Question about K2's T7 windings

[Tom Mc]

Hi,

  A recent post by Don, W3FPR mentioned that the winding of T7 in the K2 has
changed, apparently with K2 manual revision C.

  I have built K2 s/n 1103 and used manual version B.  My manual (ver. B)
states that the winding ratio should be 11:3.  Don states that newer manuals
are stating that T7 should be 20:5.

Don's post also states that an additional 3 dB of gain should be
accomplished by this change.

  Does 3dB of additional gain on the audio mean the same as 3dB on an
antenna system?  Like double the output?  That would seem to be pretty
significant.

  Can anyone let me know if they have made this change and if they have
experienced the same results which Don says (Don also mentioned that he
hasn't had the chance to make this change in his Field Test K2).

  Also, are there any other transformers in the K2 which should be re-wound
to effect an increase in gain, or for any other reason, for that matter.

Thanks in advance for your help

Tom McCulloch, WB2QDG
K2 s/n 1103




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[Vic K2VCO]

Tom Mc wrote:

>   I have built K2 s/n 1103 and used manual version B.  My manual (ver. B)
> states that the winding ratio should be 11:3.  Don states that newer manuals
> are stating that T7 should be 20:5.
>
> Don's post also states that an additional 3 dB of gain should be
> accomplished by this change.
>
>   Does 3dB of additional gain on the audio mean the same as 3dB on an
> antenna system?  Like double the output?  That would seem to be pretty
> significant.

Actually, 3 db of audio gain is not signigicant in the same way that a 3 db
increase of antenna gain is.  In the case of the antenna, the gain improves the
overall signal/noise ratio, since the internal receiver noise remains the same.
  If the antenna gain comes from directivity (it can also come from increasing
efficiency), the result is even more significant, because desired signals are
stronger while undesired signals and noise from other directions are weaker.
Obviously, increasing audio gain increases internal and external noise as much
as signal.

>   Can anyone let me know if they have made this change and if they have
> experienced the same results which Don says (Don also mentioned that he
> hasn't had the chance to make this change in his Field Test K2).

I made this change to my K2 #709.  While I didn't notice an increase in overall
gain, it did decrease the additional loss I had when the crystal filter was set
to 200 Hz by a couple of DB.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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[G3VVT]

3dB gain reference to what?
 
Without a reference decibel values are meaningless. With voltage, 3db gain  
is a value greater by one half and is only applicable if the impedances  remain
the same across the item of equipment you are measuring. When  referred to
power, 3dB gain is greater by x 2 (double). Most  of the measurements carried
out professionally in telecoms are  related to power and do have a specific
reference such as dBm (reference to  1mW), etc. which makes life a whole lot
easier. Traditionally in the Cable  TV industry the decibel is related to the
voltage at 75 ohms.
 
Bob, G3VVT
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[Jim Brown-10]

On Wed, 13 Oct 2004 07:27:52 EDT, [hidden email] wrote:

>3dB gain reference to what?
>
>Without a reference decibel values are meaningless.

Horse puckey.   Without a reference they do not tell you the voltage or power in a circuit,
but they can certainly tell you the differences between two voltages or powers, and that
can be QUITE meaningful. For example, you are in the UK and I am in Chicago, and I
increase my transmitter power by 4:1 (6 dB) and change nothing else. My signal at your
location will get 6 dB stronger, independent of the reference used by the voltmeter you
choose to measure them, and the signal to noise will also improve by 6 dB, again,
independent of that voltmeter's reference. If you must have a reference it is the signal
before I increased by power!  

Once you use the word "gain" you make dB quite meaningful. The most formal definition
of gain using dB is for power -- 10 * log (P2/P1), where P2 is the output power and P1
is the input power. But if the impedance is the same for the two powers,  the gain in dB
is 20 * log (V2/V1). BOTH of these expressions are VERY meaningful, and no
reference must be defined!  Or, looking at it another way, P1 (or V1) is the reference.

Also, many circuits operate as voltage amplifiers, where power is essentially
insignificant. Thus it is also entirely correct to speak of the voltage gain in dB, and that
is also 20 * log (V2/V1). And, again, no reference level need be defined.

Further, if we wanted to MEASURE the gain in dB, we could measure the voltages and
do the math, or we could use a voltmeter calibrated to ANY voltage reference for dB,
subtract the input voltage in dB from the output voltage in dB, and that would be the
gain. Again, the reference is unimportant (so long as we don't change streams in mid-
horse).

On the other hand, if we want to measure power gain, we would, indeed, need to
consider the resistance at both input and output.

While the telecom industry was the first to define the Bel and did so in terms of power,
virtually all modern instrumentation that gives a readout in dB with respect to any
reference is REALLY a voltmeter. That voltmeter may include a load resistor (with a
switch to remove it from the circuit), or it may not.

Hope this helps.




Jim Brown  K9YC


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[G3VVT]

There you go Jim shooting yourself in the foot straight off by referring  
your dB to power!
 
> and I increase my transmitter power by 4:1 (6 dB)............
 
Of course the two entities voltage and power are mathematically related,  but
unless you say to what reference you are using, they can be somewhat  
meaningless. In the 17 years I spent in the Cable TV industry pre the advent of  
satellite technology, the dB was always related to voltage at 75 ohms and was  
understood to be so. The only occasion I remember using dB related to power was  
when using multiple phased antenna arrays to work out the total output as the  
gain was increased by a ratio of 3dB in power every time the number of
separate  arrays was doubled.
 
With a sideways shift into telecoms for the last 20+ years I had to  start
thinking about dB related to power where 4 times gain or  loss when related to
power is 6dB as you say. The same 4 times gain or  loss is 12dB when related to
voltage as used in Cable TV. Irrespective  of how the measuring systems for
the two industries work, they are  calibrated in dB relative to the system you
are working with and as such are not  directly compatible, thus making a
reference when referring to dB gain or loss  essential.
 
Perhaps a topic to be discussed further off the Elecraft Reflector if  needed.
 
Bob, G3VVT
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