Underwater Antenna Experiment

[NCtoad]

33 minutes ago, Chase Goldman said:

Gotcha.  I just figured the the further you run it up the metal shaft, the more attenuation occurs, with the mid point probably being the worst.  The closer you are to either end of the upper less attenuation.  Couldn't say where in the upper Parkgt terminated the waveguide so I just assumed it was being attenuated by the upper shaft metal.  I'll have to run some tests myself because the "cut to suit" coax solution seems best.  It also looks like that's where XP is headed with their new antenna pack.

I’ll probably do what Carolina has done and coil the cable so it contracts as the rod is collapsed into its travel configuration.  That way I could hunt in water that’s as deep as the top of the upper rod.  

[PimentoUK]

Apologies, I skimmed the thread, but ...
The length of the stripped section of cable should ideally be 31mm, which is a quarter-wavelength at 2.4 GHz. That then makes it an impedance match to 50 Ohms cable.

It's probably not going to be far 'off-spec' using 25mm / 1 inch, but the correct length may just give more leeway on antenna placement etc.

[matt]

6 hours ago, CPT_GhostLight said:

Well people have been complaining about the clear and super long antenna wire that has to be wrapped aound the outside of the shaft, so XP is making a black antenna wire that is much shorter and presumedly can be run inside the shaft. My take away is that they are listening and trying to to respond customers requests, and that's a good sign.

Not everybody wants the same things on their detectors and some have no problems with stock detectors, but if XP is going to be responsive to customers needs and wants, then that's a win. I'll take it, thanks XP!

It makes me wonder what else they may be working on. 😏

I agree!

A lot of people asked for a dive shaft and now they sell one. I purchased a dive shaft mainly because I plan to use the Deus ii for underwater diving in Hawaii. The antenna that came with the dive shaft is routed inside the shaft and is really short. I can't remember what color it is, but I'll check when I go over in a couple weeks. XP seams to have really good customer service.

[F350Platinum]

On 1/25/2023 at 4:29 PM, PimentoUK said:

The length of the stripped section of cable should ideally be 31mm, which is a quarter-wavelength at 2.4 GHz. That then makes it an impedance match to 50 Ohms cable.

Appreciate the additional info. 👍

My question is does the coax need to be stripped at all? There is no physical electrical connection, the wave is passed via induction, isn't it?

I'm considering shrink wrapping the end of the coax, putting ~30mm next to the coil transmitter bump, and letting the sheilding do the work. 🤔 I'll let y'all know how it goes. 

 

[PimentoUK]

No, don't consider it.
The exposed 31mm length ( in conjunction with the first 31mm of the shield ) acts as a centre-fed half-wave antenna. This in theory has an input impedance that is purely resistive at the intended frequency. This resistance is 50 Ohms, which matches the characteristic impedance of 50 Ohms co-ax cable. So maximum signal transfer from the antenna to the cable occurs. The cable is low loss at 2.4 GHz, so most of what 'went in' comes out at the other end - all you need to do is match it to another similar antenna, and the signal gets re-transmitted.
To be extra nerdy, the length of the co-ax cable matters, and it should ideally be multiples of half-wavelength long, ie. 62mm. It matters because the RF signal current flows in the inner core AND the outer screen.

( Anyone familiar with CB radio, for example, would know the ideal co-ax cable length ( rig to antenna ) is 18 feet in Imperial units ( 5.5metres ), and even if your vehicle installation only required 12 feet, you don't cut down the cable, just leave the slack in your trunk etc.)

[F350Platinum]

2 hours ago, PimentoUK said:

No, don't consider it.

Thanks. I do understand this, having done a bit of microwave transmission study in the military. 👍 Just a bit tho. 🙂 I'll see what works best with this in mind.

[NCtoad]

3 hours ago, PimentoUK said:
To be extra nerdy, the length of the co-ax cable matters, and it should ideally be multiples of half-wavelength long, ie. 62mm. It matters because the RF signal current flows in the inner core AND the outer screen.

( Anyone familiar with CB radio, for example, would know the ideal co-ax cable length ( rig to antenna ) is 18 feet in Imperial units ( 5.5metres ), and even if your vehicle installation only required 12 feet, you don't cut down the cable, just leave the slack in your trunk etc.)

So, a multiple of 62 mm, which to us still using the dinosaur units of inches and feet, is 2.44”.  Which in multiples is:  2.44, 4.88, 7.32, etc.   The ideal lengths depending on how tall you are and how far your shaft is extended will  most likely fall in the following ranges: 31.72, 34.16, 36.6, 38.04, 41.48, 43.92.  All in inches.  I’m not so nerdy as I am anal, so I would use the length that best fits my needs.  Thanks for the specs Pimento!

[F350Platinum]

3 hours ago, PimentoUK said:

( in conjunction with the first 31mm of the shield )

If we're getting into specifics here, does this mean leave the shield on or strip it off? 🤪

Then we get into the same infiltration issue we have with water on the original antenna. 😵 Depending upon the type of coax used, assuming it is the equivalent stuff, if the materials used are mangled by salt water (even my rivers are brackish) a cable probably won't last too long under constant use unless it forms the oxides Chase mentioned. 🤔

Here's an article I found that may 'splain things a bit better:

https://oscarliang.com/repair-2-4ghz-antenna-rx/#:~:text=The 1%2F4 wavelength of,receivers have different antenna length%3F

Mentions shrinkwrap effects too. 

Just for giggles, and since I already did it, I'll try my setup in the river near my house and let y'all know if it worked or not.  I'll also check the cable length, I certainly want to give my rig a fighting chance! 😀 I can always strip it later. 👍

35 minutes ago, NCtoad said:

So, a multiple of 62 mm, which to us still using the dinosaur units of inches and feet, is 2.44”.  Which in multiples is:  2.44, 4.88, 7.32, etc.

Thanks for the Imperial conversions, NC. 👍

[PimentoUK]

The problem with the 'multiples of 62mm' bit is that your cable is likely to be 1000mm long. Maybe it's more correctly multiples of 61.3mm - that's about 1/2" difference over 1 metre. So it's not practical to get the length correct, when the cable is many wavelengths long. And the [i]exact[/i] frequency used is somewhat unknown. The 2.4 GHz ISM band specifies 2.412 - 2.484 GHz as the 'nominal' channels; this gives half-wavelength values of 62.2mm to 60.4mm.

For HF/VHF radio gear, we're talking half-wavelength multiples of 1 foot or more, so there is a real chance of getting it 'right' . And also the real-world physical opportunity to fine-tune the cable ( shorten it, anyway ) at the antenna end.

Edit:
"Does this mean leave the shield on or strip it off?"
In this application, simply removing it is the practical solution.
The case for unshielding the inner-wire, but leaving the stripped outer attached, is when you want to make a T - shaped antenna feed. Where the co-ax is the vertical of the T, the two top branches are the inner conductor and the removed shield. This is not appropriate for a water-antenna, as encapsulating it in a water-tight way would be a headache, for a start.

[ColonelDan]

Parkgt has a super idea there!  I never understood why XP provided such a long antenna in the first place.  Parkgt's design should have been close to the initial factory design.

To their credit, XP is coming out with a shortened antenna that I intend to snatch up as soon as they're available.  Such a shortened antenna should be perfect and much more convenient for internal use with Steve Goss' CF shaft.

Good job Parkgt.  I love to see such ingenuity like this!!!👏