HT1000 VHF Spkr/Mic & Measuring RF Power Out

cracked · Post by **cracked** » Sat Oct 20, 2001 9:33 pm

The problem here is with a HT1000 VHF radio equipped with a speaker/mic with antenna. I can measure RF power out just fine at the antenna point of the radio, but my service monitor sees almost no power out of the speaker/mic antenna port. Can anyone explain this beyond the "you just can't do it" explantion several motorola support reps have already given me? I understand these speaker mics aren't recommended for use with VHF radios, but I also know they transmit and receive fine. Not as well as a standard sized antenna attached to the radio, but they work. I've also seen that they come from the factory with a note attached stating "do not attempt to measure power out" etc. i'm just curious why a wattmeter can't see the power that's obviously making it out the antenna somehow.

Alan · Post by **Alan** » Sat Oct 20, 2001 9:56 pm

I assume that you are using a "Public safety" speaker mic (the one with the antenna on the speaker mic head).
When you attach the public safety speaker mic, can you measure any power out of the radio antenna connector or does it drop to zero when you attach the mic?
With the public safety speaker mic, there is a small tit that depresses a button (switch) on the side of the radio near the top of the accessory connector. It is this switch inside the radio that diverts the path for the antenna on the radio to the antenna on the speaker mic.
Not all radio's had the switch installed inside of the radio. If you do not have the switch installed, you will not measure power at the speaker mic antenna but rather there will be transmit power present at the radio antenna with or without the speaker mic attached.
Hope thi helps, Alan

cracked · Post by **cracked** » Sat Oct 20, 2001 10:19 pm

On 2001-10-21 01:56, Alan wrote:
I assume that you are using a "Public safety" speaker mic (the one with the antenna on the speaker mic head).
When you attach the public safety speaker mic, can you measure any power out of the radio antenna connector or does it drop to zero when you attach the mic?
With the public safety speaker mic, there is a small tit that depresses a button (switch) on the side of the radio near the top of the accessory connector. It is this switch inside the radio that diverts the path for the antenna on the radio to the antenna on the speaker mic.
Not all radio's had the switch installed inside of the radio. If you do not have the switch installed, you will not measure power at the speaker mic antenna but rather there will be transmit power present at the radio antenna with or without the speaker mic attached.
Hope thi helps, Alan

right, this is the speaker mic with the stubby antenna on it. these radios have the button ("tit") on the side that disables the standard antenna, so no power is measured out of the regular antenna port when the speaker mic is attached. actually, the only antenna even attached is the one on the speaker mic, which transmits and receives fine. but no service monitor i've tried has been able to measure any significant amount of RF power out from that port on the speaker mic head.

Reslock · Post by **Reslock** » Sun Oct 21, 2001 6:58 am

I have experienced this problem as well with a UHF MTX. It did not have the RF switch installed. I also have a VHF HT1000 and it does have the switch and I can measure power at the mic antenna port. It reverts all of the power to the mic port. You probably do not have that little RF switch.

Good luck
RL

cracked · Post by **cracked** » Sun Oct 21, 2001 12:27 pm

the RF switch is there, i'm sure of it. these radios transmit and receive for miles and miles with that antenna on the speaker mic. these mics even come factory with a note attached stating that you can't measure power out from the antenna port. for some reason you just can't.

cracked · Post by **cracked** » Mon Oct 22, 2001 12:08 pm

here's some additional information. the speaker mic, part NMN6251A, comes with a note that reads:

>>>
Technical Notice
The microphone antenna connector is NOT wired as a coaxial power connection. Therefore conducted power measurements are not possible from the connector. Refer to service manual (P/N 68P81085C36) for RF measurement procedures.
<<<

we could always go get the service manual, and probably will, but i'm still curious with the technicalities behind why we can't measure RF power out from that antenna port.

Will · Post by **Will** » Mon Oct 22, 2001 7:25 pm

Some public safety speaker mics depend on radiating some RF from the cord as well as the antenna connector, a TNC connector.
You can measure continuity with an ohm meter from the radio side connector to the antenna jack ground side, not sure on the center pin.
Also some PS mics have chokes and capacitors in the rf line to "tune" the antenna.

TheMotorolaMan · Post by **TheMotorolaMan** » Tue Oct 23, 2001 8:09 pm

The RF connector on the remote mic is not wired the same as the one in the radio. The one in the radio has two conductors (shield and center). When you connect it to an service monitor you're able read power.
RF connector on remote mic is wired as center and shield shorted together and connected to the coax cable center only. The coax shield is connected to pcb ground only.
When you connect the remote mic to the service monitor, the input is a dead short and hence no power reading.
Hope this helps