There have been a number of people who state that you MUST NOT operate an Icom IC-9700 with a linear amplifier, as the spurious emissions generated by this combination will contravene various regulations. I have even seen social media posts that suggest as much as 50 KHz wide "hash" can be generated in this situation, particularly on 432 Mhz (70cm)
As the owner of both an Icom IC-9700 and a 432 MHz capable amplifier (Linear Amp UK Gemini 70), and also being in posession of a Marconi 2945 test-set, I thought that I would test this much mentioned "fact" to see how much truth there is to it.
Firstly, I must point-out that my test-set has not been recently calibrated, but I don't think that this is an issue for amateur use, as I am not attempting to provide any certification. All of my tests were performed using an un-modulated FM carrier. I used the spectrum analyzer function of my test-set for the majority of tests, the base-line being the following (with no radio connected):
The reference level is 54dBm (approx 250w) as my amplifier tended to go into protection mode above this! I decided to perform all tests at the same reference level, as that allows easier comparison of each graph (I would be interested to hear any comments/suggestions on the test methodology used though).
As you can see, the current level is -18.5dBm, which I will take as the noise-floor of the spectrum analyser. Obviously this is due to the analyser only having 8 x 10dB divisions, below are the spectrum plots for similar output levels both barefoot and through the linear:
Firstly, 45dBm (32w). The left-hand image is barefoot, with the radio running at 61% of maximum power, the right-hand image is through the linear, with the radio running at 0% of maximum power:
As you can see, the plots are almost identical, which suggests that the linear is not adding any additional noise/spurii to the signal.
Similarly at 46.5dBm (45w). This was with the radio running at 88% on the left, and 1% through the linear:
Obviously, it isn't possible to test the radio running barefoot much further, as 100% produced 47.5dBm (56w) and 2% drive on the linear, is 49dBm (80w).
A final graph shows the absolute minimum drive that the radio can deliver barefoot (at 0% drive) 25.5dBm or 0.36w compared to the maximum drive that I was able to deliver to the linear, which was 10%, this resulted in 53.5dBm (220w)
As you should be able to see, although the noise levels are significantly higher (around 23dB) through the linear, the output level has increased by 28dB, which suggests to me that running the IC-9700 is absolutely fine through a linear as I still have 50dB between the maximum signal and generated noise. The manufacturer quoted specification for the linear is -70dB of spurious output at 300w, so while the IC-9700 is obviously generating some unwanted signals, I don't believe that it would be sufficient to cause "hash all over the band"
A quick word about regulations. The UK license states that no-more than 1% of mean power of the transmission falls outside of the frequency band. By my (rough) calculations, looking at the above graph where the wanted signal is 53.5dBm and the unwanted signal is peaking at around 3.5dBm, going back to the original base-line of -18.5dBm, this suggests that 22dBm of "spurious" emissions are being created, which is 0.158w or 0.07% which is well below the UK limits.
