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.
73,
Maynard
W6PAP
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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)
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.
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-----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:
I suppose the network although more work would yield better results?
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.
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
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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:
Hi Patricio
Thanks for clarification , I do not understand this graphic zone circled on red color below
73's Nizar
<Capture d_????cran 2025-04-06 174532.png>
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Re: Dummy Antenna and FM Alignment
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
On Mon, Apr 7, 2025 at 09:47 PM, Terry W7AMI wrote:
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.
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".
Joe, K1ike
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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.
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.
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-----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.
Joe, K1ike
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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.
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.
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-----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,
True beginner in RF here for a classic alignment question. Sorry if this
is inappropriate or the wrong group, I'm not sure where to ask such
question. I suppose the NanoVNA would be a great tool to solve the question
and you guys seem to know a thing or two about RF.
I'm working on FM alignment for Citroëns car radios from the late 80s and
early 90s. For this I would suppose matching the alignment to the OE
antenna and cable are ideal for best tuner performance and sensitivity.
Therefor I also suppose it would mean making something such as a dummy
antenna/aerial or a network so that the RF generator looks exactly like the
antenna does to the radio, right?
I have the cable, base and antenna of the car taken out. I also have
access to a bunch of cars if averaging is necessary.
My particular generator is 50ohm. I have no clue what the OE antenna and
cable impedance are, as nothing is printed on them. Reading online it seems
most automotive radio cable are 93 or 100ohms?
For the extra information: there are two models I would like to align, a
Blaupunkt which would have a 150ohm impedance, but the other which is a
Clarion does not give any information regarding its impedance... it just
says to plug the generator into the radio and voila.
Thanks for looking into my question.
-- *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,
True beginner in RF here for a classic alignment question. Sorry if this
is inappropriate or the wrong group, I'm not sure where to ask such
question. I suppose the NanoVNA would be a great tool to solve the question
and you guys seem to know a thing or two about RF.
I'm working on FM alignment for Citro?ns car radios from the late 80s and
early 90s. For this I would suppose matching the alignment to the OE
antenna and cable are ideal for best tuner performance and sensitivity.
Therefor I also suppose it would mean making something such as a dummy
antenna/aerial or a network so that the RF generator looks exactly like the
antenna does to the radio, right?
I have the cable, base and antenna of the car taken out. I also have
access to a bunch of cars if averaging is necessary.
My particular generator is 50ohm. I have no clue what the OE antenna and
cable impedance are, as nothing is printed on them. Reading online it seems
most automotive radio cable are 93 or 100ohms?
For the extra information: there are two models I would like to align, a
Blaupunkt which would have a 150ohm impedance, but the other which is a
Clarion does not give any information regarding its impedance... it just
says to plug the generator into the radio and voila.
Thanks for looking into my question.
-- *Dave - W?LEV* -- Dave - W?LEV
|
Dummy Antenna and FM Alignment
Hey everyone,
True beginner in RF here for a classic alignment question. Sorry if this is inappropriate or the wrong group, I'm not sure where to ask such question. I suppose the NanoVNA would be a great tool to solve the question and you guys seem to know a thing or two about RF.
I'm working on FM alignment for Citro?ns car radios from the late 80s and early 90s. For this I would suppose matching the alignment to the OE antenna and cable are ideal for best tuner performance and sensitivity. Therefor I also suppose it would mean making something such as a dummy antenna/aerial or a network so that the RF generator looks exactly like the antenna does to the radio, right?
I have the cable, base and antenna of the car taken out. I also have access to a bunch of cars if averaging is necessary.
My particular generator is 50ohm. I have no clue what the OE antenna and cable impedance are, as nothing is printed on them. Reading online it seems most automotive radio cable are 93 or 100ohms?
For the extra information: there are two models I would like to align, a Blaupunkt which would have a 150ohm impedance, but the other which is a Clarion does not give any information regarding its impedance... it just says to plug the generator into the radio and voila.
Thanks for looking into my question.
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Re: NANO VNA H4 , Implausible measurement results?
If you used this BNC to Banana adapter, you are adding about 2 pF or so of shunt capacitance across the feed point of your test dipole. At 500 MHz for example, this is about Xc = 159 ohms. This is significant and will impact where the min SWR will be.
Not sure if this was mentioned above or not. Sorry.
I determined that many of these adapters disrupt the characteristic Z. I came across this years ago as I have old GR bridges. GR, I think, set the spacing at 0.75 inches many decades ago and it was even significant when tinkering with EFHW antennas (the feed point is on the order of 3000 ohms so even a few pF at HF can be significant).
There are different versions of these BNC/Banana adapters and different mfrs so your adapters might be better or worse. I use them mainly for low frequency applications.
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Re: Measurement correction for Zc Coax caracteristic Impedance
This is the part of LF model that don’t work basically because is the wrong frequency region…
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On 6 Apr 2025, at 1:49?PM, Team-SIM SIM-Mode via groups.io <sim31_team@...> wrote:
Hi Patricio
Thanks for clarification , I do not understand this graphic zone circled on red color below
73's Nizar
<Capture d_????cran 2025-04-06 174532.png>
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Re: Measurement correction for Zc Coax caracteristic Impedance
Hi Patricio
Thanks for clarification , I do not understand this graphic zone circled on red color below
73's Nizar
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Re: Measurement correction for Zc Coax caracteristic Impedance
This chart shows the typical behaviour of a loosy coaxial cable over frequency. Basically there are two models , the low frequency model in this case the dielectric loose are very small then G tends to zero and is removed from the Zo formula. The serial equivalent loose resistor this is basically the resistance of the metallic boundary of the cable. It drops to lower frequency to a minimum this is determined by the conductor resistivity and uses to be very small but never zero , L goes up from the High Frequency model because appears magnetic fields inside de conductors… as frequency goes down the Zo goes up. In the other hand on high frequency region L an C are dominant on Zo formula and results on a more stable value. Skin effect reduces de magnetic field inside de conductor and L becomes defined basically by metallic boundaries.
At all this is not a problem in the practical world because the coaxial cables are used on high frequency regions , at audiofrecuencies becomes a shielded cable and Zo concept are irrelevant. In metrology these variations are taken to correct high precision measurements and standards characterization.
At very low frequencies where transmission lines are required usually higher Zo is adopted too . This is the frequency where LF and HF curves crosses are higher as Zo is lower. This is of course with the same materials on each case.
I?m sorry for my engllsh is not very good. If you have any difficulties to understand that I say please let me know.
Regardd, Patricio.
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On 6 Apr 2025, at 5:31?AM, Team-SIM SIM-Mode via groups.io <sim31_team@...> wrote:
Hi Patricio
I do not understand this chart, can you explaine it more pse.
73's Nizar
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Terry W7AMI
