Here's my thoughts. And if you know all this, forgive me. Also, to start off, I just got a 3G radar and a HDS-7 display and will kinda use that.
First there's the radar unit resolution. That's listed as a solid angle sent out from the radar antenna, which rotates inside the dome. By and large, that's determined by the antenna length and height. Obviously you can't have a wider antenna than the dome diameter. So that defines an area that's "painted" with the radar transmission. For the 3G their specs are 5.2 deg horizontal and 25 deg vertical. For the Garmin HD18, those numbers are 5.0 and 25, the same as the 3G. For a HD24, those numbers are 3.6 and 25 deg., a goodly increase in resolution at some cost in size.
Next, still with the radar, one has to make sense of the returned signal (extract data). I can find no published spec for this. BUT, that's where the HD comes in. One hopes that the signal processing electronics can pull more data from the returned signal than a non-HD radar can. And I think that's what we're all looking for. I'm happy with the antenna resolution (5x25 deg) but I want to know if there's something there. THAT'S what a HD radar gives you and it's important. In addition the 3G radar uses a different signal technique which, I think, allows them to pull more data out of the return, a frequency difference instead of a time difference. Better at close ranges.
Now, to display these data. An SVGA display is 800x600 pixels, which beats the 640x480 resolution of VGA. Now, all the display tells you is if there is a return signal at that pixel. It does NOT process the data, just displays it, all the processing is done in the radar dome these days. The radar outputs the pixels and the display cannot give any more resolution that it receives, and I'll bet that SVGA is all the radar puts out.
So, the important part of radar is to extract the data from the radar return signal. The display isn't as critical. The HD is important, and I assume that SVGA is all you get out of the radar.
Two additional thoughts. OpenCpn has the capability to import Garmin and Lowrance radar data into a laptop computer. This give you a possible resolution of 1340x770, minimum. This will give you more distance, not more resolution, the radar data will still be within a 800x600 pixel frame. But the PC is a cheaper navigation display. For a C-dory, it can sit inside the cockpit.
And finally, I agree with Bob (as usual,) get the largest unit you can. Though 18 is really good these days 24 is better.
A C-Dory doesn't need range, because our boats are close to the water so the horizon is the limiting factor. I figure for Journey On, the unit is 8 ft above the water and the horizon is ~4 mi away. So I got the unit which promises the best short range resolution, 3G. Both Garmin and Lowrance are good, I'm not going to get in that discussion.
Boris
