900 MHz Marine Antenna 1

[Tofflemire]

Hi All,

I am looking for a 900MHz marine antenna with a bayonet type mechanical mount and a central antenna connector. We are willing to work with a company to possibly to design such a thing.

Tofflemire

 

[Higgler]

Is this a high volume item? Or do you just want a few of them.
Do you have some other spec's. Bandwidth (900 Mhz +/- ?Mhz), polarization, physical size, power handling, VSWR, Mounting structure(flat metal surface vs. metal tube of X" diam.) etc.

I work in the antenna group of our small company (90 employees) and have recently made for the Navy, a Vertically polarized 960 Mhz to 40,000 Mhz antenna that's relatively small, 3 inches tall and 5 inches diameter, but it's not marine ready. The weight can be very light since it's mostly foam, i.e. only a few onces if we use metallized plastic to create a conducting surface.
kch

 

[Tofflemire]

Hi Higgler,

This antenna would be mounted on a product used by the US Navy. See link

http://www.metocean.com/corp.asp?v=69200440927&item_id=88

. This antenna would be used with a FreeWave radio modem. We would be making 6 prototypes and would require 94 others for retrofitting.
The antenna length (one piece)would be in the range of 8-10 feet long with a tapered mast section to reduce wind drag. The antenna gain would be 3-6 db. As far as mounting we are trying to make it a bayonet style vs. screw on or clamping.
We are still in the information gathering stage; I was trying to gather antenna manufacturer’s interest in potential collaboration on this venture.

Thanks

Tofflemire

 

[Higgler]

Your website reference doesn't actually specify the frequency range of the antenna, what is it?

Frequency Range 902-928 MHz ?

The wavelength at 900 Mhz is just over one foot long, hence an actual antenna is only 6-7 inches height maximum.

I assume that the 8 foot size is only to set the height above the water at some reasonable value.

Typically for an antenna above the water, your reception at the ship will have peaks and nulls due to bounce off the water, i.e. you will have some varying signal strength to your receiver. Hopefully you store the data and send it repeatedly to the receiver to catch a peak wave (that'll help maximize communications range).

In a recent (past 3 years) wireless connectivity program between ocean buoy's, a company consultant was surprised by the very short range reception that they found. The cause was multipath off the uneven ocean. Range was 1/4 that they expected. Hope your spread spectrum alleviates some of their multi-path short range headaches.

Do you want the bayonet mount for quick connection and easy replacement?

This website

http://www.rfwiz.com/Comtelco/ComtelcoMarine.htm

seems to have what you're looking for.

Is this design so special that you can't buy a commercial unit and change the mechanics of it? If so, my company may be willing to help you.

kch

 

[VEBill]

Multipath interference can be reduced by means of circular polarization (assuming the unwanted reflected path is a single bounce, it most likely is). The single bounce will be of opposite polarity (RHCP <-> LHCP) and will be rejected by the CP antenna at the other end. Apparently, at low grazing angles this beneficial effect is somewhat reduced - I'm not sure by how much.

There are omnidirectional antennas that are also circularly polarized (the 'quadrifilar helix antenna (QHA)' is the best example). You can 'squish' the QHA to put more power towards the horizon and less straight up. Maybe a slightly cone shaped ground plane would help to put more power on the horizon (plus and minus the buoy's pitch and roll motion).

The reception end could perhaps use an axial mode helix or crossed yagis (needs to be matching CP at both ends) - such antennas would obviously need aiming. To improve reception, a remote mounted preamplifier (right at the antenna) with an excellent noise figure wouldn't hurt.

900 MHz will provide antennas of convenient size.

Avoid too much gain on unstable platforms.

 

[Higgler]

The circular polarization thing works worst mathematically at around 6 degrees incidence angle since all of the vertical polarization energy is absorbed at that angle by a flat ocean. Hence if you send circular you receive horizontal polarization over the water at that incidence angle.
One thing I've often thought about but haven't seen, is to have a receive antenna aboard ship for a program such as this using two antennas atop each other. Combined them into a magic tee (zero/180 degree outputs) to have sum and difference outputs. If you can point the null of the 180 degree output properly (between the tranmitter and the ocean), then the energy bounce off the ocean will always help one channel and always hurt another channel of the magic tee output, hence dual receiver front ends to detect the highest signal would stop multipath fading. I believe the antenna separation must be large if the transmit antennas in this system are close to the water. It works best for picking up low flying transmitters approaching ships that pop into and out of dead spots due to multipath on a calm ocean day.
Antenna diversity would help too, but not as reliably at the magic tee, or so I think. Proof is always in the measurements, and sometimes in the paperwork. I've seen alot of trees killed for nothing by useless ink.
kch