David, G0MRF, provided the following assessment of the Icom-7300 after being modified for use on 630-meters. He was kind enough to send these details and allow me to publish them so that others might be able to make an informed decision when faced with the purchase of a rig for use at 472 kHz. Enjoy!
A couple of weeks ago I ‘invested’ in a new IC7300. Mainly to experience the SDR aspects of the radio, but also to see how it performs on 630m. My local dealer was more than happy to feed me a cup of coffee while his engineer removed a couple of diodes to broadband the transmitter.
After a few days of informal testing, the results are an interesting mix – Which is to be expected as the radio was never designed for transmitting below top band. Note. All IC7300 radios may not be the same as this one.
The transmitter, once broadbanded, operates nicely down to about 700kHz. At 640kHz the indicated SWR increases to 2:1 and as you reduce the frequency further the radio’s automatic protection system begins to reduce the output power. At 630m this IC7300 produces 10 Watts output with a DC input of 7 Amps at 13.8V. The indicated SWR is around 5:1 into a Bird 50 Ohm load. The RF output does needs a dedicated low pass filter as the 3rd harmonic is only 18dB down on the carrier and everything up to the 5th harmonic exceeds FCC limits.
I compared the receive performance on 475kHz with 1850kHz using CW mode and a 1.2kHz filter. The audio was fed to my PC running SDR Sharp to look at audio output level and the background noise. If operated below the AGC operating point the dB change in RF input signal is reflected directly in the audio level visible on SDR sharp.
Input for an indicated S5 on the radio:
1850kHz = apx. -92dBm
475kHz = apx -74dBm – ( A difference of 18dB )
The radio appears to have a 7dB higher background noise on 630m when compared to 1850kHz. The reason for this is not immediately clear but it does mean that to overcome the extra noise the input signal must be 7dB higher than on top band.
After some calibration and checks on the AGC action I decided to measure the signal level necessary to produce an audio output that indicates -44dB on SDR sharp.
On 1850kHz the background noise level was indicated as -67dB and a -44dB audio signal was produced with an RF input of -130dBm. ( S:N = 23dB). On 475kHz an indicated -44dB audio signal on SDR sharp was produced with an RF input of -113dBm. ( A difference of 17dB ).
1) The receiver is around 17dB less sensitive at 475kHz when compared to 1850kHz. This is consistent with the 18dB difference necessary for indicating S5 on the signal meter.
2) If the apparent 7dB higher background noise is real, then this increases to 24dB down when looking at signal to noise ratio.
3) My IC756 and many other radios have better receive performance than the IC7300
Is the IC7300 the ultimate answer for a commercial radio that works on 630m straight out of the box? Clearly not. The receiver requires a reasonable preamp to make its sensitivity comparable to many 20yr old radios. But on the positive side, the modern DSP filtering, inbuilt waterfall display and ease of interfacing mean that it’s probably worth the effort to make the 7300 work rather than building a transverter or a dedicated transceiver for 630m. When 630m is finally widely available to the amateur community it shouldn’t take ICOM too long to bring this radio up to a reasonable standard. I get the feeling that the transmitter really does want to work on the band, but some little detail in the protection circuit is holding it back.