A different direction
I started (as most users of this frequency would) with a small transverter kit (thanks Steph!). While that worked admirably well, it was a little cumbersome to integrate into my setup, not to mention that my old radio was drifting in frequency quite a bit. So I happened to come across another solution, from a completely unrelated project. The IC-7300 - which I had bought primarily as an IF demodulator) was able to transmit at 475KHz.
This radio has an SDR architecture, a really nice waterfall display and a built in USB soundcard. Most interesting is that there was some talk of a simple modification enabling transmit capability at 475KHz. I have tested this out, and whilst I can confirm that it is indeed true that this radio will transmit at 475KHz, it does require some additional filtering to meet harmonic content limits.
I will not post any modifications required to be made internally to the radio here (they are easily found with a google search), but I must emphasise:
- Modify your radio at your own risk - and void the warranty.
- DO NOT transmit outside the permitted Amateur bands. Why give amateurs a bad name?
- DO set the User Band Edge to ensure that your transmissions are within the permitted band.
- It is your personal responsibility to ensure that your transmitted signal meets all specs in terms spectral purity and linearity. By modifying any equipment, you are taking it outside its design specifications. Be aware of this.
If you cannot measure your transmitted signal to ensure compliance to your license conditions then do not attempt this modification.
You cannot rely on a power meter alone.
I cannot stress this point enough.
Transmit Low Pass Filter
The IC-7300 does not have a dedicated LPF for 475KHz. As a result the output rf has significant harmonic content at this frequency. More than enough to cause considerable problems for your PA and VSWR meter, not to mention the fact that it is very many dB greater than regulatory limits for harmonic content.
You don't see this on a power meter, but as you can see in the screenshot on the left, it is there and it is bad. The second harmonic is only 13dB below the fundamental, and there is significant energy up to the 5th harmonic.
The first job is to clean up the IC-7300 transmit RF, before any amplification. The best way to to do this is with a low pass filter. This one is nothing special - just a 5-pole Cauer (Useful Links) filter, with the added benefit of a 1-pole absorptive filter section. The schematic of the filter is shown right.
The inductors are wound on T68-2 iron powdered cores, and the wire size is chosen to fill up the core as much as possible (this limits leakage inductance). Feel free to substitiute other cores, but do stay away from ferrites (for obvious reasons). Also be aware that there will be considerable variation amongst cores, so take the number of turns as a suggested starting point only.
The capacitors are SMD NP0 low ESR types.
The absorptive filter basically shunts any harmonic energy to a dump resistive load, rather than reflecting back to the radio. Probably not strictly necessary, but it was a fun thing to design and build.
At these frequencies, you can get away with practically anything. But I felt that it would still be nice to have a reasonable layout, just to keep everything neat and tidy. So a basic 2-layer PCB was designed and a prototype built. The filter design was tweaked a little so that I could arrange the poles to fall very close to the 2nd and 3rd harmonic of 475KHz.
Here is the transfer function of said filter, as measured on my spectrum analyser. Insertion loss is around 0.4dB, and stopband attenuation is about 60dB. Attenuation of the second and third harmonics is better than 75dB. I think this will do the job nicely.
The return loss for the filter is better than -20dB, so the radio will not see any reflected energy from any of the harmonics.
I should point out that this filter is not symmetrical. So input and output are not interchangeable. Just in case someone tries it....
As it stands this filter should handle around 30W of power at 475KHz, the limitation being the power in the harmonic content being dumped into the 2W resistors.
I anticipate a drive power of around 2.5 - 5W being required for the PA (still to be built), so there is plenty of headroom here.
Still to come...
This is a work in progress, still to come are: