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Re: Measurement correction for Zc Coax caracteristic Impedance
As someone who does both measuring and modeling.. Modeling is what gives you insight into things like sensitivity and variability. Measuring, to some extent, confirms what you modeled, or, that you’ve got something wrong in the model.
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Modeling also can give you a much wider tradespace - building a new model is essentially free. I build a fair number of antennas for various uses - Here’s an example: I’ve got a spacecraft that has two dipoles about 5 meters long, crossed at right angles. Easy enough to model, or build a mockup and measure (a bit harder, but doable). But I might have a question about “if there’s an angular misalignment of 10cm at the tip, does that make a difference in my measurement”. Easy question to answer with modeling, very, very difficult with measurement (time consuming, if nothing else). Consider someone putting up a LPDA with 10 elements - modeling can tell you what happens to the performance if the elements are skewed by 10 degrees pretty quickly. Testing would be a pain. On Apr 9, 2025, at 10:05, Team-SIM SIM-Mode via groups.io <sim31_team@...> wrote: |
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Re: Measurement correction for Zc Coax caracteristic Impedance
Hi Maynard
Electrical modelisation is appreciated as a first approche of computing in the history, after what we procede with computers simulation last twenty years, in present time there is no better then a good measurement at the desired frequency, its become possible with the cheap devices as nanovna , just need some good method and practice , lt should defeat all mathematical modelisations or PC simulations. 73s Nizar |
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Re: Measurement correction for Zc Coax caracteristic Impedance
I would say that L remains constant (it's mostly determined by the physical construction, and the length), as long as it's not one of those funky delay line coaxes where the center is a spiral wrapped on a ferrite core. Same with C - it's all about the two diameters, and epsilon, which for most popular dielectrics is pretty constant with frequency. Unless there's water or a liquid involved.
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The things that change with frequency are R (skin depth) and G (dielectric loss) -----Original Message-----
From: <[email protected]> Sent: Apr 9, 2025 7:42 AM To: <[email protected]> Subject: Re: [nanovna-users] Measurement correction for Zc Coax caracteristic Impedance If you need to calculate the characteristic impedance over a range of frequencies that overlaps the curved segment of the figure and, if you can assume that G=0 for all frequencies of interest, a further simplification is possible: use the low frequency approximation at all frequencies. If you are looping through tabular values of C, R, and L, or using approximating expressions, then as wL / R becomes very large, the low frequency approximation approaches the high frequency approximation as a limit. Although R and L are generally variable with frequency, it is often possible to assume that C is constant over a wide range of frequencies. 73, Maynard W6PAP On 4/8/25 07:45, Maynard Wright, P. E., W6PAP via groups.io wrote: True! The three expressions in the figure represent the exact formula, > a low frequency approximation, and a high frequency approximation. On > the logarithmic scale of the figure, the low frequency approximation is > asymptotic to a straight line, approaching that line very closely at low > enough frequencies.In the figure, the straight line representing the low frequency > approximation is extended below the horizontal straight line > representing the high frequency approximation. But the conditions that > make the low frequency approximation reasonable, R >> wL, are not true > above around 300 kHz for virtually all transmission lines and the actual > impedance begins to move toward the high frequency approximation through > a curved region for which you must use the exact expression if you want > accurate calculations.W6PAP |
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Re: Measurement correction for Zc Coax caracteristic Impedance
If you need to calculate the characteristic impedance over a range of frequencies that overlaps the curved segment of the figure and, if you can assume that G=0 for all frequencies of interest, a further simplification is possible: use the low frequency approximation at all frequencies.
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If you are looping through tabular values of C, R, and L, or using approximating expressions, then as wL / R becomes very large, the low frequency approximation approaches the high frequency approximation as a limit. Although R and L are generally variable with frequency, it is often possible to assume that C is constant over a wide range of frequencies. 73, Maynard W6PAP On 4/8/25 07:45, Maynard Wright, P. E., W6PAP via groups.io wrote:
True!? The three expressions in the figure represent the exact formula, a low frequency approximation, and a high frequency approximation.? On the logarithmic scale of the figure, the low frequency approximation is asymptotic to a straight line, approaching that line very closely at low enough frequencies. |
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Re: Dummy Antenna and FM Alignment
The only adjustment the antenna will affect is L2. Here's an alternative to measuring and averaging the impedance of several antennas: Put a short whip on your signal generator and radiate a signal into the car antenna. Then adjust L2 with the receiver in the car and connected to the antenna as normal but open so you can get at L2. I see no tracking components so the instructions probably just say to peak L2 at some frequency. You can do the rest of the alignment on the bench with the signal generator connected directly to the antenna input. AGC appears to detune the L2 tuned circuit on strong signals. This means you should use a weak signal when adjusting L2. To avoid an in-car adjustment for each radio, readjust L2 on the bench after having set it in the car. Note any difference from the in-car setting and apply it to other radios. This is only an approximate procedure, but it may be good enough.
Rereading your first post, you mention 150 ohms in the Blaupunkt instructions. If the instructions tell you to put 150 ohms in series with the generator, then just do it. Use it for both radios. If a more complicated dummy antenna was really necessary, the instructions would have said so and given a schematic. Good luck. Brian |
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Re: Dummy Antenna and FM Alignment
I've forgot to add AM is sadly out of the picture. There are no audio AM broadcasting left in France, we still receive BBC very well but not for long. So this is 100% an FM matter. Most use the AM amp signal as a way to wire a bluetooth receiver anyway.
I've added the schematics of the front-end. Once again I suck at reading schematics. It's built around a TDA1575T from Philips which is a front end chip housing the LO, Mixer and IF Amp. I think BF999 would be the RF amp? The alignment instructions say to tune L6 which would be the local oscillator I think, L2&L4 which would be bandpass filter?, and F1 which would be the IF filter before the amplifier, and after that the rest of the IF alignment. Let me know if I got everything right or not, it's always a great exercise. I'll do the antenna measurements mounted on cars yes. It's also useful as in averaging the antenna because of production variation, but also because being 30 years old I except some to have aged quite a lot compared to new in terms of characteristics. Regarding external signals, how can I avoid nearby stations from interfering with the VNA reading? |
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Re: Dummy Antenna and FM Alignment
I assume that you will be aligning this antenna in the broadcast band. So be careful about strong signals from nearby stations.
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73, Maynard W6PAP On 4/8/25 05:28, Ed AB1XQ via groups.io wrote:
It's best to measure the antenna with the VNA as the antenna will be used. Mounted on the car, outside, doors closed, away from large metal structures, etc. |
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Re: Dummy Antenna and FM Alignment
That antenna looks like a 100k reactance capacitor at 1 MHz. (about 1.5 pF)
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The coax probably is something like 50 pF, so even without the radio in the mix, you're looking at a 1:30 voltage divider. Thankfully, AM radio signals are strong. At 1 km from a 1kW station at 1 MHz with a 75 meter 1/4 wave monopole, the field is about 300 mV/m. At 10 km, 30 mV/m, etc. A typical AM MW receiver has a sensitivity of say, 30-40 microvolts/meter. So even with a 30 fold hit you've still got plenty of signal. -----Original Message-----
From: <[email protected]> Sent: Apr 8, 2025 9:15 AM To: <[email protected]> Subject: Re: [nanovna-users] Dummy Antenna and FM Alignment Thanks for answering my post guys. Some additional information: here's a photograph of the whole antenna network. The antenna is about 43cm in length from the central stud of the base to the tip of the antenna. The base is grounded (interior side) to the car body directly. The cable is 327cm (10.7ft) and is ended by a ISO male plug, possibily ISO 10559 but not sure. The radio is totally unbalanced. It seems some of you guys think the impedance mismatch won't be much of an issue, while others suggest taking measurements of different cars, average, and build a matched network to emulate the antenna to the radio and a 50ohm load to the generator. I suppose the network although more work would yield better results? And also perhaps allow me to tune the radio regardless of its impedance? Thanks a lot. |
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Re: Dummy Antenna and FM Alignment
On Tue, Apr 8, 2025 at 09:15 AM, AArnaud wrote:
It will let you align the front-end more accurately. Check the radio alignment instructions. If there are no adjustments for the front-end, then it doesn't matter. Just connect the generator directly to the receiver. If you measure the antenna impedance with a NanoVNA, do it with the antenna mounted on the car in its normal hole and oriented as it would be when driving. Brian |
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Re: Dummy Antenna and FM Alignment
Thanks for answering my post guys.
Some additional information: here's a photograph of the whole antenna network. The antenna is about 43cm in length from the central stud of the base to the tip of the antenna. The base is grounded (interior side) to the car body directly. The cable is 327cm (10.7ft) and is ended by a ISO male plug, possibily ISO 10559 but not sure. The radio is totally unbalanced. It seems some of you guys think the impedance mismatch won't be much of an issue, while others suggest taking measurements of different cars, average, and build a matched network to emulate the antenna to the radio and a 50ohm load to the generator. I suppose the network although more work would yield better results? And also perhaps allow me to tune the radio regardless of its impedance? Thanks a lot. |
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Re: Measurement correction for Zc Coax caracteristic Impedance
True! The three expressions in the figure represent the exact formula, a low frequency approximation, and a high frequency approximation. On the logarithmic scale of the figure, the low frequency approximation is asymptotic to a straight line, approaching that line very closely at low enough frequencies.
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In the figure, the straight line representing the low frequency approximation is extended below the horizontal straight line representing the high frequency approximation. But the conditions that make the low frequency approximation reasonable, R >> wL, are not true above around 300 kHz for virtually all transmission lines and the actual impedance begins to move toward the high frequency approximation through a curved region for which you must use the exact expression if you want accurate calculations. So the extension of the low frequency approximation represents a segment of the curve which will not be useful for representing most, if not all, actual lines. It is important to note that below about 300 kHz, the imaginary component of the characteristic impedance is not insignificant and, in the limit as the frequency goes lower, will be equal in magnitude to the real component so the impedance will have an angle of -45 degrees. This is true of telephone cable pairs at voice frequencies, almost all of which exhibit a phase angle of between -44 and -45 degrees. Since the high frequency approximations are not applicable where the phase of the characteristic impedance departs significantly from zero degrees, telephone engineers working on voice frequency facilities rarely use SWR and reflection coefficient, and use instead return loss and reflection loss. That's not very important to most of us in radio work unless we are reading material that was originally intended for folks working at lower frequencies. 73, Maynard W6PAP On 4/6/25 10:42, Patricio Greco via groups.io wrote:
This is the part of LF model that don’t work basically because is the wrong frequency region…On 6 Apr 2025, at 1:49?PM, Team-SIM SIM-Mode via groups.io <sim31_team@...> wrote: |
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Re: Dummy Antenna and FM Alignment
On Mon, Apr 7, 2025 at 09:47 PM, Terry W7AMI wrote:
The impedance presented to the front-end can affect tuned-circuit tracking and Q in the RF amplifier, which can affect receiver sensitivity. It can affect receiver selectivity if the tuned circuits have high Q and the tracking is off. Even if the car antenna is resonant, the impedance at the receiver input can be far from 50 ohms due to impedance transformation from the low-capacitance cable, which isn't anywhere near 50 ohms. Brian |
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Re: Dummy Antenna and FM Alignment
If you need to do an alignment on an FM radio you need an RF signal generator not a VNA. In my case I used the TinySA to generate the IF frequency, probably 10.7 MHz in your case, and the RF frequencies. It also needs to be able to generate an FM signal as well as un modulated carrier. I just did the AM and FM alignment on a 1960's transistor radio and the TinySA was perfect for the task. I wouldn't worry to much about the impedance miss match for doing an alignment. Once you are past the 1st RF amplifier it won't make any difference.
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Re: Dummy Antenna and FM Alignment
I remember a form of low loss cable that was popular in the 1980-90's the size of rg-8 (can't remember the brandname). It had a spiral plastic dielectric and I used a run of it for a UHF antenna.? One day I walked in the shack, only to find water on the floor.? It had rained that night and the outdoor connection to the antenna had a leak.? Water easily found its way in the outdoor leak and down the center of the coax.?? I called this my "garden hose feedline".
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Joe, K1ike On 4/7/2025 6:09 PM, Jim Lux via groups.io wrote:
You want low epsilon of the dielectric and foam tube or a spiral of plastic meets the requirement nicely. |
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Re: Dummy Antenna and FM Alignment
exactly. You want low epsilon of the dielectric and foam tube or a spiral of plastic meets the requirement nicely. To that extent it's like minimizing dielectric losses in microwave cables, but for a different reason.
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I asked a coax mfr about options for a custom coax for an application in a spacecraft receiver for low frequencies. We wanted something shielded for a run about 10-15 cm long get from point A to point B with minimum capacitance. -----Original Message-----
From: <[email protected]> Sent: Apr 7, 2025 1:51 PM To: <[email protected]> Subject: Re: [nanovna-users] Dummy Antenna and FM Alignment Maybe that explains an automobile antenna I found once in the past. I was surprised that what looked like coax was actually a piece of plastic tubing, maybe 1/4" I.D., with a thin wire running through the center. Joe, K1ike On 4/7/2025 1:05 PM, Jim Lux via groups.io wrote: The big requirement for coax on voltage probes is low capacitance - you generally don't worry about impedance. |
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Re: Dummy Antenna and FM Alignment
Maybe that explains an automobile antenna I found once in the past.? I was surprised that what looked like coax was actually a piece of plastic tubing, maybe 1/4" I.D., with a thin wire running through the center.
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Joe, K1ike On 4/7/2025 1:05 PM, Jim Lux via groups.io wrote:
The big requirement for coax on voltage probes is low capacitance - you generally don't worry about impedance. |
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Re: Dummy Antenna and FM Alignment
The big requirement for coax on voltage probes is low capacitance - you generally don't worry about impedance.
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That's because the antenna is largely capacitive (so it looks like a voltage in series with a capacitor), and shunt C at the receiver reduces the signal level (think of it as a capacitive voltage divider). In some ways, this is similar to high performance audio cables with high Z sources and amplifier inputs. -----Original Message-----
From: <[email protected]> Sent: Apr 7, 2025 9:31 AM To: <[email protected]> Subject: Re: [nanovna-users] Dummy Antenna and FM Alignment The high-Z coaxial cable is more for the AM portion of the radio. On AM frequencies, especially the European version, the "stinger" antenna is nothing but a capacitive probe. Consequently, it presents an extremely high impedance. The loss in the 93-ohm line is considerably less at the AM broadcast frequencies than a standard 50-ohm cable would offer. However at FM broadcast frequencies, the external "stinger" antenna is a reasonable portion of a wavelength, although still a bit short. Therefore, it offers something more manageable considering losses. If you're just doing FM alignment, 88 to 108 MHz, a 50-ohm source/load is appropriate. Some older units may have a 300-ohm balanced input. In that case you need a transformer/balun to convert from 300-ohms balanced input to an unbalanced 50-ohm load. That's the purest approach and theoretically correct. HOWEVER, if its 300-ohmsbalanced input, just take a visit to Home Depot, Lowe's or any big-box distributor and buy a cheap 300-ohm to 75-ohm converter and use your 50-ohm equipment. Dave - WØLEV On Mon, Apr 7, 2025 at 2:43?PM AArnaud via groups.io wrote: Hey everyone,-- *Dave - WØLEV* -- Dave - WØLEV |
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Re: Dummy Antenna and FM Alignment
What I would do is use a NanoVNA to measure the impedance over 88-108 MHz right at the connector that plugs into the radio. Average the measurements for several cars. Then design a network that approximates the average when driven by a 50 ohm generator. To ensure generator flatness, make the network show 50 ohms to the generator. Network loss shouldn't matter since you can set the generator level so resistors are fine.
Brian |
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Re: Dummy Antenna and FM Alignment
The high-Z coaxial cable is more for the AM portion of the radio. On AM
frequencies, especially the European version, the "stinger" antenna is nothing but a capacitive probe. Consequently, it presents an extremely high impedance. The loss in the 93-ohm line is considerably less at the AM broadcast frequencies than a standard 50-ohm cable would offer. However at FM broadcast frequencies, the external "stinger" antenna is a reasonable portion of a wavelength, although still a bit short. Therefore, it offers something more manageable considering losses. If you're just doing FM alignment, 88 to 108 MHz, a 50-ohm source/load is appropriate. Some older units may have a 300-ohm balanced input. In that case you need a transformer/balun to convert from 300-ohms balanced input to an unbalanced 50-ohm load. That's the purest approach and theoretically correct. HOWEVER, if its 300-ohmsbalanced input, just take a visit to Home Depot, Lowe's or any big-box distributor and buy a cheap 300-ohm to 75-ohm converter and use your 50-ohm equipment. Dave - W?LEV On Mon, Apr 7, 2025 at 2:43?PM AArnaud via groups.io <a.cabiyaud= [email protected]> wrote: Hey everyone,-- *Dave - W?LEV* -- Dave - W?LEV |
Terry W7AMI
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