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



OK,  I need to fall on my sword..  Either too old or too early in the AM..  I lean towards too old.

The electrical length is longer so the short occurs closer to the 40 meter band. While the wavelength is reduced in a cable, this makes the "electrical length" longer.

According to our own tutorial on the website - which Don Jackson and I wrote, 

"If resonant lengths of RF coax cable are to be used, then it is necessary to know the velocity factor of the coax cable. It is often possible to determine this to a sufficient degree of accuracy from just knowing the type of the dielectric material. Coax cable electrical length:  Wavelength in Cable = Wavelength in Air * Velocity Factor (< 1)

One important factor of a coax cable in some applications is the wavelength of the signals travelling in it.  In the same way that the wavelength of a signal in free space is the speed of light divided by the frequency, the same is also true in any other dielectric.  Since the speed of the wave is reduced, so is the wavelength reduced by the same factor. Thus if the velocity factor of the coax cable is 0.66, then the wavelength is 0.66 times the wavelength in free space.  Or, In some instances, lengths of coax cable are cut to a specific length to act as an impedance transform element, or a resonant circuit. In such applications, this velocity effect needs to be taken into consideration when determining the required length of coax cable. The advantage of using a coax cable with a low velocity factor is that the length of coax cable required for the resonant length is shorter than if it had a velocity factor closer to unity. Not only does this save on cost, but it can also be significantly more convenient to use and house. There is a flip side to using cable with a low velocity factor. Such cable, by definition, has a higher capacitance per foot, and this must be factored into the selection process when the cable is being used for some applications. These applications usually involve shorter lengths of cable employed for shielded runs in equipment where the source, or termination, impedance does not match the cable impedance (ZO)."

Bottom line is I tossed out that comment too fast and should have said that the wavelength is cable is less so the electrical length is longer than 20.5 feet or about 27 feet making the short somewhere between the 30 and 40 meter band.  The conclusion is the same..  Bad plan using cable to couple a driver and a PA for the earlier stated reason..

Credit to both Bob, WE3TOU and Kurt, W6PH for this slip of the tongue and slide rule.  Thanks guys for the correction.

Bill

-----Original Message-----
From: Robert Thomas [mailto:ve3tou@xxxxxxxxxxxxxxxxxxx] 
Sent: Saturday, May 30, 2015 9:53 AM
To: Bill Carns; collins@xxxxxxxxxxxxx
Subject: 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%207t
> h%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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