[TowerTalk] Understanding Coax Loss Measurements

[jimlux]

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)
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>