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Rumblefish
Junior Member
Username: Rumblefish
Post Number: 44
Registered: 12-2002
| | Posted on Monday, December 13, 2010 - 8:57 am: | 
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i was going through some sensitivity listings for different radios. why do the older sets "courier" for example, have better sensitivity ratings .25uv vs. .1uv. for the newer radios? Almost looks like we've traded selectivity over sensitivity.would it be that difficult to have the better numbers in the newer radios? what can i replace in the new ones for better sensitivity? thnx. rumblefish. |
Ronin
Member
Username: Ronin
Post Number: 85
Registered: 8-2006
| | Posted on Tuesday, December 14, 2010 - 5:44 pm: |
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There are receiver mods for a number of radios that are available online or on ebay, or you can buy the parts yourself. The idea is to replace the RF amp transistor with one that has a lower noise factor and better amplification as well as the diodes in the AM detector and NB circuit. I have performed this mod in my Galaxy DX-949 and Uniden Pro-810e with very good results.
The stock transistors in both radios were the 2sc1674L and were replaced with an NTE 107... you can also use the 2sc2999 as well. The 4 diodes are replaced with ECG583 Schottky barrier diodes. The end result in the 949 was good..about 1/2 S-Unit lower noise and an increase in signal of about 1/2 S-unit as well. I checked the results against a DX-2547 (same pcb) and there is a noticable difference in not only the noise floor, but the signal to noise ratio on the received signals as well. With the Uniden 810, the results were more dramatic...1 S-unit less noise and signal increase of 1 S-unit as well. The receive mod took the 810 from what I considered a very good SSB performer and mediocre AM performer to excellent on both AM and SSB.
If you have the schematics and the block diagram for your radio you can find what components are used in the above circuits. BTW...I live about 3/10th of a mile (and within eyeshot) from a heavily traveled state highway and do not have anymore bleedover now than I did before the mods.
Jim
CEF-813
K3ZOR
OT-235
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Rumblefish
Junior Member
Username: Rumblefish
Post Number: 45
Registered: 12-2002
| | Posted on Friday, December 17, 2010 - 8:56 am: |
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hey ronin, thanks for the reply. I had a cobra 139xlr and I did the rx mod to it. I saw the same result as your pro 810. Looked up the schematic for a friends courier centurion. this radio has the best rx I have ever seen. He uses an a-99 antron besides!! the courier sports 3 fets ,real crystal filtering and more disc capacitors for filtering.I imagine the fets combined with the true crystal filtering made the difference. rumblefish |
Tech833
Moderator
Username: Tech833
Post Number: 1895
Registered: 8-2002
| | Posted on Friday, December 17, 2010 - 8:55 pm: |
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For receiver sensitivity specs, the lower the number, the better.
What you want to look for is the 10 dB or 20 dB SINAD figure. That is a true indication of how well your receiver will hear signals coming in from your feedline including internal noise and front end sensitivity.
Some older equipment (tube type) can be much more sensitive than newer gear because they lack internal processors, and manufacturers really went out of their way to create sensitive gear. The extra sensitivity could be utilized because the bands had less noise; There were no noisy plasma TV's, no noisy computers, very few noisy light bulbs, traffic lights, etc. These days, making a receiver with sensitivity down in the -120 dB (or better) range will not make the user very happy.
It is highly advised to purchase a radio that has a double conversion (or triple conversion) receiver. The single conversion receivers might look good on paper in the sensitivity area, but in the field, sensitivity is greatly reduced by images and out of band noise. A double or triple conversion receiver is almost immune to out of band noise.
73,
833
Your radio 'Mythbuster' since 1998
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