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

[Brian Alsop]

As Tom said.

The cause of the signal "gain" when using a tuner is really a matter of 
having a badly matched situation without the tuner.

In the old days (with manual tuners!), it was easy to demonstrate how a 
mismatched load seen by the RX degrades signal.

Just start with random tuner settings.  Then tune to maximum RX noise. 
This ends up being darn close to a perfect match to the RX.  It's pretty 
  close for TX too for most RX's.

In fact, there was a tuning method using a milliwatt level noise 
generator that some advocated be used for setting the tuner.  Their goal 
was to eliminate any significant RF being transmitted during the tuning 
process.  Don't forget with manual tuner, it wasn't a fraction of a 
second process.  Depending on the skill of the user it could take some 
time-- unless he had some presetting info available.


73 de Brian/K3KO



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
>