Using an IC-R3 to receive SSB from DC to 2.450GHz including the AO-40 satellite
Created 25 June 2001
Updated 26 June 2001
Disclaimer: if you break your radio, your pride or anything else remotely to do with this procedure, you're on your own.
Here's a mod I've been wanting to do for a while.
Why? The Icom IC-R3 covers upto 2.450GHz, so off the shelf you have a means of receiving S-Band and the AO-40 satellite downlink - err, not quite.
Problem #1: the IC-R3 is only FM, WFM or AM. You need SSB to receive the AO-40 transponder.
Problem #2: the IC-R3 is VERY DEAF at 2.4GHz. It's not quite so bad below 1GHz.
OK, so how do you get the IC-R3 to do SSB? The answer is you don't directly... what you do is tap off the IF and feed the IF into the antenna socket of your General Coverage All Mode HF receiver. This is not a new technique, what's presented here is how you butcher your IC-R3 to do it.
1. Remove belt clip, carry strap & antenna.
2. Remove battery cover and battery (and the AA spacer if you're using AA batteries).
3. Remove the four screws marked in fig. 1. (the top right screw is slightly shorter than the others).
4. You can now remove the front of the radio.
5. Remove the speaker lead and the screw marked in fig. 2.
fig. 2 Remove the speaker leads and the screw marked
6. You can now remove the front of the radio.
7. With the antenna connector and tuning knob facing towards you, attach some sub-miniature coax (RG-174/RG-178) to the two points indicated in fig. 3 with the outer shown to the left attaching to the metal shield and the inner attached to the left most pin of the row of three.
fig. 3. The location of the IF tap
8. Make a hole big enough for the coax to exit the radio - I made a hole near the tuning knob. Alternatively there's probably just room inside around where the carry strap attaches to put a sub-miniature socket. I just left a short SMA lead pigtail fly out.
9. Double check there's no shorts.
10. Reassemble the radio.
In a perfect world...
(a) Capacitively couple your General Coverage all mode HF radio to this IF output with, say, a 1nF cap.
(b) Use an impedance matching attenuator on the IF output.
I didn't bother with (b), and I'm not sure it made a lot of difference as the IF is so strong at this tapping point.
Make sure that having done the mod you don't transmit into the IC-R3, and make sure that if there's any DC short or DC preamp voltage etc on your IF General Coverage All Mode HF Receiver that you capacitively couple (see (a) above) the IF out. I STRONGLY suggest that you either use a receiver (as opposed to a transceiver), or if you do use an HF transciever, use an RX only (or 'beverage') antenna socket - transmitting directly into an R3 will cost you dearly.
The problem #2 of the deafness of the IC-R3 in the GHz can only be rectified with a low noise preamp with at least 20dB gain to overcome this receiver's poor sensitivity. Yes it really really is very very deaf. I listened to AO-40 with a signal strong enough to decode with a 25dB preamp switched in at the radio, and it was not possible to even tell there was a signal there without the preamp.
The IF tap's centre frequency is 26.050MHz, and is fairly wide so you tune your IC-R3 to within a few tens of kHz, and fine tune your HF RX around 26.050MHz (see fig.4.).
fig. 4. Receiving AO-40 (at MA=13)
One last problem is that the VCO is rather sensitive to temperature, which with a unit like this at this frequency is hardly a surprise.
Mail Howard, G6LVB