(Vehicle) Diversity FM Antenna Systems - why not more popular ?

[kfbkfb]

Mobile FM reception is problematic, I heard about 2 antenna FM car radios many years ago.

FM tuner tech can only do so much in "dead spots" and/or severe multipath spots, switching to another antenna in another location on the vehicle seems like an easy way to try to improve reception, anyone know why 2 antenna FM systems weren't/aren't more popular in vehicles?


Kirk Bayne

 

[PTBoardOp94]

If you have terrain, buildings, etc. causing multipath, both antennas will encounter it, at different (random) moments in time. A tuner could never predict which antenna to use.

You can mix the two antennas, rather than switching, but that kind of combination is very much not free, and probably more harmful than just using a single antenna. An ideal combiner will lose 3dB (or half the power) in the combination, practically the losses are even higher.

 

[Kelly A]

Mobile FM reception is problematic, I heard about 2 antenna FM car radios many years ago.

FM tuner tech can only do so much in "dead spots" and/or severe multipath spots, switching to another antenna in another location on the vehicle seems like an easy way to try to improve reception, anyone know why 2 antenna FM systems weren't/aren't more popular in vehicles?

There were a couple manufacturers way back in the 80's which tried diversity reception. The result was: a more expensive and elaborate system, with unpredictable performance. That, and after doing consumer research, it was determined that occasional random multipath reception areas wasn't that much of a concern to an average consumer.
Instead, modern SDR receivers baked-in automatic stereo blending and audio filtering to temporarily reduce the impact when rolling through a multipath event.

As an aside: Why is this a 'National Radio' question?

 

[landtuna]

I had a diversity antenna system in my 2012 Genesis and it seemed to work very well because in all the time I owned that car I can remember only one dead spot and that was right next door to a general aviation airport.

 

[kevtronics]

Back in the '90s, Nissan radios used to advertise Diversity Antenna as a feature (along with C-Quam AM Stereo).

I believe most diversity antennas used the main telescopic antenna together with the rear defroster grid acting as the second antenna. Now, for the sake of a sleeker look, better aerodynamics, and cost-cutting, virtually all cars and many trucks no longer have an external antenna and only use the rear defroster grid as the antenna. (Pickup trucks which don't have a rear defroster being the exception.)

 

[ZantennaG1]

only use the rear defroster grid as the antenna. (Pickup trucks which don't have a rear defroster being the exception.)

I did not realize that. So the windows really do help with reception!

 

[Mikey Radio]

Now, for the sake of a sleeker look, better aerodynamics, and cost-cutting, virtually all cars and many trucks no longer have an external antenna and only use the rear defroster grid as the antenna. (Pickup trucks which don't have a rear defroster being the exception.)

Is that still the case with more modern "Infotainment Systems" that have much more than just AM/FM radio to pickup? Even my last GM vehicle which was a 2013 model had AM/FM radio, SiriusXM, OnStar, interactive maps and could act as a WiFi hotspot. It had one of the shark fin antennas above the rear window, supposedly for all those things, including terrestrial broadcast.

 

[ZantennaG1]

Is that still the case with more modern "Infotainment Systems" that have much more than just AM/FM radio to pickup? Even my last GM vehicle which was a 2013 model had AM/FM radio, SiriusXM, OnStar, interactive maps and could act as a WiFi hotspot. It had one of the shark fin antennas above the rear window, supposedly for all those things, including terrestrial broadcast.

I rented a 2021 Jeep Grand Cherokee and spent most of the time figuring out where the whip antenna was at! (It didn't have one, to my surprise). The infotainment stuff is through that shark fin while the defroster is for the radio, most likely. I also had rented a 2017 Dodge Charger, and it did have the tiny whip antenna on top.

 

[Greg Strickland]

As I understand it- In analog FM receiver system a diversity receiving antenna system would be choosing one antenna at a time, based on a decision made by polling reception quality from the receiver associated with each antenna. In historic analog this could be done reasonably well by examining the composite audio spectrum and choosing the antenna-receiver with the cleanest composite spectrum. In an FM analog receiver with digital signal processing (in this case I don't mean HD Radio), it may be possible to process a combined RF signal from multiple antennas and/or the demodulation process in a way that is much more effective than the historic analog execution of the diversity antenna concept.

Here is something to consider: FM wavelength is approximately ten feet. Because of this I think an effective diversity antenna system would require antenna spacing of at least five feet. Empirical example of this: No doubt many of you have heard FM analog multipath distortion at a stop light. How far to do have to creep the car to reduce or eliminate it?

So far, wavelength is the heartbeat of radio.

 

[kevtronics]

Modern DSP-based car radios have so many neat tricks that could be provided to the user, if only they were programmed to do so. For example, a real-time spectrum analysis graph of the entire band. I know it's possible because the radio in my 2018 VW provides an instantaneous list of all the AM and FM stations that it thinks are strong enough to be listenable, including RDS and HD Radio text data.

And why not NOAA weather radio? It would be just a few more lines of code to add. I think Subaru and Volvo have/had it in some of their models, but it has never been a common feature. And I remember in the '80s and '90s some car radios had a button to listen to CB channel 19 (the truckers' channel).

 

[ZantennaG1]

For example, a real-time spectrum analysis graph of the entire band. I know it's possible because the radio in my 2018 VW provides an instantaneous list of all the AM and FM stations that it thinks are strong enough to be listenable, including RDS and HD Radio text data.

Awesome!

And why not NOAA weather radio? It would be just a few more lines of code to add. I think Subaru and Volvo have/had it in some of their models, but it has never been a common feature. And I remember in the '80s and '90s some car radios had a button to listen to CB channel 19 (the truckers' channel).

Yep, 2002 Subaru Outback had NOAA Weather Radio on it, and a deck for cassette tapes!

 

[Kelly A]

Modern DSP-based car radios have so many neat tricks that could be provided to the user, if only they were programmed to do so. For example, a real-time spectrum analysis graph of the entire band. I know it's possible because the radio in my 2018 VW provides an instantaneous list of all the AM and FM stations that it thinks are strong enough to be listenable, including RDS and HD Radio text data.

My Volvo radio does the same thing. If on a local station, I can just press a jog wheel on the steering wheel, and literally scroll across the FM band within range, listing all the station call letters, and even PAD and RDS data like song title or host. The screen lists not only the station I'm listening to, but every station, including HD subchannels around it. I can sweep the jog wheel to a station, and press enter to select it. The list changes as I drive while stations go into and out of receivable range. Same goes if I select SXM. AM used to be something similar, but less. I could see the call letters of the station if they ran HD.

And why not NOAA weather radio? It would be just a few more lines of code to add. I think Subaru and Volvo have/had it in some of their models, but it has never been a common feature. And I remember in the '80s and '90s some car radios had a button to listen to CB channel 19 (the truckers' channel).

My 2019 Volvo doesn't have NWS, but I do get SXM weather.

 

[DaveBayArea]

Here is something to consider: FM wavelength is approximately ten feet. Because of this I think an effective diversity antenna system would require antenna spacing of at least five feet. Empirical example of this: No doubt many of you have heard FM analog multipath distortion at a stop light. How far to do have to creep the car to reduce or eliminate it?

So far, wavelength is the heartbeat of radio.

Most of the early diversity radios relied on the signal received from a windshield antenna vs a vertical whip. Since those two have opposite polarization, and multipath interference arrives at random oblique angles, it worked amazingly well. I actually hunted down one of those old Clarion FM Diversity radios to use as a replacement for the factory radio in a Chevy. The result was astounding in mountainous terrain. The only trick is, you need the preamp module that came with the radio. The design included a varicap-tuned preamp that works with the windshield antenna.

Dave B.