[Towertalk] Antennas over Salt Water.
Thu, 14 Mar 2002 07:00:04 -0800
It says two things: a beam's life in rough seas would be approximately five
minutes, and omnidirectional performance is critical. Because it would be
mounted on a rapidly rotating platform (the ship) keeping a beam pointed in
one direction would be a complex and expensive proposition. A few
special-purpose communications and command ships have been fitted with small
to medium sized log periodics for use in port, but I don't know much about
how well they hold up. For broadband performance, most ships employ a single
large wire "cage" type discone, usually on receive only, and mounted as far
forward (thus as far from the RFI envelope) as possible. The wire cage is
constructed of 3/8" or so phosphor bronze or stainless wire rope. A few
horizontal wires are used, always end fed. All the ships I served on ran
extremely heavy duty 25' - 35' whips on HF, if you can call a vertical with
a 5" base diameter a whip. They are not "phased", but are mounted wherever
space allows after best engineering guesses. On carriers, the whips are
built on fold-down bases so they can be lowered to horizontal during flight
ops, because F14's/F18's are hard on whips (the whips are even harder on the
aircraft, though). Needless to say, they don't work very well in that
position. Remote autotuners are usually included as part of each whip's base
assembly. They are weather sealed using cast aluminum housings with multiple
O-rings, and filled with nitrogen under a slight positive pressure at all
times. I've opened them up after they spent 10 years at sea, and the guts
looked like they came from the factory yesterday. The Navy, of course,
always had a bigger antenna budget than me.
If you think you had RFI problems during field day last year, consider a
dozen 1KW HF rigs pumping full blast within a couple of hundred feet of each
other, each nestled into a mass of steel and aluminum wires, cables, pipes,
and reflecting surfaces of all shapes and sizes. Then throw in another dozen
VHF and UHF rigs, half a dozen radars (some running several megawatts), IFF
transponders, navigation beacons, etc., and you begin to get the idea. A
Navy ship under full ops is a seething mass of RF. It's always been a wonder
to me that they can talk at all, or that any of us had kids, but we managed.
The advent of satellites has helped, but much of the lower band stuff is
Milspec comm equipment is expensive for good reason, considering the
environment it must operate in. My FT-1000D would likely melt into a puddle
under those conditions. :-)
73, Jerry W5KP LT (LDO) USN, Retired
> Hi guys.
> All the navy and coast guard ships I have ever seen, and been on many, all
> have vertical antennas for HF. They usually have two, on opposite sides,
> one on starboard and one on port.
> I have never seen a beam on one for HF. Doesn't this say something?