Angle of attack for VHF aerials and radar domes?

gravygraham

gravygraham

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Correct me if I'm wrong but your average VHF set will enjoy best range with it's aerial set dead vertical - and so presumably radar domes will perform best when mounted on the true horizontal. So, my question is to you folks with planing hulls which of course at speed ride nose up (let's pretend trim tabs haven't been invented here), is your kit set for when your mobo is level, or when it's up on the plane? Or do you not bother or worry about being such an anorak?
 
gravygraham said:
Correct me if I'm wrong but your average VHF set will enjoy best range with it's aerial set dead vertical - and so presumably radar domes will perform best when mounted on the true horizontal. So, my question is to you folks with planing hulls which of course at speed ride nose up (let's pretend trim tabs haven't been invented here), is your kit set for when your mobo is level, or when it's up on the plane? Or do you not bother or worry about being such an anorak?
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Radar domes are normally angled.. you can see that when the boat is stationary.
Given h/helds were covering a good ten miles in a recent test, I am not too concerned about a few degrees on my main VHF aeriel.
As it happens, coming back from Guernsey is some grim weather last summer, it was only as we got into Solent that I saw the aeriel locking lever had worked its way loose, and the top end of the aeriel was now in the sea ! Didnt seem to stop hearing the maydays and AIS from working.
 
It's a long time since I wrote an article on aereals (read 30 years+) but IIRC 10 to 20 deg lean has little effect on the performance of a vertical one.
 
prv said:
Lifeboats set theirs so that they're vertical under way.

Pete
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Yup, they need to ensure best comms possible as of course they make VHF calls when seriously moving, us mortals, as Paul said, would probably throttle back to make an emergency call... mine are set vertical.. always makes me smile when you see some sweptback back at around 45 degrees... in a go faster style
 
Can't say I've noticed domes in the marina leaning forward - maybe I need to open my eyes. When I did my Short Range Certificate I remember teacher saying about aerial position - and thereafter noticing boats with Firefly's boy racer styling.
 
Radar = slight angle downwards when lying still (2 -3 degrees)
VHF Aereals = Pretty straight up.....

... worth noting that our normal running angle is low (typical Italian of the age) and do not run with high bow....
DSC_0121.jpg
 
Vertical beam width on most small marine radars is 25 degrees I believe so I wouldn't get hung up on the radome. Most small sail boats don't have self-levelling mounts and I've found that when the angle of dangle is big enough to impact the radar you're usually too busy hanging on to look at the damn thing anyway! In general the idea with MoBo's seems to be set things so that they're level when at cruising attituse so it looks good. Probably saves a fraction in wind resistance too.
 
More relevant than vertical beam width with radar inclination is how high above the wheelhouse roof and how far forward/aft the scanner is mounted. I did an own boat instruction last year on a well known make of large sports cruiser. When steaming and bows trimmed as low as possible, there was good target response from astern & either beam, but nothing from ahead because the wheelhouse roof blocked the signal as the scanner was mounted low and well aft.

Paul
 
gjgm said:
...Given h/helds were covering a good ten miles in a recent test, I am not too concerned about a few degrees on my main VHF aeriel.
As it happens, coming back from Guernsey is some grim weather last summer, it was only as we got into Solent that I saw the aeriel locking lever had worked its way loose, and the top end of the aeriel was now in the sea ! Didnt seem to stop hearing the maydays and AIS from working.
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Depends on the aerial.
Any VHF antenna tends to transmit most efficiently at right angles to the line of the antenna - i.e if the antenna isvertical, it "focusses" most of its power towards the horizon
Low gain aerials (generally short ones) transmit are less focussed than hi-gain antennas so they still transmit towards the horizon even when they are tilted. That is part of the reason hand-helds have short antennas.
But hi-gain (generally long) antennas are much more tightly focussed, so they are much more susceptible to tilt.

Almost anything can be used as a receive antenna (remember the wire coathangers we used to use for car radios?) so its ability as a receive antenna tells you almost nothing about its ability as a transmit antenna.
 
I have two separate systems.
One with a long aerial one with slightly shorter.

Upstairs has the shorter one which should transmit more of a ball to allow for pitching/roll and is slightly angled for use on the plane.

downstairs is the longer one and upright when stationary to provide a theoretical longer and focused range.

Frankly they look a bit of a mess and while playing in the solent they tend to be both raked back a bit in the interests of beauty .

I recommend you dont go for a combined system with a remote mic for upstairs.

two totally separate systems, when one breaks it doesnt matter.

have your downstairs one repeat to a speaker upstairs with a toggle switch.

you can listen to 16 while chatting to mates on 6 and listen to 16 while hearing the weather reports.

you just need to remember to turn the speaker off when transmitting on 16 or you get a screeching sound to remind you.

Another reason for a shorter one is that it will not be snapped off along with the long one when you forget to lower it under bridges.
 
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