Sorted by IM3
Sorted by Intermodulation. Best receiver first.
This is a measure of how badly a close-spaced signal will trash your receiving with spurious signals or splatter sounds, and you might blame it on the other guy thinking your receiver is just great. Remember that even in my very quiet rural location where I have to "beam" through the east coast to hear Europe, 80dB or so IM3 is probably more than enough. The reason my Drakes are so good is because they were built to duplex on the same band with my own transmitters running!! Unless you are in a situation like that, anything over 80dB is likely wasted.
Most transmitters are really the source of problems. Exceptions to transmitters being a primary source of off or adjacent channel interference occurs if:
1.) You use a noise blanker. Most noise blankers kill receiver IM and blocking performance.
2.) You don't have good narrow IF filters.
3.) You use too much gain in the receiver front end.
4.) You have a receiver with a design problem (less than 80-85dB close-spaced IM3 DR) .
Beyond the above, most transmitters are the real problem. Look at transmitter IM tests on SSB and read about keyclicks.
Poor designs include:
1.) Many different Yaesu's without noise blanker mods. NO, changing the diodes doesn't do a thing! Been there, done that. The hissing noise comes from a FET following the narrow filters, the IM comes from the silly Yaesu noise-blanker design that hangs an FET right on a wide IF line with the potential for that FET to be at full gain even when the NB is OFF!!!
2.) Early DSP filter only radios. Many aren't that good. If you have a DSP radio without a roofing filter that matches the mode, you could have a poor receiver. There are exceptions, but they are still not as good as a better narrow crystal or mechanical filter radio.
3.) Radios with spurious response problems. The JRC JST-245's I've tested and used all test good, but they have so many spurious synthesizer responses on 160 meters I'd never consider using one for serious weak signal work. I don't know how they are on other bands, but the spurious responses make them poor performers on 160.
You can see how the noise blanker FET in the Yaesu hurts the performance. The W8JI modified Yaesu's have the noise blanker transistor modified to turn fully off. The stock early R4C uses a dual gate MOSFET mixer that is subject to overload, and is one of the worse receivers manufactured. 10KHz or wider tests are almost meaningless. Almost anything is good at 10kHz, even some very poor receivers. Here's how a few receivers stack up:
Note 1: The FT1000MP has the same inherent problem as the FT1000 and FT1000D. A noise-blanker amplifier FET is left on even when the noise blanker is turned off with front panel NB and NB-W switches. This FET connects directly to the IF, and overloads when strong signals are present. Unlike the FT1000 and FT1000D, the front panel noise blanker level control affects the FT1000MP IMD performance. * is with the NB gain control fully advanced, ** is with the NB gain control fully off. In ALL cases the NB off-on switch is OFF!!
Note 1A: Yaesu copied the same basic NB from the FT1000D, and so this radio also has a NB problem even when the NB is switched off. This test was with NB gain menu set for minimum gain. The radio is worse when NB gain is increased in the NB gain menu.
Note 2: This is a MOSFET 2nd mixer R4C, aligned and in good shape but with no changes. This receiver is almost useless for weak signal CW work of any kind because of poor IM performance and filter blow-by.
Note 3: This is a tube 2nd mixer R4C, aligned and in good shape but with no changes. This receiver is almost useless for contesting or crowded bands because of poor IM performance and bad filter blow-by.
Note 4: This is the same receiver in note 2, but with the following changes:
Note 5: Gutted and rebuilt with solid-state double balanced high-level mixers. No vacuum tubes in signal path. Sherwood 600Hz roofing filter. Experimental receiver for full-duplex operation on 160 meters. This receiver allows me to hear weak DX while I am transmitting, as long as they are a few kHz off frequency.