Powering a mobile FM transmitter

[JohnnyRocketsk8]

So next week I'm hoping to bring a portable FM transmitter along on a group bicycle ride, in order to bring a single music mix to dozens of FM radios and boom boxes on a group ride. After some research, I've determined that a 7-watt transmitter on the back of a bike trailer ought to do a decent job, hopefully broadcasting to radios in a ~500 meter radius––especially if we can mount our antenna up high. This is the transmitter I have in mind: http://amzn.to/1SMYqLV

My problem is, this thing comes with an AC adapter that spits out DC power at 12 volts, 6 amps. My question––and please excuse my ignorance––is this: Is there an easy way to power this thing using batteries? If so, how should I go about rigging such a thing up? I'm hoping to do this at a minimum of cost. The event should last about 3-4 hours, so I figure I'd need at least 25 amp hours of charge. (Do I have that right?)

Thanks in advance for your help!

 

[JohnnyRocketsk8]

Oops, looks like it's actually 12 v 2 amps. If that makes any difference. Like fewer amp hours of battery life needed.

 

[duke37]

If it runs off 12V, then a battery will provide this directly, however, a battery will provide very high currents if allowed which can cause damage and fires.

The battery connection should be fused close to the battery and all cables should be properly insulated and protected.

Make sure the polarity is correct.

 

[Colin Mitchell]

The actual current required by the Tx will be just over 1 amp. Use 2 x 12v batteries in parallel of about 3-4 amp-hr.

 

[KMoffett]

Why would you recommend two 12v batteries in parallel as opposed to one 12v battery with the needed AHr rating?

Ken

 

[Colin Mitchell]

Small 12v batteries are much handier to have around, easier to buy, cheaper, and will be lighter than a 7AH battery.

 

[AnalogKid]

If the AC power adapter is rated for 2 A continuous duty, then the radio must draw less than that. 24 W of DC for a 7 W transmitter means that the unit is less than 30% efficient, a low number. 1.5 A for 4 hours is a 6 A-h, 12 V SLA battery. Digi-Key has a 5 A-h unit for $25, and a 7 A-h unit for $30. A lithium battery will weigh much less and cost more. An alternative is a replacement 12 V portable electric drill battery from Lowes or Home Depot. More money per watt that an industrial one, but you don't need an account with a distributor, you probably can buy a matching replacement charger at the same store, and you can return all of it if the battery if it doesn't work out.

Also, 7 W seems like a lot of RF power for less than 1/2 mile radius. A 3 W radio would cut your battery in half.

Now a REAL biker would be talking generator, voltage regulator, some blinking LED's, and, of course, a 555...

ak

 

[BobK]

And you have a license for broadcasting in the FM band at 7W?

Bob

 

[davenn]

I've determined that a 7-watt transmitter on the back of a bike trailer ought to do a decent job, hopefully broadcasting to radios in a ~500 meter radius––

And you have a license for broadcasting in the FM band at 7W?

Bob

indeed !!
7W could easily cover up to 100km at times the cycling crew are going up over hilltops etc

@JohnnyRocketsk8 you are likely to find that somewhere around 0.5W is about the max power
you can transmit without a licence


Dave

 

[Martaine2005]

@JohnnyRocketsk8 you are likely to find that somewhere around 0.5W is about the max power
you can transmit without a licence

I think he is fast on his bike!!!
The small villages they pass wont mind a little "pop" music interference!

 

[KJ6EAD]

Your transmitter can be set to 1W output and that will be more than enough. Use an automotive-style whip antenna instead of the included telescoping one which is not suitable for mobile use.

There is a user review for your transmitter that warns of a pre-emphasis mismatch as the CZE-7C is hard-wired for 50μs as is used in most of the world and this will "muddy" the sound when listened to on US receivers that use a 75μs standard. If this is correct, this model may be a better option since it's designed for the US market, is less expensive and closer to legal for unlicensed use. It also comes with a rubber duck antenna so you won't need to buy a whip.

http://www.sainsonic.com/sainsonic-...gHTam_kso7ySm18Bequ3nwbXiOXxsMaAn3m8P8HAQ#top

After reviewing the data sheet for the Rohm BH1415F which is used in all of these transmitters, I have good news. The pre-emphasis time constant is fixed by capacitors connected from pins 2 & 21 to ground according to the formula τ = 22.7kΩ × C so you can modify it by replacing two capacitors with 3.3nF capacitors!

http://pdf.datasheetcatalog.com/datasheet/rohm/bh1415f.pdf

 

[AnalogKid]

That's 50 us (microseconds) and 75 us.

ak

 

[KJ6EAD]

That's 50 us (microseconds) and 75 us.

ak

Good catch; thanks; fixed in post and added a happy note.