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N3FIX > TECH 31.07.19 18:54l 74 Lines 2387 Bytes #999 (0) @ WW
BID : 7683_N3FIX
Read: DJ6UX OE7FMI GUEST
Subj: SB-200 input impedance correction on 15 and 10m
Path: DB0FHN<OE2XZR<OE5XBL<F1OYP<IZ3LSV<IR1UAW<IW2OHX<IQ2LB<IR2UFV<N3HYM<
N3DWB<N3FIX
Sent: 190731/1436Z 7683@N3FIX.#EPA.PA.USA.NOAM LinBPQ6.0.18
I had known that both the 10 and 15 meter band input circuits on my old
Heathkit SB-200 linear amplifier were not well matched to my transceiver.
Using this amplifier with a newer radio, would cause the transceiver
to fold back on these bands with high SWR.
Referring to the schematic we see that these employ an L-network to
attempt to match the input impedance to standard 50 ohm load.
L2
--UUUU------
|
= 75pF
|
gnd
L1
--UUUU------
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= 68pF
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gnd
Going through the mathematical exercise for a PI network, which I won't
trouble you with here, it stands to reason this would probably work
better just like the PI network does on the lower bands.
The limited number of turns on the 10 and 15 meter coil meant that
they required some fairly small capacitors to attempt to make the
matching come out correctly. The problem is that those size
capacitors are about the same value of some of the stray
capacitance I believed to be in and around the spaces in the wiring.
The plan was to double the number of turns and increase the capacitance
required on the input side of the network. I then stumbled on an
article by PA0FRI who came to the same conclusion.
I stripped the 10 and 15 meter coil forms and re-wound
with 1mm diameter magnet wire. Scrounging through the junk box
yielded the capacitors that I calculated and were confirmed by PA0FRI.
I didn't have any tuning caps available, so trial and error yielded
appropriate values for the output capacitors which had to factor out
the stray capacitance in the wiring.
The trick was how to validate and tune the slugs in the inductors.
I've lost the source from a forum post on eHam, but the procedure is
to simulate the resistance of the active tubes per their data sheet.
So we temporarily place a 220 ohm resistor between the filament lead
and then to ground on the chassis. While manually pressing in the
transmit / receive relay an antenna analyzer is used to measure the
impedance at the input connector. After few tweaks of the slugs both
bands yielded acceptable impedance at less than 1.5:1 SWR reading.
The final results are below:
L2 = 10 turns
----UUUU-----
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= 150pF = 50pF
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gnd gnd
L1 = 6 turns
----UUUU-----
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= 100pF = 33pF
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gnd gnd
de
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