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New user
I got my first Nano VNA yesterday, a friend who used to live 10 miles away, now lives 1000 miles away bought it for me along with jumpers and adapters. Today I set up "presets" for 80, 40, 20, and 2 meters.
I researched a bit and found out it could be a small SD card, all I had was some 64-gig and one 16-gig card, well the 64 failed but I did manage to format the 16 to fat32 and that just got finished can see this device is going to be very handy, and a bit of a challenge, but that is OK. I have always believed that learning is a lifelong experience, and if you stop learning you stop living. As I try to learn more about this thing, I am sure I will be visiting this page frequently. |
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Re: Measuring Impedance for the 433 MHz RTL-SDR Antenna
QUOTE: You will do far better to connect the right most antenna and
collapse it ***toa 1/4-wavelength at 433 MHzWhy do you say that ? Can't find the extensible antenna anyway. Because the other two antennas shown in the image will not work well if at all on 433 MHz. The extendable antenna in the right-most antenna in the image. Dave - W?LEV On Sat, Mar 8, 2025 at 12:39?AM Roger Need via groups.io <sailtamarack= yahoo.ca@groups.io> wrote: On Thu, Mar 6, 2025 at 03:38 PM, kellogs wrote:--it *Dave - W?LEV* -- Dave - W?LEV |
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Re: Smith Chart accuracy - what am I missing ?
Hi Donald,
Thanks for that, it was pretty much what I was expecting, and ties in with what I am getting with my DG8SAQ VNA. I am also using a silver mica capacitor with about the same lead lengths. Also when using a a Sark 110 VNA I am able to get sensible readings. To get 11 ohms resistive just doesn't make any sense, and I am beginning to suspect that I have a defective NanoVNA. Thanks for the info. 73 Stewart G3RXQ |
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Re: Smith Chart accuracy - what am I missing ?
Hi Stewart,
Out of grins and giggles I took a 470 pF silver mica cap which actually measured 454pf using smart tweezers. I then just used a test jig with my NanoVNA which was a white plug in experimenter board. Using an 18inch cable connected to my NanoVNA on one end and an SMA adapter on the other with 0.75 inch wires soldered to the SMA adapter that I plugged into my experimenter board. I then just did a simple open, short, load calibration in which the open was just that (complete open), then I used a short jumper wire plugged into the board, and then a 50 ohm resistor plugged into the experimenter board. I used a resistor with legs versus a nice calibration resistor that comes with the NanoVnA because I wanted the calibration to include the component legs (zero out the effects of the complement legs). I then measured my silver mica cap and it measured 845 m - 46.29 j ohms at 7.285 MHz which is pretty close to the Xc value of 48.12 ohms based on a 454pf cap. Pretty crude fixture but does a decent job at 7 MHz. P.S. I’m using a NanoVNA-F V3.1 Just FYI, Don wd8dsb |
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Re: Measuring Impedance for the 433 MHz RTL-SDR Antenna
Uhm, right. This is over my head...
I was looking to find another impedance measurements for the said antenna, so I can check my measurements against. It looks to me unlikely to obtain such, so how about this: use a few resistors after calibration and verify I am reading their resistance with 5% margin and +/-j10 reactance ? I have done so with two thick film resistors, 24 ohm and 220 ohm; results are within the limits above. Call it good? |
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Re: Measuring Impedance for the 433 MHz RTL-SDR Antenna
it depends ... what about a vertical 2x 5/8 wave ?? also with a loading coil at centre but with some gain over a quarterwave ... and cause its endfed it may be halfwave on 2m and 2x 5/8 on 70cm as dualband antenna its more or less ground independant (so called "earthless")
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dgt9bfc sigi Am 08.03.2025 um 01:38 schrieb Roger Need via groups.io: On Thu, Mar 6, 2025 at 03:38 PM, kellogs wrote:Because the antenna that is center loaded with a coil will have less gain than a 1/4 wave marconi antenna.You will do far better to connect the right most antenna and collapse itWhy do you say that ? Can't find the extensible antenna anyway. |
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Re: Smith Chart accuracy - what am I missing ?
I get accurate results when measuring capacitors with my NanoVNA so I assume it is something with your setup or calibration. Without photos or screenshots we don't have much to work with here.
Here is a measurement of a ceramic cap marked 485 pF that I made with a NanoVNA. I compared it to a measurement made with a DE-5000 LCR meter at 100 kHz. The estimate of Rs is poor under 500 kHz when the reactance is very high. But capacitance measurement is quite good. |
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Re: Smith Chart accuracy - what am I missing ?
Hi Jeff,
Yes, you are right about the strays of the test jig, and about the capacitor lead length. However, I am using the same test jig and capacitor that, although not perfect gives milli ohms resistance when using the DG8SAQ VNA and 11 ohms with the NanoVNA. So I am comparing Apples with Apples, and not with Oranges. 73 Stewart G3RXQ |
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Re: Smith Chart accuracy - what am I missing ?
Hi Stewart
Even at 7MHz it is unlikely that you will not be seeing some stray inductance and stray capacitance, so I am more surprised, at the 0 -j50 ohms. Unless your test jig is very good and the actual capacitor is leadless you are unlikely to get exactly 0 -j50 ohms. 73 Jeff G8HUL ________________________________________ From: nanovna-users@groups.io <nanovna-users@groups.io> on behalf of Stewart Baker <baker@...> Sent: 08 March 2025 14:51 To: nanovna-users@groups.io Subject: [nanovna-users] Smith Chart accuracy - what am I missing ? Hi, I am trying to get to grips why I am not getting the same Smith Chart results with my NanoVNA as I am with my DG8SAQ VNA. I have got to the stage of trying to make everything as simple as possible. I am using the same connectors, cables and calibration standards. To make things even simpler I am just using a quality HV mica capacitor which measures 442 pf. According to Pasan software this should be 0 -j50 ohms at 7.2MHz. When I use the DG8SAQ VNA I get milli ohms -j46 ohms, which to me is close enough. Using the NanoVNA with an identical setup I get 11 ohms -j49 ohms. Which is pretty useless. What is going on ? T.I.A. Stewart G3RXQ |
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Re: Smith Chart accuracy - what am I missing ?
Using the NanoVNA with an identical setup I get 11 ohms -j49 ohms. Which is pretty Indeed 442 pF at 7.2 MHz, that's 0-j50 ohms No need for software to calculate that. Z=1/Cw where w=2*pi*F C in Farad F in Hz pi 3.14 If with your NanoVNA you read: 11 ohms -j49 ohms, it's probably because you have badly calibrated your nanoVNA. You have to do OSL at the location where you place your capacitor -- Fran?ois -----Message d'origine-----De la part de Stewart Baker Envoyé : samedi 8 mars 2025 15:51 |
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Smith Chart accuracy - what am I missing ?
Hi,
I am trying to get to grips why I am not getting the same Smith Chart results with my NanoVNA as I am with my DG8SAQ VNA. I have got to the stage of trying to make everything as simple as possible. I am using the same connectors, cables and calibration standards. To make things even simpler I am just using a quality HV mica capacitor which measures 442 pf. According to Pasan software this should be 0 -j50 ohms at 7.2MHz. When I use the DG8SAQ VNA I get milli ohms -j46 ohms, which to me is close enough. Using the NanoVNA with an identical setup I get 11 ohms -j49 ohms. Which is pretty useless. What is going on ? T.I.A. Stewart G3RXQ |
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Re: Measuring Impedance for the 433 MHz RTL-SDR Antenna
Hit send too quick.
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The way these work is the bottom 1/4 wave has a high Z at the top. The coiled section moves that up, and ideally is a 1/2 or 1 wavelength that does not radiate, then you drive the bottom of a half wave dipole at the base (another high Z location). So you wind up with two antennas spaced a bit more than a half wavelength apart, phased so that they radiate in phase. Since there is something loading the top of the bottom monopole, it’s not exactly like a monopole. I should note that these things are tricky to model, and a lot of times, it’s just done empirically, often starting with a known design that’s scaled - someone builds a prototype out of copper wire, stretches and shrinks the middle coiled section (or add/remove turns) and change the top and bottom lengths until it works right, then that’s what gets produced in steel. The other thing you want to do is make it reasonably broadband, and that can also be done by adjusting the lengths of the 3 sections. Tuning one of this without a VNA is a pain. And it’s even better if you can set up the second input as a field strength measurement - you want both an ok match and good gain. On Mar 7, 2025, at 17:31, Jim Lux via groups.io <jimlux@...> wrote: |
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Re: Measuring Impedance for the 433 MHz RTL-SDR Antenna
Not necessarily - depending on the lengths. What you might wind up with is essentially a colinear array, with either a dipole stacked over a monopole or dipole stacked over a dipole, and that coil in the middle is essentially a phasing/matching section to get the currents right.
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This kind of thing was very common with cellular antennas back in the 80s - where you want a patten that is high gain at the horizon and lower gain at elevations above that. Cellular is ~850 ~900 MHz so 33-35 cm wavelength. A half wave is about 15 cm (6”) long. On Mar 7, 2025, at 16:38, Roger Need via groups.io <sailtamarack@...> wrote: |
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Re: Measuring Impedance for the 433 MHz RTL-SDR Antenna
On Thu, Mar 6, 2025 at 03:38 PM, kellogs wrote:
Because the antenna that is center loaded with a coil will have less gain than a 1/4 wave marconi antenna.Why do you say that ? Can't find the extensible antenna anyway. |
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Re: how good is a rain gutter antenna
"You'd be surprised how sensitive modern electronics are to
stray RF fields!!" Not at all surprised. I recently had to remove the flatbed scanner from the desk where the computer it is connected to sits, which is near the 530. Every time I tripped the transmitter to load the thing or test something, the scanner would do a start-up cycle. "I used to do just this for Uncle. I can't confirm or deny anything......." Yeah. Me too. Never happened. |
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Re: how good is a rain gutter antenna
Just simple overload of the sensors. They typically are tied together
throughout a single installation which may consist of a whole apartment complex. If one is tripped, the whole complex is tripped. For example, if you connect a fast diode to a couple of clip leads and measure the developed (rectified) voltage while you're transmitting, it's easily enough potential to "trip" about any modern semiconductor into conduction. You'd be surprised how sensitive modern electronics are to stray RF fields!! I used to do just this for Uncle. I can't confirm or deny anything....... Dave - W?LEV <> Virus-free.www.avg.com <> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2> On Fri, Mar 7, 2025 at 4:24?PM Jim Blanchard via groups.io <proptop12= gmail.com@groups.io> wrote: Just want to understand correctly why the fire alarms went off.-- *Dave - W?LEV* -- Dave - W?LEV |
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Re: how good is a rain gutter antenna
Just want to understand correctly why the fire alarms went off.
I'm thinking the fire alarms are on the 450MHz band somewhere. So are they set off due to some harmonic from the HF bands you all are trying, or is it just plain RFI from the non-resonant attempt for an antenna? Thanks and 73. Jim - N?TRP |
Roger Need
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