Hi,

Just wondering if anyone knows how strong of an FM signal
is required for the average car stereo to be able to lock on
to the broadcast while the user is scanning through frequencies
to find a station.

How long is your piece of string? Unfortunately, that's a pretty meaningless question - it depends on the receiver sensitivity and selectivity, on the receiving aerial, on the path between the transmitter and the receiver, on the transmitting aerial, on the transmitter power and many other things - too many to mention!

Look at a receiver sensitivity specification - usually quoted in µV for full noise-quietening (limiting in the IF). Commercial receivers have widely differing specifications. My old analogue-tuned Sharp car radio could receive a perfectly good quality stereo signal from a low powered pirate 40 miles away. The first digitally-tuned Blaupunkt receiver I got couldn't receive the BBC without hiss....

As the Yanks say - Your Mileage May Vary!

the 2 car radios we have here are both different one auto seek turning works about 44db, the other the signal has to be head stomping, consider the first one the best for testing. If your radio can lock onto signals that are out of your area in spots where the signals are favoriable then it will be fine.

i think you said your using a loft aerial it will struggle unless the loft is high up, may get lots of fluctuations in the signal, you might struggle getting it locked on.

44dB sounds quite high like when a radio has Local/DX setting and it's been set to Local sensitivity.

Thanks for your replies.

I think the best thing is for me to do is drive around my local area
and test if I can get a lock when i'm scanning the dial. If not
I guess I will have to push up the wattage.

I increased my TX power to 7 watts last night for the first time so will
do again over the weekend and see how it goes.

Radionortheast, Yes my aerial is still in the loft, but I'm now using
a ground plain antenna instead of my dipole.

teckniqs is problaly right problaly is lower, anyway hope the gp works well for you!

Some radios also require RDS TP before they'll lock on to stations


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halfwave wrote:I increased my TX power to 7 watts last night for the first time so will
do again over the weekend and see how it goes.

Yes my aerial is still in the loft,

You'll be lucky to be radiating 700mW of your 7 Watts - most of the energy will just be heating the roof tiles. It will be even worse of it's raining!

You really do need an outdoor aerial!

Some (but not all) car radios are fussy about the RDS signal when scanning. They will not lock onto a station, no matter how strong it is, unless the RDS signal is phase-locked to the pilot. Surprisingly some RDS encoders - even from a certain big supplier have free running RDS and suffer from this problem.

44dBuV/m sounds right to me for the weak scan level. 54 is Ofcom/BBC target for coverage area, but most car radios can receive beyond that area.

Albert H wrote:

halfwave wrote:I increased my TX power to 7 watts last night for the first time so will
do again over the weekend and see how it goes.

Yes my aerial is still in the loft,

You'll be lucky to be radiating 700mW of your 7 Watts - most of the energy will just be heating the roof tiles. It will be even worse of it's raining!

You really do need an outdoor aerial!

Exactly this


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I may look and see if I can mount it on the side of my house and get a pole up
to roof level, as much out of sight as possible. Problem is that it's probably going
to look a bit out of place with other peoples std TV and FM aerial's and It will put
it in very close proximity to one of my neighbors TV aerial.

At the moment no one can see the aerial as it's in the loft so i'm kind of undercover.
But if I put it out in view, people may start asking what is it for etc.

If you build a ⅝ or ⅞-wave vertical, you can tell people that it's for CB!