OT Radio antenna (John T)

Mahalo,
doogdoog

Now to all that want to know why, I run an XM and use the fm channels. A lot of times there is stations in the area so I have to keep changing the fm and XM so it will work. If I can just shut the antana off it would slove that problem. Thanks again BUDD

Here is what I think your problem is: A car radio relies on the external antenna, and it uses a shielded cable to cut out interference from various sources in the car. So when you disconnect your antenna, you're attenuating both the broadcast signals and your FM receiver's output.

I would try to make up a dedicated antenna for the XM radio. You could make one up from a piece of cable from a car antenna; just strip off the shielding. This antenna would be inside the vehicle, preferably right next to the XM receiver. Ideally, if you can easily get to the back of your FM radio, I would just unplug the exterior antenna and plug in the interior (XM) antenna. You can get short antenna cable extensions to make that easier. Don't use a switch if you can help it; as you've seen they don't work well and will attenuate your signal due to impedance mismatch.

Good luck, let us know how it goes.

Gerald J.

I think its what I experienced before n several other gents below thought. I had one of those antenna tuner switch boxes once but regardless how I trimmed it there was nooooo difference. The problem was without using and properly bonding n grounding the shield any all unshielded hook up wiring into or out of the box was acting as a stray antenna so alllll ll frequencies were getting to the receiver n the tuner had no affect.

I bet yours is the same situation i.e. all that wire acts as its own antenna and it will unless the shield is bonded at both ends and the actual switch is inside a faraday shield i.e. a bonded metallic enclosure.

If an antenna is grounded out right where it connects to a receiver any signal strength should be shunted but if theres much unprotected unenclosed unshielded wire or anything lying around loose YOU HAVE AN ANTENNA

PS my best area is DC tractor electricity n charging n ignition and secondary AC power distribution but if you get into electronics or radio or RF Im pretty much "out to lunch" so take this for what you paid NOTHING lol My old buddy Gerald is a practicing engineer (Im long retired) so better listen to him n others also

John T

You want the antenna to stay up when the radio is off.

If so the trouble lies in the "spliced in". You will need to disconnect the power source from the radio to the antenna. Having no idea what car or even if the power is seperate or not, this one is going to be tuff to answer.

You need to see if the antenna motor (drive) has power wires to it. If so, you will have to cut them loose from the radio or it will just back feed to the radio (radio just stay on). Conntect you switch from a power source (key or not) and the wire to the antenna motor. Switch only to raise antenna.

If you want to still have control of the antenna with the radio "and" an exteranl switch, you will need to add a realy. The wire from the radio would pass thru a diode. That way when you power it with the external swicth it won't feed back to the radio. This way either can raise the antenna (both off to lower). I'd run the swicth power through a diode as well not to have the radio feeding back to the rest of the vech.

Now if the power source for the antenna is in the coax, well you are going to have to do some shopping. Find a device to seperate it out (dc from radio signal), put in your switching, then recombine it.

If I don't get it, forget everything I just said.....

Your wire leads to your switch are small antennas so you get pickup, worse, there is capacitance between those wire leads and in the open switch that at 100 MHz couples considerable signal. Especially the capacitance of the switch (probably as much as 10 picofarads) is comparable to the antenna coupling capacitor in the radio, so it doesn't cut the signal much.

Gerald J.

If you really want to kill the signal (and I'm not sure why you want do that), try hooking a a switch that shorts the antenna to ground. And I doubt if that will work, either.

Just a guess here but I believe the wire from the switch to the radio is enough reception to make the radio work.

Gary

Another thought, maybe the signal is jumping thru the switch.

Gary