Extending an RF ground plane outside a small PCB

Question

I am making an RF PCB with a surface-mount antenna on it. It is a quarter-wave antenna, so the ground plane should also be as long as a quarter wavelength for good RF performance.

For example, a 915MHz antenna needs a ground plane 3.3 inches (8.4cm) long. Let's imagine my PCB is 1 inch wide. As a 4 layer PCB, it would cost $10 * 3.3in^2 = $33 at OSH Park. Every time I screw up a design, that's $33 more wasted.

I read an article by Antenova about tiny ground planes. I noticed this method for dealing with small ground planes:

Your ground plane does not have to be your host PCB. A copper extension can be added. However, care must be taken not to loop this piece of copper around and underneath the existing ground. An example of this is a watch application. In this case, the ground extension can be run into the watch strap.

Copper extension? I have free access to CNC machines that can mill FR-1 copper boards! I can extend my ground plane past my tiny PCB... right?

Option 1: My first idea was to place a copper sheet (FR-1) under the RF PCB and use through-holes to solder the PCB to the copper sheet. Here's a side view and a top view of that stack-up:

That FR-1 copper sheet should extend the size of my RF ground plane. I then remembered this bit from the article:

However, care must be taken not to loop this piece of copper around and underneath the existing ground.

Uh-oh. I now have a small ground plane (PCB layer 2) with a large ground plane (FR-1 copper sheet) underneath it! There will be about 60 mils (1.5mm) between the small ground and the large ground, with only the border of through-holes connecting them. So I had another idea.

Option 2: I can connect the copper sheet to the edge of my PCB with a row of through-holes. This will avoid overlapping any of the ground planes.

This fixes my last issue, but that edge connection is weaker and could easily snap.

The big question: does expanding a PCB ground plane with another copper plane tend to help or hurt RF performance? If so, can I get away with Option 1? It's my favorite.

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I am just worried that the through-hole connections between the two planes will look like a big impedance bump to the RF signals. If through-holes are a bad choice, I can always get creative with land pads under the PCB, but those will make bottom-layer routing complicated.

I cannot find other examples of this practice on the internet. If you ask me to tune it with impedance matching, there is only so much I can do with a tiny ground plane. Also, getting access to a VNA would be a long and tedious process.

I can't use flexible dipole antennas because I have size and rigidity constraints.

This is a problem that I am trying to solve with creativity. I can't keep throwing money at my problems.

Answer 1

Option 1 should be best. It has less GND impedance than Option 2 because it is closer to the antenna and it has better capacitive coupling to the other GND layers.

I do not understand the cited text and I can not think of a single reason why you not should connect all the GNDs you have together as good and close as possible.

An antenna is like a children swing: The better it is fixed on earth, the better and stronger children can swing with it.

Answer 2

Once happened something similar to us, also at 900MHz. We had a ceramic antenna soldered in our PCB with a very small ground plane. As the performance was very poor we contacted the antenna manufacturer and they propose adding a copper tape directly to our ground plane, that improved a lot the efficiency.

The copper tape was something like this: https://www.sparkfun.com/products/10561

So yes, increasing the copper plane length improves the efficiency of the antenna. From this presentation from Laird you can see: https://connectivity-staging.s3.us-east-2.amazonaws.com/s3fs-public/2019-04/engineers-approach-antenna-selection-implementation-webinar.pdf

About option one or two. The option two sure works as it is the same that we did. But I am not sure that the option one doesn't work. Hope it helps!