Aerial placement

[CraigP]

I've never seen a coax cable used as an aerial... The only antennas I have seen are a solid, bare wire (old school) and a stranded cable with a plastic insulation on it. A coax is designed to transmit a signal from one end to the other over a controlled impedance to arrive on the receiving end as undistorted as possible. The shield as two functions, as an external shield for external fields not considered as the signal and to set the impedance of the cable, usually 50 or 75 ohms. The shield is usually grounded to drain extraneous energy and to set the impedance relative to ground, which is usually established as a ground plane. If a coax works as an aerial for you then that's a good thing!

[fweasel]

Craig, modified coax cables are used as antennas all the time and I don't see how the 2.4GHz receivers using them are any different, in your eyes.

[CraigP]

The length of an antenna is 1/4 the wavelength of the desired signal to receive. The older transmitters, running 500khz or so, needed at least a 20" antenna. As the frequency increases, the antenna grows shorter. I agree that you could modify a coax for GHz reception. But that wasn't the point Paul was making. You still need 1/2 a 1" unshielded portion even for GHz. That would be trick to raise the reception point over the boat and still make it effectively receive the carrier. My reciever has the antenna inside of it on a PCB strip line. It's attached with Velcro on the under side of the deck, pointing up. I'm getting full range for a boat, but wouldn't trust it in a plane. Your cell phone has the same type of antenna. The benefit of a PCB strip line is that you can set the capacitance of the antenna, and make a better tuned circuit. I have designed antennas for a living, working for Wilson Electronics. If you use a coax, you should be well versed in antenna theory or you could easily make a unidirectional antenna out of it, something you really don't want. These antennas need to be omnidirectional in RC craft. Some of these problems being posted could be suffering from that.

[CraigP]

Im just going to leave this topic for others to discuss... This is some pretty deep theory, so just keep doing what you do. I shouldn't have said anything

[rol243]

the coaxial cable is like an extension lead for the main antenna as used for television reception, you need both the inner copper wire and the woven copper wire mesh to connect , just like positive and negative on battery terminal connections.

[fweasel]

The length of an antenna is 1/4 the wavelength of the desired signal to receive. I agree that you could modify a coax for GHz reception.

Agreed

But that wasn't the point Paul was making.

What specifically do you not agree with what Paul said? Because, it all sounds in line with my understanding of the topic, from a non-EE, practical application point of view.

[CraigP]

Paul is saying that the exposed end of the coax is the only part that is acting like an antenna, which IS true, IF the shield is tied to the ground reference of the antenna, inside the receiver. This is where I may be wrong... iF the receiver comes with an SMA or other, high speed, impedance controlled connector, then you can use the shield as he references. But I've never seen a boat receiver with a SMA connector on it.

My point is, if the shield is not tied to a good, antenna referenced ground, then the shield will be capacitively coupled to the inner conductor and it will move with the signal but not in phase, so it loads, or makes the signal weaker. So where is the shield being tied, is the big question. The grounds on the servo ports are no good, because most GHz antenna front end circuits are Capactively coupled to an isolated ground within the IF radio section of the receiver.

I'm sorry, complex explanation! But it's a complex subject matter and just recommending these generic uses can invite more trouble than it fixes.

But this discussion has made me think. IF we had that SMA connector, it would be great to feed the non-connector end of the coax up through the middle of the vertical stabilizers found on many hydros and just expose the 1/2" of bare metal at the top of the stabilizer. That would be a trick antenna position! Also, for my education, are there receivers that come with a micro coax cable coming out of it? Again, I've only seen stranded single conductor, but there are new things out everyday...

[buckman]

Howdy boys, I use 30 feet of coax cable to receive 2.4 ghz signal underwater from enclosed GoPro to I-pad, How you may ask, simply bare 6inches of copper wire from both ends of coax and tape one end to GoPro and other end to I-pad and you have 2.4ghz antenna.
lol
Buckman

[Bp9145]

[QUOTE=CraigP;701862]" But I've never seen a boat receiver with a SMA connector on it."

Actually I thought that is why some receivers have interchangeable antennas because it has some sort of SMA connectors on it? Others have modified their receivers just for that purpose to put whatever antenna they choose to use. For example, a guy who had a metal boat had a hard time receiving RF signals so what he did was open the receiver and inside the receiver the original wire antenna is connected with a connector style called U.FL and using a short U.FL to SMA cable he was able to use two matching antennas on the deck, one for his Futaba, and one for his XBee communications. I don't have extensive knowledge about 2.4ghz antennas but just enough to understand how it works. But here's an image of it. . .a boat receiver with a SMA connector on it. Check our Degi-key they lots of antennas with SMA connectors on it.Screen Shot 2017-10-04 at 11.44.47 AM.jpg

[CraigP]

Yes, I see... I owe Paul an apology, I didn't realize they had this. Man, thanks for letting me know, it gives me some ideas. I use Digi-Key a lot! Also Mouser Electronics. With that connection, you should be able to get any type of antenna you need!

[CraigP]

I'd like to hear more about this "metal boat", sounds cool!

[Bp9145]

YIPPPEEEE. . .we're all in agreement. Now let's brainstorm some really cool ideas we could all implement in this crazy hobby of ours.

[NativePaul]

Craig, I did say earlier that the sheath of the coax is grounded, like you say it wouldnt work otherwise, but they are, so they do. SMAs are pretty big for model RX usage, I did say earlier that they use the U.fl connector which much smaller but does the same thing. For comparison between the 2 this is a U.FL to SMA adapter.

And for scale, the surface mount u.flplug.


All the top brands ie Futaba, Spectrum, Sanwa/Airtronics, JR, Multiplex/Hitec and Frsky all use RX aerials based on 50ohm coax with the U.fl connector, and likely more that I have not seen inside too. I have seen some budget off brand RX that are still based on the 50ohm coax but soldered to the RX, this is inferior as the connection is not shielded as it is with the U.fl socket, and it is not replaceable in the event of antenna damage, but on the other hand some are so cheap you can see why they dont include a connector. I did also allude to Rx with non coax internal aerials with slightly longer full 31.25mm 1/4 full wave, my mistake not to specify that the aerial would be A PCB trace not a wire, I thought that would be implied by the ease of manufacrure, considering that there is nothing to gain by adding a wire while still keeping it internal to the case.

In case you still dont believe me here are a few replacement aerials from well known manufacturers.

[Bp9145]

Ok then we're all good. This was a learning experience for everyone involved and for future boaters to learn from. . .like I've said in the past this is a place that we can share our knowledge with everyone and that's why I love it. Some will have more knowledge than others but we all contribute somehow. . .peace I'm out my fellow boaters.

[CraigP]

Oh I more than believe you Paul! I have used this stuff in radio gear for our remote telemetry systems running all over in Texas, Colorado and New Mexico. This is what I'm used to seeing. Impedance matching is critical... Our competition takes it rather lightly, and we can pluck out cell signals where they can't using some pretty high gain boosters. I'm sorry I was so forceful, it's just that I know how finicky it all is.

We have our field guys put too tight a bend radius on a coax and what do you know, it don't worky no more! I just spent yesterday on this very problem, with raking with a guy that doesn't "get it", so I was sensitive to the subject. But no excuses, I've got to keep my mouth shut, lesson learned, trust the Paulman!