Understanding the Practical Application of a Simple Antenna made from Coax

I am here today to gain a deeper understanding of the fundamentals of using antennas in RC applications. I am trying to understand what exactly is going on with a particular 2.4GHz receiver a picture of which is attached.

It's somewhat difficult to see, but these are 2 small coax wires soldered to the board with the center conductors soldered to pads separate from the shielding (obviously) which I assume is just grounded. So I have 2 questions:

1. Would these 2 wires be 2 separate antennas or 2 halves of a single dipole?

2. In what way does the shielding effect the antenna? Specifically, if you were to cut the wire down (significantly changing the overall length of wire) but strip away shielding such that the exposed length of center conductor is unchanged, how would this effect the performance of the antenna?

What these two do, whether they just form a dipole together, or are two separate monopoles (with bad ground) is impossible to tell based on the info we have.

I find it less likely they form a dipole.

Maybe there's just one receive and one transmit antenna, to save on a directional coupler or antenna switch.

Maybe these antennas are there for diversity reasons – for example, to decrease the likelihood that all antennas you have are subject to fading, you just use more antennas. $P(\text{all antennas faded}) < P(\text{at least one antenna is OK})$

1. In what way does the shielding effect the antenna?

I'll nitpick on your terminology for a second, because it makes a difference later:

That's not primarily a "shield", that's the outer conductor of the coax cable, and just as important as the center conductor at transporting energy. (In fact, you can have a hollow waveguide without the center conductor.)

Together with the center conductor and the dielectric material, it forms a waveguide.

Specifically, if you were to cut the wire down (significantly changing the overall length of wire) but strip away shielding such that the exposed length of center conductor is unchanged, how would this effect the performance of the antenna?

Impossible to tell. The coax might be somewhere between impedance of the trace on the PCB and the monopole feedpoint impedance, and act as e.g. a $\frac\lambda4$ impedance transformer. In that case, the length of the isolated part is critical.

Maybe it's just a best-effort matched transmission line, and changing the length of it doesn't affect the impedance.

But: in my theory (two separate monopoles), these antennas are separate for a reason (typically, diversity to increase robustness or speed of transmission); making their leads shorter would put the antennas closer together, which would have negative effects on channel independence, and hence, on performance.

• That looks like typical FHSS hobby receiver. They use two antennas for diversity. You can find them with coax part in anything from zero to 20", but the actual antenna (stripped part) is always the same. So, if one considers "performance" as distance then they can be shortened. But for reliability during 3D flight they better left long and placed at an angle. Aug 19, 2018 at 10:10
• Thanks, that is basically what I was thinking. And sorry for the bad terminology, literally like 5 minutes after posting I thought that was probably wrong. So @Maple, the reason I ask this question is specifically because my antennas got cut short by my props. My hope being that I cold just strip away the outer layer so that the original length of inner conductor would be exposed without altering the performance too much. Is this correct? Aug 20, 2018 at 16:28
• @JoshWiens it's at least likely. Again, the reason that you have two antennas in the first place is for them to pick up different paths that the RF wave can travel, so to decrease the likelihood of all paths being bad. Now, since putting them closer together will decrease the indepence of paths, but restoring the original antenna shape will probably give you more dB of SNR back than you lose in diversity gain. So: go for it! Aug 20, 2018 at 16:30
• Second that. And a bit of advice - put one antenna along the fuselage, the other one along the wing (assuming neither is carbon coated). And if you are using electric motor then put receiver as far from it as possible. Aug 20, 2018 at 16:38