Batboard

hey folks got a question for someome smarter than me... upon reviewing all of the radios specs that motorola has to offer... they never show how much wattage the radios input... like as in receive power. i realize that the radios will put out typically 25 or 40 watts of transmit power, but what about input power. the reason i ask is that i recently purchased a cdm1250 off of ebay and i only have a mobile antenna on top of the roof with my other antennas becuz i dont feel like paying 550 dollars for a base antenna, but seems like sometimes my scanner will pick up as good of a signal as the mobile but other times my mobile will pick up signal better than the scanner. now i have a scanner base antenna on the same pole mast so was jus curious if anybody knows of how to make a mobile surpass the scanner as far as receive reception. thanks in advance, kent.

Do you mean receive sensitivity and rejection? I thought those numbers were published for the modern equipment, but I could be wrong.

As for receive performance differences, there are some good scanners out there. Amazingly good. And there are some fair to poor mobile receivers out there. The Maxtrac comes to mind. Both a Maxtrac and a CDM will test similarly on the bench, but in the real world, in my experience, the CDM performs much better.

A receiver only has to do two things - receive the frequency of interest, and reject all others. Reception is easy. It simply a matter of amplifiers. Rejection is the hard part. That involves precision selectivity which also usually means physical size, but is also a matter of engineering and method. Fascinating topic, but beyond the scope of your question.

Bill is correct - the receive sensitivity specs are published in the literature. To answer your question directly(although I'm not sure it means much to you), the specs typically call for 0.25 to 0.35 uV for 10 dB sinad(depending on the frequency band), and this equates to signal levels of -116.1 dBm to -119 dBm at the receiver input. That's 1.26 picowatts to 2.5 picowatts(i.e., 1.26 with 12 zeroes in front of it).

ok yeah in a round of a bout way... u answered my question... i just know the the portables typically put out 4 transmit watt of power but never say input watt of power... (at least in plain english) so idiots like myself can understand it haha

The fact is that the power in amps required by your receiver has nothing whatever to do with how well your receiver hears a signal. The power required is relatively small and does not affect how the receiver works, so you should not be considering it as a factor as to why your receiver many not be up to par.

OTOH, the power figures given for the transmitter are listed so you can adhere to the licensing requirements you have, and also so you can know how much power you need to supply to the transmitter. The input power to the transmitter will always be more than the output power because the final amplifier is not 100% efficient. As a very rough example, if the output power is 48 watts, which means the transmitter needs 4 amps at 12 volts to do that, the actual power being drawn by the transmitter is probably 5 amps or so, and you size your power supply according to the larger figure. Commercial radios are usually listed by their output power, and ham radios are usually listed (or more accurately most of them used to be) listed by the input power.

Just for FYI...I currently use a CDM1250 with a blown PA as a receiving only radio. Its hooked to the same antenna as a BC796D scanner through a stridsburg multicoupler. The CDM always hears the weaker signals much better than the scanner if the scanner hears them at all. I find the same results with the MCS2000 series radios also. You also don't need to spend $500 on an antenna, I just put up an off brand $100 dollar 17' dual bander and its performance is great so far with excellent swr on both bands. It replaced a homemade version that performed almost as well (cost $30.00 to build) but the weather took its toll on that one.