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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