RE: Week 1: Making it Portable

jc2048 — Wed, 12 Dec 2018 23:35:57 GMT

I'm not an expert in rf stuff, so get a second opinion on anything I say here, but...

Wouldn't it be better with the dipole vertical? I think the radiation pattern has the highest energy coming off perpendicular to the elements and tapering off as you move round to being on-axis - there's a null along the axis itself. As it stands, at the buoy end of the link you'd lose contact as soon as it turns through 90 degrees and the signal would be quite variable generally as it swung back and forth.

Other things to maybe think about:

There's a 'ground effect' with antennas in that if power is launched horizontally close to the ground then there's some absorbtion by the ground - at the bouy end that's an approximation to what's happening [basically, some of the photons that make up the radio waves will be able to 'see' the free electrons in the ground and give up energy to them]. I think you're meant to try and get the antenna at least several wavelengths above the ground and preferably more. (At 900MHz that probably just means a minimum of a metre or so, though the more height the better.)

When the sea is calm and the surface is flat you'll get a strong reflected wave which will interfere with the direct wave. I'm not sure if there's anything you can do about that, but be aware in your testing that rough conditions may not be the worst case. [To complicate things, that interference can be constructive as well as destructive.]

That dipole looks short to me for 900MHz. Is it foil on a plastic substrate? If so, you might get slightly better performance from a metal rod dipole. If it's shorter than you'd expect, then the substrate is a mild dielectric (slowing the wave velocity, which is why it can be shorter [good in a confined space], but also absorbing some of the energy [less good if you want the best range]). However, if the transmitter has only been certified with that antenna, then you're probably stuck with it.

If the house antenna only needs to see the area in front of it, then there might be some benefit in putting a reflector behind it to push more of the energy forwards. (The antenna would then look like an 'H'.)

RE: Week 1: Making it Portable

genebren — Thu, 06 Dec 2018 21:10:56 GMT

Nice work on your project. It was good that you decided to test the communications link before moving further into the project. Hopefully you can figure out a antenna configuration that will improve your link. You might also be able to use a repeater of some sort to hop between the units.

Good work and good luck!

Gene