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Measuring Impedance for the 433 MHz RTL-SDR Antenna
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Hi,
i am trying to get a grip on using the nanoVNA. Does anybody have the 433 MHz antenna + base from the RTL-SDR kit ? Firstly calibrated at device port (no cable), then measured the loaded monopole through its magnetic antenna base at 433 MHz (first picture, antenna is on the left):? Nooelec - Nooelec NESDR SMArt v5 Bundle - HF/VHF/UHF (100kHz-1.75GHz) RTL-SDR Kit with 3 Antennas. RTL2832U & R820T2-Based Software Defined Radio Mounted on car hood:? 8 ohm at resonanceMounted on bicycle:? 7.5 ohm at resonance Does it sound right ? |
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No images attached.
Dave - W?LEV On Thu, Mar 6, 2025 at 8:22?PM kellogs via groups.io <mihai0226= [email protected]> wrote: Hi,-- *Dave - W?LEV* -- Dave - W?LEV |
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Whoops!
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(first picture, antenna is on the left) On Friday, March 7, 2025 at 12:31:47 AM GMT+2, W0LEV via groups.io <davearea51a@...> wrote:
No images attached. Dave - W?LEV On Thu, Mar 6, 2025 at 8:22?PM kellogs via groups.io <mihai0226= [email protected]> wrote: ? Hi,-- *Dave - W?LEV* -- Dave - W?LEV |
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Sounds quite low. A monopole against a ground plane will be in the 25-35 ohm range. That one is obviously center loaded, but perhaps not for 433 MHz (more likely cell band at 850). Did you also get the extendable whip antenna? put that on the base and make it about 17 cm long. Or a piece of wire.
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-----Original Message-----
From: <[email protected]> Sent: Mar 6, 2025 2:34 PM To: [email protected] <[email protected]> Subject: Re: [nanovna-users] Measuring Impedance for the 433 MHz RTL-SDR Antenna Whoops! (first picture, antenna is on the left) On Friday, March 7, 2025 at 12:31:47 AM GMT+2, W0LEV via groups.io wrote: No images attached. Dave - WØLEV On Thu, Mar 6, 2025 at 8:22?PM kellogs via groups.io wrote: Hi,-- *Dave - WØLEV* -- Dave - WØLEV |
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None of these antennas are really any good at HF frequencies and certainly
NOT down too 100 kHz. Sure, any hunk of conductor will capture "something". But only the one on the right is acceptable for anyhthing, but not good at everything. The left most antenna is strictly narrow-band and may not even address the 433 MHz band. You will do far better to connect the right most antenna and collapse it to a 1/4-wavelength at 433 MHz. That would be about 6.5-inches long. Dave - W?LEV On Thu, Mar 6, 2025 at 10:34?PM kellogs via groups.io <mihai0226= [email protected]> wrote: Whoops!-- *Dave - W?LEV* -- Dave - W?LEV |
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I would not say that. Aside from measuring a monopole + cable and not just a monopole, it is not only loaded but also 27 cm long (17 cm for 433 MHz). Probably in order to present a good match to the receiver.
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I have found the UHF, similar but shorter antenna; this one is 433 MHz and capturing all sort of signals. Included: ? NESDR SMArt SDR in black brushed aluminum enclosure ? Antenna base w/ 2m RG58 cable ? Telescopic antenna mast (variable frequency) ? 433MHz (ISM) antenna mast (fixed frequency) ? UHF antenna mast (fixed frequency) On Friday, March 7, 2025 at 12:54:54 AM GMT+2, Jim Lux via groups.io <jimlux@...> wrote:
Sounds quite low.? A monopole against a ground plane will be in the 25-35 ohm range.? That one is obviously center loaded, but perhaps not for 433 MHz (more likely cell band at 850).? Did you also get the extendable whip antenna? put that on the base and make it about 17 cm long.? Or a piece of wire. -----Original Message----- From: <[email protected]> Sent: Mar 6, 2025 2:34 PM To: [email protected] <[email protected]> Subject: Re: [nanovna-users] Measuring Impedance for the 433 MHz RTL-SDR Antenna Whoops! (first picture, antenna is on the left) On Friday, March 7, 2025 at 12:31:47 AM GMT+2, W0LEV via groups.io wrote: No images attached. Dave - WØLEV On Thu, Mar 6, 2025 at 8:22?PM kellogs via groups.io wrote: ? Hi,-- *Dave - WØLEV* -- Dave - WØLEV |
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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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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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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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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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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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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= [email protected]> wrote: On Thu, Mar 6, 2025 at 03:38 PM, kellogs wrote:--it *Dave - W?LEV* -- Dave - W?LEV |
Roger Need
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