Very short car antennas these days

[FMEnjoyer]

Walking around I noticed that some cars have an antenna on the rear of the roof some are as short as about 5 inches/10-12cms. That seems very short even for DAB Radio. They range from 5 inches about 12cms to a full 70cms or so. The 70cms ones are rare I only saw one of them and I think it was on a small mini Toyota, impressive for the micro size of the car itself. and the average size looks like about 25cms.

Is it because they are now mainly optimized for DAB radio which is high frequency ?

And I guess this is why that TEF6686 FM/AM chip is so sensitive to help make up for the trend in very short antennas.

Most appear to be a bit of wire that is coiled around the stick to attempt to lengthen it by 3-4 cms more wire than the stick length.

Not the most interesting thing in the world I know but I did notice it. As you might imagine for an Aneckorak my old banger has a big antenna that I even extended a bit. It works superb and picks up stations a friend with his modern 8inch "stick" has no chance of hearing.

Sign of the times I suppose. Dab and modern car looks.

[radionortheast]

I got a combined fm/dab car radio to play about with a while ago, i’d have car radios which i’d plug in use them off of 12v power supply. (not been used in a car!) I was going to ask about it, as what confused me was it had separate antennas for fm and dab, it made me wonder if there was some problem picking up dab through an existing car radio aerial. It made things awkward you always use an fm aerial with dab, a telesopic rod if you’ve have a roberts portable radio or something, they don't seem to have any problem picking up. From what I make out cars tend to use those fin antenna's now, its an unknown what goes on inside, I suppose i've always assumed car antenna's work on fm their connected by leaky coax, so it would likely form part of the aerial, the car body would also be part of the aerial, you can still pick up dab with longer antenna's which makes it mondo confusing.. can only imagine someone getting one to fit in their vauxhall nova/duesenberg, then finding no dab antenna, I guess they'd need an installer/mechanic or something.

[reverend]

There is an issue with trying to receive FM and DAB off the same antenna, if the antenna is resonant. A quarter wave at 100 MHz (which will be roughly 75cm long) will be a half wave at 200 MHz. A quarter wave has a low feed impedance, but a half wave has a very high feed impedance. Thus matching an antenna on the two bands is nigh-on impossible. However, if the antenna is non-resonant this doesn't occur.

Many older cars used the rear window heater as an antenna for broadcast services and the short roof antenna was for GPS only.

[FMEnjoyer]

I have seen those little fin radomes with no stick coming out as well, where it looks like you could barely fit a coil never mind an antenna. (most have radome and stick between 12cms and 70cms)

I would have thought they would be fine for receiving irrelevant of the impedance ? Maybe not though.

One thing for sure they are very very short. I suppose you have a decent ground on a car and with an ultra sensitve receiver it may not make much difference, until you compare it with an older and decent fm radio and a decent length rod.

Must all be about compromise. I see some cars have nothing visible at all.

[radionortheast]

I suppose with a portable set they only have the telescopic aerial, extending the length beyond the quarterwave will likely improve the reception, by connecting up a wire to ground 70cms hanging down you would find that you’d have to shorten the length of telescopic rod. With it on coax with a ground connection, it would likely be the samething, there wouldn’t be much difference making it shorter.

[shuffy]

Many older cars used the rear window heater as an antenna for broadcast services and the short roof antenna was for GPS only.

Some years ago I had a Golf mk5 which had the rear heated window element as the antenna - there was nothing outside the car at all. I was gobsmacked how well it worked and in particular how there was no engine/alternator or other electrical interference on medium wave. These days I've got a mk8 with a "blade" on the roof which does everything, DAB, FM, GPS. Again perfect performance on all 3, and again, gob smacking how these things are so good.

[FMEnjoyer]

Many older cars used the rear window heater as an antenna for broadcast services and the short roof antenna was for GPS only.

Some years ago I had a Golf mk5 which had the rear heated window element as the antenna - there was nothing outside the car at all. I was gobsmacked how well it worked and in particular how there was no engine/alternator or other electrical interference on medium wave. These days I've got a mk8 with a "blade" on the roof which does everything, DAB, FM, GPS. Again perfect performance on all 3, and again, gob smacking how these things are so good.

It must be the sensitivity and selectivity of the RX that makes up for the compromises in antenna ?

I suppose for something made of carbon fiber like a Tesla a piece of copper wire can be glued or moulded into the front windscreen pillar and connected easily, one side DAB and one side FM ? Probably all sorts of implementations now electric cars are most made from plastic (at least they all look like PVC.

Anyway at least it brought a little conversation.

I found this :

[reverend]

Looks an awful lot like the 'power strip antenna boosters' that you used to be able to buy back in the day to 'dramatically reduce static and increase reception by up to 50%'.

antennabooster.jpg

I also understand that they were also good at dramatically reducing your bank balance, and reducing the pain in the diodes down your left hand side. Life, don't talk to me about life...

[FMEnjoyer]

Yes this was just a fin antenna image search. What is actually in those find on cars I do not know.
Maybe it is something like that though, cause what else useful would even fit in there ?

Only thing I could think of is maybe a copper wire glued to the inside ?

The answer is PCBs and stuff

https://www.google.com/search?sca_esv=d ... ACGAIgAigC



Lots of modern horse sh1te no one really needs and will cost a fortune to repair at the manufacturer when it all goes wrong.

[Krakatoa]

They use an approach like the mini-whip does for hf bands. A small copper structure coupled to a high impedance wideband amplifier.
Mixed with the GPS signal down the coax line to the head unit.

[Persona Non Grata]

A lot of old school purists tend to look down their noses at these "active" antennas and insist that a roofmounted quarterwave whip is the only way to go.

They're probably right even if the technology behind amplified aerials is improving.

[EFR]

I had to remove that stock non amplified 20cm whip from my car, and get old classic 80cm whip to listen anything else than 60kW govt owned stations here were I live

[shuffy]

They use an approach like the mini-whip does for hf bands. A small copper structure coupled to a high impedance wideband amplifier.
Mixed with the GPS signal down the coax line to the head unit.

I was going to mention the miniwhip also but you beat me to it. Here's a writeup of a similar design to the one used on the Twente webSDR. If you're familiar with this SDR then you might be surprised when you see the antenna!

https://www.pa3fwm.nl/projects/miniwhip/

There are 2 links early on in the article to some of the theory. The structure of the whole antenna and the earth in particular are critically important.

If anyone wants to try one, there are versions of it all over eBay, aliexpress, amazon etc for £20 or less.