[TowerTalk] Understanding Coax Loss Measurements

[Edward Mccann]

Thanks Jim:

But not sure which your “yes” addresses, part 1 or part 2?

1. “If I use the VNA with direct input of ladder line, say nominal 450 ohm Zo, but with measured parameters more near 400 ohms, should I not run OSL calibration with a 400 ohm load prior to running the sweep?”

2. “The response below seems to imply that calibration with a 50 ohm load will yield “good results for coax and two-wire line of any impedance?”

Appreciate the clarification.

For example, Sark-110 allows you to select Zo, calibrate for 50 ohms, then add or subtract feed line, with a menu of 50 to 450 ohm feed lines. 

Thanks!

Ed McCann
AG6CX

Sent from my iPhone

> On Nov 28, 2020, at 2:02 PM, jimlux <jimlux at earthlink.net> wrote:
> 
>> On 11/28/20 11:47 AM, Edward Mccann wrote:
>> If I use the VNA with direct input of ladder line, say nominal 450 ohm Zo, but with measured parameters more near 400 ohms, should I not run OSL calibration with a 400 ohm load prior to running the sweep?
>> The response below seems to imply that calibration with a 50 ohm load will yield “good results for coax and two-wire line of any impedance?”
> 
> Yes, if you use AC6LA's spreadsheet. It takes care of the impedance.
> 
> 
>> Or does the creation of a reference plane with 50 ohms in the OSL calibration yield results for a wide varieties of feed line?
> 
> Yes, you can test anything with a 50 ohm reference cal, you just have to adjust for the non 50 ohm system you're testing.
> 
> (one can spend a lot of time arguing about whether you'd be better off doing a cal with an impedance closer to your system impedance, but ultimately, it's the same measurement, just different math)
> 
> 
> 
>> Basic question.
>> Best from the West
>> AG6CX
>> Sent from my iPhone
>>>>> On Nov 28, 2020, at 11:27 AM, jimlux <jimlux at earthlink.net> wrote:
>>>>> 
>>>>> On 11/28/20 11:08 AM, Jim Brown wrote:
>>>>> On 11/28/2020 3:43 AM, Kirk Kleinschmidt via TowerTalk wrote:
>>>>> I'm trying to characterize some coaxial cables and various connectors, and the results from my two measuring methods aren't reconciling well. My understanding of the tools is also less than complete.
>>>> First, make sure that your VNA is well calibrated. For cailbration, you will need standard Open, Short, and 50 ohm parts.
>>>> Then, use it to make S11 measurements over the frequency range of interest. I'd go with 1-50 MHz. Then download a copy of AC6LA's ZPlots Excel spreadsheet, export your data to it, and follow his instructions for characterizing the cable.
>>>> This will give good results for coax and two wire line of any impedance.
>>>> 73, Jim K9YC
>>> 
>>> 
>>> A bit of background: AC6LA's spreadsheet does the renormalization for the different impedance, if your VNA doesn't have that feature.
>>> 
>>> 
>>> (you're not using the NanoVNA, but for those who are, the NanoVNA-Saver python program also has the "change characteristic impedance" feature)
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