From: pcdhSpamMeSenseless@electrooptical.net
Cursitor Doom wrote:
> On Sun, 1 Feb 2026 14:03:03 -0500, Phil Hobbs
> wrote:
>
>> On 2026-02-01 12:33, Cursitor Doom wrote:
>>> On Fri, 30 Jan 2026 11:46:03 -0000 (UTC), Jasen Betts
>>> wrote:
>>>
>>>> On 2026-01-22, Cursitor Doom wrote:
>>>>> On Thu, 22 Jan 2026 23:31:15 +0100, Jeroen Belleman
>>>>> wrote:
>>>>>
>>>>>> On 1/22/26 17:16, john larkin wrote:
>>>>>>> On Thu, 22 Jan 2026 09:29:10 +0000, liz@poppyrecords.invalid.invalid
>>>>>>> (Liz Tuddenham) wrote:
>>>>>>>
>>>>>>>> What physical properties determine the velocity factor of co-ax? Most
>>>>>>>> of the amateur radio books give around 60% as the velocity factor for
>>>>>>>> 'common' types of 50-ohm co-ax.
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> V = c/(sqrt(Er))
>>>>>>>
>>>>>>> Solid polyethylene has Er around 2.3.
>>>>>>>
>>>>>>> Foamed stuff is lower.
>>>>>>>
>>>>>>> Polyethylene is awful. It melts when you solder it. Foamed is worse.
>>>>>>>
>>>>>>
>>>>>> That's why we have crimped connectors.
>>>>>>
>>>>>>> Your VNA measurement may be suspect.
>>>>>>
>>>>>> Maybe. The VNA needs to be calibrated to move the reference plane to
>>>>>> the start of the cable, which is probably not at the same place as the
>>>>>> VNA output connector. At lowish frequencies, it probably doesn't matter,
>>>>>
>>>>> True, but the OP also wants to measure the length of the cable AIUI,
>>>>> and for that, you want as high a frequency as possible for greatest
>>>>> accuracy. It's a trade-off (as ever).
>>>>
>>>> you probably want to do wavelengths shorter than the cable for best
results
>>>> but crazy high frequencies aren't needed.
>>>>
>>>> You run a frequency sweep and record the reflected amplitude and phase,
>>>> then do a Fourier transform, (which is what the NanoVNA does in TDR
>>>> mode) This will give you good answers if the cable has a linear
>>>> response.
>>>
>>> NanoVNA?? ROTFLMAO!!!!!!!!!!!!
>>>
>>
>> I'm quite fond of mine. What don't you like about yours?
>>
>> Cheers
>>
>> Phil Hobbs
>
> Don't get me wrong. I've said here before that what they've done for
> the money is truly exceptional. But you don't get lab grade accuracy
> from those things and (the ones I've seen at any rate) give very
> misleading readings at anything remotely close to their claimed
> limits. A pro designer couldn't possibly rely on one to ensure the
> specs a prototype meets comply with a customer's (or regulatory)
> requirements.
>
How so?
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
