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



Definitely not your day, Bill -- the electrical length is always GREATER than the physical length. If you can make a cable perform otherwise, run, do not walk, to the patent office and patent faster-than-light communication! Sorry about that!

My take on it is that the ideal length is 0 feet (which, BTW, is hard to see, especially when it's coiled up). Next best is a half-wave or multiple thereof. Using a velocity factor of 0.66, I calculate that a 20.5 foot cable is a half-wave at 15.84 MHz (a compromise between 20 and 15 meters?), and near a full wave on 10 meters. But near a quarter-wave on 40!!

73
Bob

On 2015-05-30 9:59 AM, Bill Carns wrote:
By the way (with 50 ohm cable coupling the driver to the PA in a transmitter), I should have added that when you do the math on a 20.5 foot cable and take an average velocity factor for our cables of around 0.7, that cable is really about 14 feet long electrically. This equates to 4.26 meters or a wavelength of 17 meters...  So, it you were to use that transmitter on 17 meters or even 15 meters, with a 50 ohm coax connected between the driver tube (which presents what amounts to an open circuit - - 20,000 ohms terminating a 50 ohm cable), that would translate to a short across the plate of the driver tube at RF frequency (the active load line) and that tube would be very unhappy.

Bill

-----Original Message-----
From: Collins [mailto:collins-bounces+wcarns=austin.rr.com@xxxxxxxxxxxxx] On Behalf Of Gaylord Hart
Sent: Saturday, May 30, 2015 3:03 AM
To: 'Steve Beveridge'; collins@xxxxxxxxxxxxx
Subject: Re: {Collins} 30L-1 Input SWR High on 21/28 bands

I'm with you Steve.  I don't think the cable length makes a bit of difference, and if anyone here has actually tuned and loaded the XMTR and AMP combo and looked at actual IMOD performance (before and after) on a spectrum analyzer with "specified cable length" versus short lengths, I'd love to see the data.  I'm a big believer in hard data, and without it, not much matters.  Theories are wonderful, but every physicist will tell you they have to be validated with real-world confirmation from observed measurements.   Quite honestly, the measurement tools we have available today by far exceed the tools available when the equipment we are discussing was designed.  If anyone wants to spend the time to make the measurements, and has the right equipment to do so, this can be put to bed once and for all.  I have the equipment and expertise, but am not inclined to spend the time here.  I believe Steve is right when he indicates Collins dropped the issue after realizing it was a non-
issue.

If you follow the logical "theoretical" conclusions presented by some here of the need for a measured coax between driver and amp to reduce distortion, then this should apply to the driver and finals in the transmitter itself, and I've never seen a transmitter that had a 20' coil of coax between these two tubes on any of my XMTRs.

Gaylord
WB7ODD



-----Original Message-----
From: Collins [mailto:collins-bounces+gahart=ix.netcom.com@xxxxxxxxxxxxx] On Behalf Of Steve Beveridge
Sent: Friday, May 29, 2015 5:28 PM
To: collins@xxxxxxxxxxxxx
Subject: 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:
http://collinsradio.org/archives/manuals/Collins%2030L-1%20manual%207th%20edition%20February%201965.pdf

Cheers
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
www.w0btu.com

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

  [snip]

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”.

[snip]
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