Reflecting on my Collins service days.
15 meters was the band that suffered without the 21.5' cable. That's all.
The customers all expected the LOADING control on the exciters to sit
exactly on 50 ohms. Matter of fact the manual said, YOU DRIVING AMPLIFIER?
Set the loading control to the 50 OHM point and resonate the plate the ALC
will handle everything else. Well on 15 meters the exciter would load all
wacky without the "Special" cable.
So the story is: When you have ham gear costing two times as much as the
competition or more, you can't have your loading control two lever's widths
off can you?
Drake tried the same thing with their "Special" cables for the LO's on the
twins. I think mostly to IMPRESS their customers with esoteric thinking.
Hey we can engineer just as good as Collins guys, because we have special
cables TOO...
The Drake didn't care, plug whatever you want in there and it still works.
The Collins don't care either if its 6 foot or 106 foot, you'll just have
some exciter loading control issues... In today's solid state world, just
fire the B***H up and make radio.
SO now you know the actual reason behind the infamous cable.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Ian White GM3SEK
Sent: Tuesday, January 02, 2007 3:10 PM
To: amps@contesting.com
Subject: Re: [Amps] *** SPAM *** Re: Alpha 87a
Peter Chadwick wrote:
>Tom said:
>>Currents on the
>shield can't change SWR inside the cable <
>
>This is very true: however, what they can do is to change the indicated
>SWR, depending on the SWR meter. So you can have a cable that in
>reality has a 1:1 SWR, but has an indicated 1.5:1. This fact is not
>always appreciated..
That can only happen if either the cable, the SWR meter, the
transmitter, the ATU or a dummy load has a substantial break in its
shielding. Without any of those, the indicated SWR is not affected by
common-mode currents on the outside surfaces.
The one case where it definitely *does* happen is when the "load" is an
antenna. Common-mode currents on the feedline will change the feedpoint
impedance from what it should be, and that will also change the SWR; but
the actual change is taking place at the feedpoint, and not local to the
SWR meter.
The tool to detect such effects is not an SWR meter, but a clamp-on RF
current meter.
Altering the length of cable between the driver and the PA may have some
effect if the PA input does not present a good 50-ohm load. Varying
lengths will categorically not affect the SWR (unless there is a
shielding fault as indicated above) but they will transform the load
impedance that the driver sees, rotating it around a constant-SWR circle
on a Smith chart. When that impedance is not exactly 50 ohms, a
solid-state driver with no tuning or loading controls may perform better
with some lengths of cable than with others. However, a "special" length
of cable will only improve the situation on one band (unless you're very
lucky) and even then, it's only a patch. The real problem is with the PA
input matching.
The Collins book gives some justification for making the total phase
shift between the driver anode and the PA cathode into an exact multiple
of 90deg, and for avoiding odd multiples of 45deg. Part of this phase
shift occurs in the driver output tank circuit, part in the PA input
tank, and the rest is made up by the cable.
Both tank circuits are band-switched, so their phase shifts won't vary
much from band to band; but the phase shift in the cable will vary from
band to band. This means that the special length of cable can only make
up the correct phase shift for one band, and it will be valid for only
one combination of driver and PA. Switch bands, and the whole
phase-shift story falls apart.
I'm not disputing that a 20.5ft length of cable between a KWM-2 and the
input of a 30S-1 does *something*; but I don't believe it works the way
Collins claim.
--
73 from Ian GM3SEK
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