Within the past few years, tests of Collins linears has not found the
effects on linearity claimed for that magic length of coax. Even with
the same transmitters and linears used decades ago. The 30S-1 has a
tuned input, unlike the 411. And the exciters of its era used tubes and
a TUNED output, not just a low pass filter which changes the interaction
between the PA and the exciter. Those exciters also had RF feedback to
improve their linearity at all signal levels.
On 6/2/2010 12:41 PM, CSM(r) Gary Huber wrote:
Gary's observation means that no matter the band the input impedance is the same
except for the "intervention" of the coax between the exciter and the AMP.
No, that's not quiet the case... the input resistor's impedance remains the
same but impedance/reactance of the tubes will change with the operating
frequency, the load (antenna and feedline) and the tune and load controls of
the amplifier (and the quality of the amplifier's chassis to RF ground
connection). So you can get a different input impedance as well as a different
reactance which may require a different length of coax between the exciter and
amplifier.
From http://www.wa3key.com/collins.html
INPUT CIRCUITS
Pi-network broad-tuned circuits and the interconnecting r-f feed line match the
50-ohm input impedance to the cathode impedance, which is approximately 100
ohms. The 20.5-foot length of cable (furnished) is necessary between the 32S-1
(or KWM-2) driver and the 30S-1 input circuits. This is due to the necessity of
having an even multiple of 180-degree phase shifts between driver plate and
power amplifier grid. The cable length and the 30S-1 input circuits together
accomplish this. An even multiple of 180-degree phase shifts is necessary
because modulation components cause a change in the resistive PA cathode
impedance which is translated to a shift in reactive impedance at the driver
plate. The shift in reactive impedance, at the driver plate, results in phase
modulation of the driver and increases the total over-all distortion of the
system. A 2.5-foot additional length of cable is furnished to bring the total
interconnecting cable length to 23.0 feet for use with the KWM-
1 a
s a driver. Drive power required for maximum legal input on SSB is 80 watts
PEP.
I'd build a two section cable (RG-8) as above and try loading the amplifier
into a good dummy load just to see if it'd solve the problem.
Best regards,
CSM(r) Gary Huber - AB9M
9679 Heron Bay Rd
Bloomington, IL 61705
(309-662-0604)
www.csm-gh.com
glhuber@msn.com
gary.huber@us.army.mil
A choke on the coax will only show something IF the coax connectors are
not solidly grounded to their chassis. Or the coax connectors are not
solidly soldered to the braid of the cable.
73, Jerry, K0CQ
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