[AMPS] 4-1000A Project

[Jon Ogden]

on 4/4/01 3:58 PM, Wt8r@aol.com at Wt8r@aol.com wrote:

>> 
>> I used the 850A because that is what I had and I made it work.  However, if
>> you calculate the inductance values needed for a good Q, they don't work or
>> at least they don't work well.
>> 
> 
> Jon,
> 
> The above wealth of information has left me a little confused. Perhaps you
> can enlighten me concerning it.

I'll try.

> 
> You say you used the 850A because that was what you had, however, if one
> calculates the values needed for a good Q, the 850A is not very good.  I see
> later in your design and RF Engineering calculations that you assume a Q of
> 12.  

I used 12 purely as an example, not as a statement of what I actually have.
The fact is that if you look at the values of inductance in the 850A and
what is needed from calculated values of pi-net components, the two don't
match up.  This is why I don't recommend that switch.  If I had to build the
amp all over again from scratch, I would not use it.  I'd roll my own.  But
I was trying to get it done and make it work with what I had.  This is
probably why I don't have the gain or efficiencies that some people have
with their 4-1000A amplifiers.  But it works and works well for me probably
10,000 QSOs after I put it in service.

Again, for a Q of say 12 (it's just an example), the following values of
Ctune, L and Cload are needed to match 5300 Ohms (Load Z of 4-1K at 5.75 KV)
to 50 Ohms:

Frequency (MHz)    Q0    C1 (pF)    C2 (pF)    L (uH)
                
3.75               12    92.05       429.34    20.28
7.15                     48.28       225.18    10.64
14.15                    24.39       113.78    5.37
21.20                    16.28        75.94    3.59
28.30                    12.20        56.89    2.69

C1 = Ctune, C2 = Cload

Again, the B&W coil has inductances of 13.6, 6.5, 1.75, 1.0 and 0.8 uH in
each of the 5 amateur bands.

So the 850A isn't even close to what is needed.

Now if you wish to match 3000 Ohms we see:

Frequency (MHz)    Q0    C1 (pF)    C2 (pF)    L (mH)
                
3.75               12    155.42    860.74      12.56
7.15                      81.52    451.44       6.59
14.15                     41.19    228.11       3.33
21.20                     27.49    152.25       2.22
28.30                     20.60    114.06       1.66

Now here, the B&W comes pretty close on the low bands, but on the higher
bands, it's still about half of the inductance that is needed.  A lower
inductance will result in larger values of capacitance and higher Q.

At 14.5 MHz, we get the following values:

Q= 24   C1= 80.31   C2= 580.67   L= 1.76

The L=1.76 is just about right at what the B&W is at 14 MHz.  So with that
fixed value of L, the tank will match up with a Q of about 24.  Is that too
high?  Perhaps not, but it seems a little high to me.

That's why I think that the B&W coils are poor for use and it is better to
roll your own.  The nice thing about them is that they are indeed heavy
duty.

> 
> If the 850A is not very good, what (poor) value of Q did the 850A produce
> which you used in your linear?

Frankly, I don't know.  I have not measured it.  I ended up solving a lot of
my high band problems by using the step down coil that I described earlier.

73,

Jon
NA9D


-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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