For anyone using a HF rig and an SDR, or just two HF radios and wants to share the antenna so that you can listen on both, I thoroughly recommend one of these SDR TX/RX Switches. This is an alternative to the MFJ1708B-SDR, but at a much more acceptable price – I picked it up on Amazon for about £35 with a two week delivery time, they're also available with Prime Same Day for £60.

I was expecting something pretty flaky, but I was actually really impressed. It's well made and well finished, in a solid aluminium case, and seems to work very well. There's a jumper inside which lets you either have RX signal to both radios simultaneously or only to the RX radio; I have this set to RX on both.

I made a cable for my FT450AT which both powers the switch (it has a 100ma draw) and triggers it; the switch does sense RF and that seemed to work pretty damned well but I prefer it to be explicitly triggered with the RF sense as a backup just in case.

I tested it out via a power meter to see what happens on the RX side. As you can see from the attached photos, when power from the main antenna output is 80W, my power meter seems to show around 110mw from the SDR output – but I think that it's just too low power for it to measure accurately.

So I ran the SDR output into my Tiny SA. There's 40db of physical attenuation on the input, but the SA has been configured to recognise that, so I believe that the reading of -36dBm is accurate.

Here's a conversion chart showing that -36dBm is a really really small amount of power

This does shows up as a signal on the SDR (in the photo it shows on my IC705), but I think that at this level it's safe.

I'll be using this setup so that I can:

• Work stations on HF SSB using RX the IC705 but TX with the FT450AT for a bit more power (I will auto-sync the frequecies using cat control for this use case)
• Listen on HF with the IC705 while the FT450AT is running GB7WEM – WEM will interrupt when it TX's but that's a fine compromise
• While I'm away from home (using the IC705 portable!), I will have the FT450AT running GB7WEM and a WebSDR listening to HF

Overall I'm very pleased with this indeed, and I thoroughly recommend it!