Re: {Collins} 30L-1 Input SWR High on 21/28 bands

Hi folks,

Appreciate the technical analysis Don.

Taking a more simple line, one only has to look at the more recent versions of the 30L-1 manual to find that any mention of the magical '20.5 ft' cable is deleted entirely.

For example, looking at the 7th edition manual (1965), a 4 ft RF input cable (RG58) is provided by Collins. Not much attenuation there! Here's the link:

Steve VK2LW
CCA member

On 30/05/2015 8:41 AM, Don Jackson wrote:
Hi All,

Ok, I'll chime in here. First, an extra dB or so of loss in the cable is highly unlikely to have any affect on 30L-1 stability. My studies didn't show any significant affect. If the real question concerns the function of the 20.5 foot cable, here are two quotes from Warren Bruene on the subject. This design approach was originally intended for the 30S-1 in an attempt to obtain a few more dB of IMD performance.

From “SSB Principles & Circuits”, Bruene states:
“The RF coupling network between a linear amplifier stage and its driver should have a total electrical length of either zero degrees or some multiple of 90º. This is necessary to avoid phase distortion due to a nonlinear load on the coupling network.”

From Bruene’s QST article “Inside GG Amp” he states:
“...nonlinear grid-current loading causes flat-topping and becomes the major cause of intermodulation distortion at large signal levels. The effect of this nonlinear grid loading can be reduced by providing a low driver source resistance. This is the reason for the special length of coax specified to connect a Collins KWM-2 to a 30S-1 amplifier, for example. This special length, plus the phase delay in the KWM-2 output network and that of the 30S-1’s cathode circuit, approximately equals some multiple of 180º on each band. This provides a low source resistance, which reduces the effect of nonlinear screen-current loading in the 30S-1’s 4CX1000A tetrode, which is cathode-driven and operates in class AB1. The length of connecting coax is therefore important for minimizing IMD.”

For my money, this is the final word on the function of the 20.5 foot cable. My article on the cable contains further analysis, but reading it is only an option if you wish to completely understand how the cable does what it was meant to do.

Don, W5QN

-----Original Message----- From: Mike Waters
Sent: Friday, May 29, 2015 4:45 PM
To: Bill Carns
Cc: collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands

I don't have time to read all this now, but so far, so good, and I've
learned a few things. Thanks.

However, I did notice that you refer to RG-8; but Collins specified
RG-58C/U with more loss. Just food for thought. I didn't calculate anything.

73, Mike

On Fri, May 29, 2015 at 4:24 PM, Bill Carns <wcarns@xxxxxxxxxxxxx> wrote:


Re the loss of the longer cable helping with stability.  Hmmm.. The loss
of a typical 9913 type (RG-8U) cable is less than one db total PER 100 FEET at 30 MHZ. The amount of loss injected into the stability picture at the
input by using a 20.5 foot cable instead of the typical 4 to 6 feet of
cable is mice nuts – much less than one db. I suspect strongly if loss was the motivation for the recommendation to use the longer cable, the length
would not have been set at 20.5 feet.

In fact, Warren Bruene wrote an article about the use of the 20.5 foot
cable in the 30S-1. ... Gene Senti was the designer of that amp. This amp “inherited” the 30S-1 20.5 foot cable recommendation as far as we can tell.
... While there is a theoretical basis for using the 20.5 foot cable and
Don did confirm the theory, he also concluded, as did Art, that it was
unnecessary and lost in the “noise”.

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