for a low-pass "L" ATU with any given values of antenna impedances.

Hope this helps!  73,

Chuck, AE4CW

Vic K2VCO wrote

[e-mail] wrote:


> However, you can't adjust CA and CT independently. And checking all of
> my pretuned bands the values for CA and CT on any given band are
> equivalent (which seems odd). Furthermore, merely selecting between CA
> and CT with the ANT button while in LCSet mode has an effect on the
> received signal-- a couple of s-units difference between the two is
> what I noticed. (But I don't think it is the antenna relay being
> switched, as mentioned before.)

> What I want to know is, are the displayed values for CA and CT
> correct, and can they be adjusted independently of each other?

I believe this is a bug which I reported. If I remember properly, the
problem is that the values are not being displayed correctly.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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