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Dummy Antenna and FM Alignment
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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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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 |
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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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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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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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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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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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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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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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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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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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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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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. |
Terry W7AMI
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