# 12v FM Transmitter - Help finding antenna

#### spooky_pitboss

Dec 19, 2013
7
Hi, I was hoping someone can help me decipher where I can attach a copper wire to act as a larger antenna on my FM transmitter, to hopefully increase the sound quality and clarity. I bought a Excelvan aux-in FM transmitter for my work truck, and it gets power via 12vdc cigarette lighter plug. I'm very basic when it comes to electronics, and can handle minor wiring/diagnostic issues when it comes to vehicles, but I don't know small electronics well enough. I'm fully capable of soldering, and testing points on the PCB with my multimeter, but that's about it.

Here's a picture of the transmitter :

Unfortunately, there's no solder point obviously named "ANT" or anything for an amateur like myself, so any help would greatly be appreciated.

Here's a few pics I took of the transmitter after I opened it up.

Thanks for the help, I really appreciate it!

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#### spooky_pitboss

Dec 19, 2013
7
Sorry for the large image size, but I figured that might help! They can be larger if needed.

#### davenn

Moderator
Sep 5, 2009
14,126
Hi, I was hoping someone can help me decipher where I can attach a copper wire to act as a larger antenna on my FM transmitter, to hopefully increase the sound quality and clarity. I bought a Excelvan aux-in FM transmitter for my work truck, and it gets power via 12vdc cigarette lighter plug. I'm very basic when it comes to electronics, and can handle minor wiring/diagnostic issues when it comes to vehicles, but I don't know small electronics well enough. I'm fully capable of soldering, and testing points on the PCB with my multimeter, but that's about it.

Unfortunately, there's no solder point obviously named "ANT" or anything for an amateur like myself, so any help would greatly be appreciated.

Here's a few pics I took of the transmitter after I opened it up.

Thanks for the help, I really appreciate it!

Well they don't put ANT anywhere as the units are not supposed to be modified

the antenna is likely to be that upper most circuit track the one that ends in 3 solder blobs on the left side
That's not guaranteed, just the most likely

Scrub That

it looks like that's a +5V rail

without a circuit for it, I have no other ideas .... sorry

adding an external probably wont improve sound quality/clarity unless you are already using it at the limits of its range.

Soldering a wire on there isn't going to hurt it so you mite as well try

cheers
Dave

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#### spooky_pitboss

Dec 19, 2013
7
Thanks for the help! I figure it can't hurt, I'm just trying to make the unit not so touchy, since its limited to a very specific area of my truck to get the best reception. I was thinking of plugging this in with the case open, and using a test lead and alligator clip to test individual points (carefully of course not to short anything) to see if reception improves. Am I an idiot, or does that sounds like a good idea?

Thanks for the help! Keep it coming

Aug 13, 2011
1,114
This type of transmitter uses a short piece of wire in one of the attached cables as an antenna (a poor one as you've surmised). It will be connected to the circuit board but not to anything on the other end. You can use an ohmmeter to test the continuity of the wires and connectors to identify the antenna by elimination.

There's also an attenuator between the output pin on the IC and the antenna wire to reduce power so the unit complies with regulations. Unattenuated it's capable of up to 450mW.

#### spooky_pitboss

Dec 19, 2013
7
So one of the wires going towards the headphone jack and USB power output, has one wire that shouldn't show continuity to anything else on the board?

Would I simply remove the attenuator and replace it with a wire?

#### spooky_pitboss

Dec 19, 2013
7
Having a hard time getting continuity tests from most of the board covered in a film. Again, thanks for helping me out, as small electronics like this are a total learning experience to me.

Is the attenuator one of the larger black "chips" in my last three pictures?

#### davenn

Moderator
Sep 5, 2009
14,126
This type of transmitter uses a short piece of wire in one of the attached cables as an antenna (a poor one as you've surmised). It will be connected to the circuit board but not to anything on the other end. You can use an ohmmeter to test the continuity of the wires and connectors to identify the antenna by elimination.

I was thinking of that possibility also but when you look at the number of connections between the USB plug and that other 4 contact connector then only 6 wires in that cable I kinda dismissed that idea

eg the red and black most likely the +5 and 0V for the USB, 2 more wires will be data lines for the USB. So that leaves only TWO wires going to a FOUR contact connector plug
let alone another one for an antenna wire

try to figure that one out !! .... I cant without doing some continuity tests

There's also an attenuator between the output pin on the IC and the antenna wire to reduce power so the unit complies with regulations. Unattenuated it's capable of up to 450mW.

what attenuator ? which IC is the output one ? there's no markings to identify that

cheers
Dave

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#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,507
Is the attenuator one of the larger black "chips" in my last three pictures?

No, it will be on of the smaller components that have only 2 connections. But we can't tell you which one it might be.

#### davenn

Moderator
Sep 5, 2009
14,126
I don't see any obvious 3 resistor pii or T network that would indicate an attenuator

Dave

#### davenn

Moderator
Sep 5, 2009
14,126
OK going some googling on these units I discovered a few things

The USB line is likely to be power only .... it appears to be just just for charging only ( so no data lines)

The 4 contact 3.5mm plug will have microphone, left and right audio and a 0V line Its capable of hands free phone use using the mic in the transmitter unit and connected back to the phone)

The 0V line most likely the black, will be common to both the USB and the 3.5 mm audio plug. The red is most likely the 5V line

2 more for left and right audio and there would be a spare one for an antenna

SINCE it already probably has an external to the circuit board, wire antenna, lengthening that wire probably isn't going to make much difference

cheers
Dave

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Aug 13, 2011
1,114
If you manage to identify the output, you can attach a telescoping dipole antenna such as are used on portable FM receivers. It should be ~75cm or 27" long.

#### spooky_pitboss

Dec 19, 2013
7
Is my crude idea of using my mutlimeter probe attached to a short copper wire, while the unit is in my truck plugged in and being used, and probing each area (without touching anything else to not fry it) an easy way to determine the antenna?

Good idea on the the dipole, if my crude plan works, how would I attach each lead (I think there's two) to the single connection? Simply solder everything together, to a single point?

Thanks for the help!

#### davenn

Moderator
Sep 5, 2009
14,126
Is my crude idea of using my mutlimeter probe attached to a short copper wire, while the unit is in my truck plugged in and being used, and probing each area (without touching anything else to not fry it) an easy way to determine the antenna?

Good idea on the the dipole, if my crude plan works, how would I attach each lead (I think there's two) to the single connection? Simply solder everything together, to a single point?

Thanks for the help!

did you not read the last sentance in my last post ?

cheers
Dave

#### spooky_pitboss

Dec 19, 2013
7
did you not read the last sentance in my last post ?

cheers
Dave

Evidentally not, I was browsing from my phone at work lol. So this is a lost cause then I guess huh?

Maybe i'll look in my service manual, and try to find a way to pick up the audio channels for the truck, and add an aux-in headphone jack.

#### davenn

Moderator
Sep 5, 2009
14,126
experiment with setting different FM radio frequencies
tune across the FM band on your radio and find a really clear spot, well clear of a commercial broadcast station ( if that's possible)

you may find some are better/clearer than others cuz a certain freq isn't too close to one being used by a local FM broadcast station

cheers
Dave

#### dietermoreno

Dec 30, 2012
238
I bought one with a telescopic antenna:

http://www.amazon.com/Ramsey-FM-Stereo-Transmitter-Kit/dp/B0002NRIMS/ref=pd_rhf_pe_p_tnr_9

Its more expensive ($50) than the store bought ones ($5), but it has a bigger antenna in addition to you have total control over how powerful it is because your building it.

I haven't built it yet. I'll let you know how it sounds after I build it. I have already determined that the best frequencies for me to use are around 88.0 MHZ to 88.5 MHZ because there are only low powered college stations on those frequencies. That appears to be pretty much the only frequencies I can use because I get a station pretty much every where else on the FM tuner since the air waves in Chicago are so saturated with FM stations.

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