Good layout and grounding seems to be poorly understood out there so religion finds a foothold. You are right, there is really very little reason to use both the top and bottom of a two layer board for ground.
What I usually do for two layer boards is to put as much of the interconnects as possible on the top layer. This is where the pins of the parts are already anyway, so is the logical layer to use to connect them. Unfortunately you usually can't route everything on a single layer. Paying attention and thinking carefully about part placement will help with this, but in the general case it is not possible to route everything in one plane. I then use the bottom plane for short "jumpers" only when needed to make the routing work. The bottom plane is otherwise ground.
The trick is to keep these jumpers on the bottom layer short and not abutting each other. The metric of how good a ground plane is left over is the maximum linear dimension of a hole, not the number of holes. A bunch of short 200 mil traces scattered about won't keep the ground plane from doing its job. However, the same number of 200 mil traces clumped together to make one island a inch accross is a much bigger disruption. Basically, you want the ground to flow around all the little disruptions.
Set the auto router cost for the bottom layer high and don't penalize it much for vias. This will automatically put most of the interconnects on the top layer. Unfortunately, the auto-router algorithms I have seen can't seem to be tweaked for not clumping the jumpers. In Eagle, for example, there is the hugging parameter. Even if you turn this off, you still get clumped jumpers. Let the auto router do the grunt work, then you clean things up afterwards. Sometimes you can spot a case where a little re-arrangement can eliminate a jumper altogether. Most of your time, however, will be spent moving the jumpers apart to not make large islands.
As for power planes, that's mostly silly religion. Route the power just like any other signal, although in this case you have to consider the voltage drop due to the trace resistance, since power traces presumably handle significant current. Fortunately even 1 oz copper traces on a PCB are quite low resistance. You can make the power traces 20 mil or whatever instead of 8 mils for signal traces. In any case, the point is that the DC resistance matters but it is usually not much of a issue unless you have a high current design.
The AC impedance isn't all that relevant, which the religious folks don't seem to get. This is because the power feed is locally bypassed to the ground plane at each point of use. If you have a good ground plane, you don't need separate power planes for most ordinary designs, just good bypassing at each power lead of each part. The bypass cap connects directly between the power and ground pins, then there is a via right at the ground pin to connect to the ground plane on the bottom layer.
The high frequency power loop current of a part should go out the power pin, thru the bypass cap, and back in to the ground pin without ever running accross the ground plane. This means you don't use a separate via for the ground side of the bypass cap. Connect it directly to the ground pin on the top side, then connect that net to the ground plane with a via at a single point. This technique will help a lot with RF emissions and cleanliness in general.
The principal difference is the major one:
- SMB connectors are Snap-In. You push them together to mate, and pull them apart to unmate.
- SMA connectors are threaded. You have to screw them together, and unscrew them to disconnect.
SMB connectors are also slightly smaller.
SMA connectors are also far more common.
Personally, I would be much more inclined to go with SMA connectors, just because they're so much more easily available.
However, since this is going outside, I would more strongly recommend SMA, as it is much more mechanically reliable.
However, it's crucial to note that the Bulgin connector you link isn't really SMB. It's a SMB-style connector with a bunch more stuff on it. It's also exclusively available from Bulgin. However, realistically, any high-frequency IP67 connector is going to be non-standard (well, aside from N connectors), as there aren't really any standards for sealed high-frequency connectors, so the fact that it's Bulgin-Exclusive isn't really a big deal, as anything else will be similarly single-sourced.
On the whole, If you really need IP67, I can highly recommend the Bulgin Buccaneer connectors. I use several varieties of them in a device I designed, and they are very nicely made. Just be aware that you will need to make your own cables (which involves crimping/terminating coax), if the few varieties Bulgin offers do not suffice (and the longest cable they do make is 5M).
Is there any reason you can't use a N connector on your equipment? The only real downside to N connectors is they're large.
Best Answer
To electroplate the fingers with gold they must all be joined together electrically. This is done with a "plating bar" trace outside the final board area, which is cut off afterwards.
Usually the board edge will be chamfered for easier insertion in the socket. Since chamfering removes the lower part of the fingers they only have to be wide enough to carry the electroplating current. Making them narrower saves gold, which makes the board cheaper. If the board is not intended to be plugged in often then chamfering may not be applied, and then the narrow parts remain.
Gold plating for edge connectors