John Fogery can't help me. What the heck is CCR?

[SUPERG900]

Sorry - I wasn't referring to your post. My mistake. I was responding to the guy I was asking for published data from.

[Lscott]

No problem. I've had dead links before where the cut and paste from the browser's address bar didn't copy the full link. I just wanted to be sure.

[gman1971]

Yes, absolutely, Lscott.

 

Equipment needed.

-RSSI meter. (Motorola radios have RSSI meters built in, make sure its calibrated, all XPR radios I own are within 0.5 dB of each other, so they are good for the sake of this test)

-Signal Generator (use a calibrated RSSI meter to ensure the signal on the display is correct, used the XPR radios here to measure)

-50 Ohm dummy load, I use a 50W one, but for the receiver sens test only you don't need anything more than 1W. (for duplexer desense you need the large one)

-ISO-tee (directional coupler)

 

Procedure:

1) If you don't have a directional coupler and you don't want to buy one, you can build your own ISO-tee using an old SO-239/PL-259 T-adapter. You need a hacksaw and a file. Unbolt the PL-259 prong from the T-adapter first, cut it with a hacksaw so it barely touches, and then, if needed, gently file it until the measured isolation is around 20-30 dBm. Remember the T adapter already has a 3dBm isolation, and it doesn't take much to file it a bit too much, so, be careful: I ruined two T adapters before getting it right... To measure the ISO-tee isolation I used the XPR6550 RSSI meter with the Signal Generator. You can also install a 20 dB attenuator on one end too I've been told... or you can just go and buy one from eBay... 

 

2) Measure the site RSSI with no signal, this is the lowest signal the radio will be able to hear, for that I used the XPR6550/7550e RSSI meter. This is the best the radio can do, if the noise threshold is -107 dBm, it will never be able to hear a -115 dBm signal... no matter what. You want the radio to be as close to this figure. Perform the ISO-tee tests on several locations to see how different noise thresholds affect receiver. Useful for mobile application. Noise threshold is not receiver desense. Desense happens in addition to the noise threshold.

 

3) Measured absolute receiver sensivitty and squelch sensitivity on the radio. 

-Set SQL to threshold.

-Connect SG to radio directly with lowest possible signal. -130 is usually a good number to start.

-Increase signal dBm until SQL opens. Record this number.

-Reduce signal dBm until SQL closes completely. Record this number.

-Keep reducing signal dBm until RX led is no longer blinking. Record this number.

 

4) Connect ISO-tee as follows:

-The vertical part of the T-adapter to the signal generator (the one you cut/filed).

-The horizontal ends one goes to the radio, the other one to a 50ohm dummy load.

 

5) Measure dummy load sensitivity.

-Set SG signal strength to lowest possible dBm.

-Increase signal dBm until SQL opens. Record this number.

-Reduce signal dBm until SQL closes completely. Record this number.

-Keep reducing signal dBm until RX led is no longer blinking. Record this number.

 

6) Replace dummy load with the antenna the radio uses and measure sensitivity again.

-Set SG signal strength to lowest possible dBm.

-Increase signal dBm until SQL opens. Record this number.

-Reduce signal dBm until SQL closes completely. Record this number.

-Keep reducing signal dBm until RX led is no longer blinking. Record this number.

 

7) Subtract the RX led sensitivity figure recorded in step #5 minus the RX led sensitivity recorded in 6.

 

8) Subtract the result calculated in step #7 from the figure recorded in step #3. That is your "effective sensitivity" which can vary quite a bit depending on location.

 

9) Subtract the result from #7 from the noise level, that will give you the desense relative to noise floor.

 

10) Now repeat step 7 8 and 9 this for the figures calculated for SQL open and close if you operate in FM, remember to set SQL to threshold, or lowest setting possible. For FM operation Squelch Performance is critical, so you will determine using this test if the radio needs its squelch aligned, tuned... etc. 

 

--You can also use this procedure to evaluate duplexer performance and see if you need additional isolation on the RX side if its desensed b/c the transmitter...etc.

 

Once I performed this test on enough of my radios I started to see an emerging pattern, a trend. After all radios were measured, most commercial gear radios I own measured a typical 3-4 dBm "desense" over the noise floor in nearly all my use case conditions, whereas nearly all CCR stuff had an average of -13 dBm desense, in VHF, and about -17 dBm desense average in UHF, again under typical use case conditions.

 

EDIT: Another trend I noticed was that for nearly all CCRs, the higher the antenna gain, the higher the desense above noise threshold, or a decrease in effective sensitivity. Using my base antenna as the ISO-tee antenna, a triple 5/8 collinear at 40 feet up, the desense measured on several CCRs was pretty bad, but honorable supersuck mention goes to all the TYT and Retevis CCR radios tested (GD77, RT52, RT82, TYT MD760) The desense over the noise floor (which was measured at -119 dBm) was on the order of -38 dBm in VHF, and -43 in UHF... so all that gain that my base antenna, all 4.5 dBb, were demolished by the massive -43 dB desense coming from the CCR landfill of garbage... A total winner in my book to buy these pieces of garbage. With the GD77/RT52/RT82/MD760 the base antenna I couldn't hear a 50W mobile beyond 2 miles. With an EVX-539 portable mated to the same antenna I could hear the same mobile, on 25W, talking from 17 miles away. Again, same antenna.

 

 

G.

 

 

Just for my info can you provide some details on how you performed the tests? For a home made iso-tee connector they have to be calibrated, from some sources I've  read, before each use.

 

I found a nice write up on how one guy did his testing along with photos for those who might like to know how its done. Also how somebody made their own iso-tee connector. I'll admit it's done with a crappy "UHF" type connector but it's just the general idea.

 

http://www.repeater-builder.com/antenna/desense-testing/desense-testing.html

 

https://urgentcomm.com/2007/09/01/every-toolbox-needs-one-of-these/

[AdmiralCochrane]

Select, copy, paste.  

 

Thanks G! 

[Lscott]

Thanks for the detailed write up! I’ll have to go over it in more detail later. Out of all the CCR’s you tested which seems to be the best?

[gman1971]

You're welcome.

 

The best CCR I've tested is the Anytone AT-578UV...  but that is hardly a CCR clocking at 400 dollars. At that point you should consider an XPR5550e...

 

G.

 

Thanks for the detailed write up! I’ll have to go over it in more detail later. Out of all the CCR’s you tested which seems to be the best?