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Why do some radios receive better than others?


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I found this out by accident while playing around with some radios.  Two older radios, an Icom IC-F2821 and a couple of Kenwood TK880's and a newer Icom IC-2730a. 

I have been using the 2730a mostly, but stuck a cheap mobile antenna into the 2821 and started picking up a somewhat local GMRS repeater that the 2730a wasn't receiving, using an outside, roof mounted antenna. Tried the cheap mobile antenna on the 880 and also worked great, picking up the repeater. I tried the mobile antenna in the 2730 and nothing. Of course both the 2821 and 880's worked great with the outdoor antenna.

Is it because the 2821 and 880's are more of a commercial radio where the 2730 is more consumer oriented radio?

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What antenna is the 2730a hooked up to? I know (at least in terms of swr) the comet gp1 that's my ham base antenna isn't happy with much outside the 420-450 range.

The commercial stuff is also probably better optimized /filtered for the higher frequencies (possibly more sensitive, as well) where commercial and gmrs reside, where the 2730a is outside it's normal operating range

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The outside antenna is a Comet CA-712 EFC. I kind of just now reconfigured my shack. I'm now using the Icom 2821 for GMRS (and scanning) and back to only 2m-440 (and scanning) for the Icom 2730 with an outside Comet GP-6. I'm still a bit surprised at how much better the older 2821 receives signals

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Unfortunately (if I'm finding the proper specifications) Icom lists sensitivity in different units, so a direct comparison is not feasible:

F2821D

Quote
Receiver

Sensitivity
Analog (12dB SINAD)
Digital (5% BER)

 

0.25/0.3μV typ.
0.25/0.3μV typ.

Squelch sensitivity
 


0.25/0.3μV typ.
73dB typ.

Adjacent channel selectivity
Analog
Digital


80/75dB typ. (Wide/Narrow)
73dB typ.

IC-2730A (and the web page has red notices that specifications are only guaranteed for 2m/70cm bands)

Quote
Sensitivity (at 12dB SINAD) Less than –15dBμ
Selectivity

More than 60dB/55dB (W/N)

   
   
   
Receiver sensitivity
FM/FM-N (12dB SINAD)
137–159.995MHz*
375–399.995MHz
500–550.000MHz
160–174.000MHz
400–499.995MHz*

AM/AM-N (10dB S/N)
118–136.991MHz
*Except amateur bands.


–10dBμ
–5dBμ
–5dBμ
–5dBμ
–10dBμ


0dBμ

 

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Lscott and gortex2 are both correct.  But there is a bit of explanation that needs to be done to have a better understanding of it.

First is receiver technology.  Back in the day, and in the present, a standard analog FM receiver has a number of stages that the RF goes through as its stripped of it's intelligence (the voice communication) and presented to the user via the speaker.  First is an RF stage, this is connected to the antenna and amplifies the signal (and ALL other signals it can hear) to be shipped up the line.  These are typically fairly wide band amplifiers that will amplify much more than just the frequency range of the radio.  Here's the first part of receiver degradation.  That first RF stage has a design gain of X number of dB gain for the stage.  So, lets say that's 10dB.  If you connect said radio to a signal generator and feed it a signal level of -110dBm the output of that state SHOULD be -100dBm.  An increase of the 10dB of the stage.  BUT, if you feed that antenna with 2 signals from a generator that are different frequencies and also are -110dBm and look at the output of that stage, the signal level of each frequency will be -105dBm which is half as much as the single signal gain because the state can only generate 10dB of TOTAL gain. We are talking GMRS, so 462Mhz.  But that RF stage exhibits gain at 140 Mhz too.  To deal with out of band frequencies a GOOD receiver will have a band pass filter that will only let in 460 to 470 blocking the out of band stuff.  Cheap receivers don't have expensive filters.  And receiver sensitivity is measured with a SINGLE frequency in a controlled environment at the BEST measured level.  Meaning they will sweep around and find where the radio performs it's very best and that's the spec. 

Second is shielding.  Keeping the signals that are floating around in the radio out of other circuits in the radio requires shielding and isolation.  Go find an old Motorola Spectra and take it apart, look at the designed isolation.  Then take a part a kenwood TK860 or other radio from another manufacture of the same era.  Spectra's were a BEAST for performance.  Other's not so much.  But you could buy 4 TK-860's for the cost of ONE Motorola Spectra. 

So stage two is the IF stage,  a second signal is created in the radio and mixed with the RF.  This mixing creates two frequencies.  The sum of the two frequencies and the difference between the two frequencies, which is the one that's important here.  Of course, what ever else has leaked through beyond the primary frequency of concern (what the radios actually tuned to) is also present and ends up converted as well.  This is where the 10.7 IF frequency is created and passed to the IF amplifier.  Stuff is cleaner now and more filtering happens.  Then it's stripped of the intelligence from the IF and sent to the amplifier for presentation by the speaker. 

Digital processed radios.

These are analog radios that operate in the digital realm.  An SDR receiver if you want to think of it that way.

Some of these directly take the RF into the digital processing chip and some operate at the IF frequency mentioned before.  Because the RF is directly converted to a data stream it's now easily processed and can be filtered much better than even the best designed analog receiver could hope for.  If it's not specific to the frequency of concern, it's simply ignored.  And 'amplification' is as simple as changing the bit stream.  All amplifiers can mix frequencies.  so RF. IF whatever, mixing can occur.  If there isn't any amplifier in the sense of a stage with an analog transistor that exhibits gain in the stage, then no mixing happens.  Remember that the RF is now a digital stream of date, even in an analog radio.  The SDR chip take the RF in and outputs the intelligence directly to the audio amplifier.  All the A to D and D to A (analog to digital and digital to analog) happens in that chip along with all other signal processing.  This allows for greater First RF stage gain, better filtering and error correction (something that CAN"T happen in an analog receiver at all) and a whole host of filtering that's done in the digital domain.

And that's how we are seeing a huge increase in receiver sensitivity in these newer radios that are Digital and analog like the XPR Motorola offerings.  They are barely analog at all.  Doing all RF and IF processing in the digital domain while being 'analog' radios.

 

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