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Hi all,long story my homebrew pi network atu wasnt tuning so stuck my v2 vna on it to have a look,at one point in the c2(variable cap nearest the output jack for the rig)looked as iff the vanes were shorting to ground,after removing the said cap it actualy looked ok but both sides looked like shorting to ground,the cap was still wired to the atu at this point, also the vna was conected to the atu,my meter was belling out between both legs of the cap and ground,i disconnected the vna and it stopped buzzing,i then measured the resistance on the s11 port on my vna,ie the center pin and outer of the s11 plug,it read 49 ohms low enough to make my fluke 87 dmm buzz on resistance mode,is that normal or a diode sc in my vna front end?.

On Thu, Nov 24, 2022 at 05:47 PM, Paul M3VUV wrote:

measured the resistance on the s11 port on my vna ... it read 49 ohms ... is that normal ... ?

All RF devices have input and output resistance of 50 Ω - unless you're in the TV business (they have 75 Ω) - otherwise youi'll get reflections (bad SWR).

Martin

On 11/24/22 9:29 AM, Ho-Ro wrote:

On Thu, Nov 24, 2022 at 05:47 PM, Paul M3VUV wrote:

measured the resistance on the s11 port on my vna ... it read 49 ohms ... is that normal ... ?

On a NanoVNA the CHO port connects to a bridge with ~50 ohm DC resistance. See the schematic of at least one version.

CH1 connects to a attenuator, also with approximately 50 ohm DC resistance.

All RF devices have input and output resistance of 50 Ω - unless you're in the TV business (they have 75 Ω) - otherwise youi'll get reflections (bad SWR).

That's not generally true - A lot of devices will have a RF impedance of 50 ohm resistive, but that's also only if the power is on and if the other ports are terminated in their expected impedances. And at the design frequency.

For example, a Bias T might show infinite DC resistance, because there's a blocking capacitor.

a typical power divider will NOT show 50 ohms DC resistance, depending on the configuration. A Wilkinson, certainly not, since it depends on the impedance transformation of the 1/4 wavelength lines. Same with a Lange coupler.

A "triangle" resistive power divider might show 50 ohms DC, if the output ports are terminated in 50 ohms.

One does want to be careful measuring the ports of RF devices with a ohmmeter, too. For instance the NanoVNA2 has some RF switches connected directly to the ports, and depending on the voltage of your ohmmeter, those could be damaged.

On Thu, Nov 24, 2022 at 07:25 PM, Jim Lux wrote:

That's not generally true

Jim,
Ok, my bad, my answer was too short, I should have written: Don't be surprised if you measure 50 Ω - and if you do not see 50 Ω think twice what could be the reason. You gave good examples of influence:

On Thu, Nov 24, 2022 at 07:25 PM, Jim Lux wrote:

only if the power is on
other ports are terminated
blocking capacitor
design frequency
...

On Thu, Nov 24, 2022 at 07:25 PM, Jim Lux wrote:

be careful measuring the ports of RF devices with a ohmmeter

You could measure the port resistance of the VNA with a VNA - oops, catch 22 :)

Martin

General advice, and please do not be offended, Paul.
If you have a long story, divide it up into several shorter sentences. It will be easier to get your message across and you will find it helps you to consider the problem more clearly yourself.

General advice, Ken G3YKI, identify who you are responding to correctly! Just for your information, I know this wasn't directed to me but there could be a great number of people called Paul who could be subscribed to this group and all will be scratching their heads at seeing your post hit their inbox!

sorted,it was the vervier dial on cap 2,the shaft was making intermitent contacts to ground,was using a solid brass coupler not insulated,now sorted,cheers 73 de m3vuv Paul.