[Elecraft] K3: PRE/ATT Bug or feature?

[Vic K2VCO]

The K3 ATU is not removed from the circuit when it is set to bypass. It is set to 
compensate for stray reactance in traces, etc. so that the SWR will be close to 1:1 for a 
true 50 ohm load.

So any variations in MDS are not caused by losses inherent in the tuner.

On 8/11/2010 6:34 AM, Tom W8JI wrote:
>
>> Actually, you will probably LOSE a couple of db by switching on both
>> preamp and attenuator compared to having both switched off. (Measure
>> your MDS and compare results if you don't believe...)
>
> MDS, like dynamic range, can go either direction with changes in gain. It is
> possible to have a gain increase and lose MDS.
>
> This is why it is important to know how things really change in abnormal
> configurations, not how we "feel" they change.
>
>> Regarding weak signals, one factor that is overlooked is the ATU. By
>> switching it to bypass you can gain 5 or 6 db in MDS. Of course, you
>> would want to switch it back on when you transmit.
>
> That large amount surprises me, although a smaller amount certainly would
> not. Systems with loss are often not bilateral. For example on receiving
> feedline losses associated with feedline SWR are determined by the radio's
> input impedance, not by antenna SWR. On receiving the receiver impedance
> determines feedline SWR.
>
> On transmitting, the antenna is the load and sets feedline SWR....affecting
> feedline losses.
>
> 6 dB is a pretty big change. Maybe your receiver input impedance is not so
> close to 50 j0, or your 6 dB is not 6 dB though some sort of errors in
> calibration????
>
> 73 Tom
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html


-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/