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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