VHF Antenna

Jinks

Jinks

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Further to my earlier post - I have now purchased a fixed VHF with Antenna.

Dumb query - does it matter where I put the antenna? In my ignorance, as the set is going next to the GPS I was going to put the aerials next to each other as well (i.e. on the rail adjacent to the cockpit). This is, as near as makes no difference, the highest point on the boat so is this ok or would I be better off having greater distance between them?

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all i read with my instructions was to keep the ariel at least 50cm clear of the compass, and avoid the cable lying along side other power cables. Havent heard that vhf transmission can interfere with gps signals.

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It is possible for the VHF signal on transmit to 'wipe out' the GPS signal, so it would be best to have some separation if you can. Get the VHF aerial as high as possible (more height gives greater range) but the GPS aerial doen't need height just as clear a view of the sky as you can reasonably find.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>
 
Your gps is a radio receiver just like the VHF, albeit on a much higher frequency band.
All radio transmitters can cause (a) splatter, where the modulation ie the information on the carrier extends many megahertz in the spectrum out from where it should be, (b) harmonic radiation, where multiples of the transmitted frequency are spuriously radiated getting weaker as they increase in frequency, (c) front end overload, where the sheer power of an adjacent transmitter de-sensitises a local receiver, and in extreme circumstances could cause the delicate mosfets in the antenna amplifier and Rf circuits to fail.
(d) intermodulation products, usually the result of two or more adjacent transmitters on close frequencies transmitting at once. results are unpredictable.

I would not put a gps antenna within a couple of feet of a 25W VHF if at all possible.
ie at opposite ends of the radar bar if practical to do so. Getting a height separation is even better.
It really relies on how "selective", ie the ability of your gps RF amplifier to reject unwanted signals, is.
One assumes that these devices have been rigorously tested in RF chambers specifically to reject marine VHF?


Steve.


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Steve

We had a clubmate whose brand new NASA DSC VHF wiped out his GPS signal on transmit, not a lot of help if the set was transmitting a DSC Mayday! The problem was actually the set rather than the aerial, but it set me thinking and I tested our 3 GPS sets with our old VHF, result all 3 wiped out, the VHF aerial is at the masthead, the GPS aerials on a stern gantry 45ft below and the GPS sets at the chart table 50ft below! We bought a new NASA DSC VHF (NASA were supposed to have solved the problem) and sure enough now have no problem at all. It just shows it pays to be careful and avoid locating the aerials close if it can be avoided, not to mention how much spurious crap old sets can send out!

Robin

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>
 
I had rfi problems with an SX35 and Navman plotter, ferrite beads solved nothing permanently, I had to move the antenna from next to the helm and mount it at the stern, on a davit.

No rfi now, to date...

<hr width=100% size=1>Two beers please, my friend is paying.
 
So on a 25 foot boat it would appear that I am almost certain to get some form of interference wherever I put the aerial? The point about the DSC link is valid; I assume that the VHF would send the signal without a GPS generated position leaving the onus on the sender to have noted position and send it verbally - which sort of defeats the object really. Does anyone have an idea how long after transmit the GPS signal would be re-established or is it a case of switch off and start again?

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No I don't think so, but try to keep the two aerials apart, preferably putting the VHF higher (maybe on an extension, some grp aerials have extenders available). In our case the VHF that caused the problem was a very old one, though otherwise worked very well, and the GPS signals returned within a few seconds of transmission stopping. Transmissions from our new NASA DSC VHF do not affect the GPS signals at all, whatever the earlier problem NASA had that affected our friend's set has obviously been resolved.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>
 
DSC Sets are constantly fed position information and time by your GPS, if by transmitting your Distress Alert and Call you wiped out the GPS, the DSC set would still have the information sent a few seconds before.

If the GPS does loose its position it does not need to be switched on and off again, it will find itself soon enough.

GPS aerials are best placed where the aerial head will remain horizontal and where they will suffer the least movement due to pitching, rolling and healing, for this reason they are best placed on or near the centre line, as low as possible (while still having a good view of the sky), and ideally towards the back of the boat. In the manual it will normally give a safe distance from VHF antennas and VHF feeder cable.

VHF/DSC antennas are best placed as high as possible to gain a better radio horizon. VHF antennas for powerboats are usually high gain but rely on being as near as vertical as possible, sailors usually prefer low gain or unity gain as this design combats the healing problem and due to their mast height do not require such a tall aerial to improve range.

Hope this helps

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<A target="_blank" HREF=http://www.stormforce.biz> Stormforce Coaching </A>
 
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