Tune VHF antenna for AIS TX

G12 said:
That's exactly what he's answered by using the difference in the two VSWR's to calculate the loss in TX between the two different antennas. 0.8dB in that particular instance wasn' it? That's about 27%.
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But those weren't the two VSWRs corresponding to the two antennae you suggested, were they? Or were they? Anyway, 0.8dB is 18% if you're talking power. Is that "much better"?
 
Didn't meant to delete the last message, I was trying to edit it.

So, I checked the numbers with a calculator rather than my brain and you are correct about 18% ish. So that's a significant improvement isn't it? Any engineer would be delighted with that, just for setting it up properly....

Not sure what I've done to upset you but I've had enough of the attitude and this is the last contribution I'm going to make to this thread.

73.
 
G12 said:
18% ish. So that's a significant improvement isn't it? Any engineer would be delighted with that, just for setting it up properly..
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From an engineering gains perspective I suspect you are right. Not bad improvement for chopping 15mm off a bit of metal. BUT

What is that in *real* terms and by real terms I mean distance on the water? That's what the OP wants to increase. And 18% increase in range? Sqrt of 18%? I dunno. But the OP was talking of not being seeing at 2miles and wanting to be seem at 5 times that... Adding 18% will not even be noticed!
 
G12 said:
Didn't meant to delete the last message, I was trying to edit it.

So, I checked the numbers with a calculator rather than my brain and you are correct about 18% ish. So that's a significant improvement isn't it? Any engineer would be delighted with that, just for setting it up properly....

Not sure what I've done to upset you but I've had enough of the attitude and this is the last contribution I'm going to make to this thread.
Click to expand...

I'm not upset at all I was just hoping that you'd put some numbers to your claim. Were those VSWRs supposed to relate in any way to the two aerial lengths/frequencies you suggested?
 
Apologies for bringing back a fading thread, but having just gone through some AIS antenna troubleshooting (VSWR 3.4 : 1), I'll put here some hopefully useful bits of information for future readers:

A VSWR of lets say 3.0 : 1 will translate to a transmit power loss of 25% or 1.249dB. There's a online calc for that here: http://www.csgnetwork.com/vswrlosscalc.html

Now the real-life question is, how much less range that causes? Luckily someone has created an online calc for that too (scroll way down to the 'Times Further' calc): https://www.immersionrc.com/rf-calculators/
A power loss of 1.249dB will give 0.87x of the expected range. So if for example with an ideal antenna the transmit range would be 10nm, a VSWR of 3.0 : 1 will reduce that to 8.7nm. If that is acceptable, is everyones own decision.

About the VHF or AIS-tuned antenna debate: I have an em-trak B100 connected with 7m of RG58 to a stern rail mounted Scout KM-3F VHF (not AIS-tuned) antenna and the em-trak shows me (after reinstalling the connectors on the cable) VSWR of usually 1.0 : 1 and a few random transmits go to 1.1 : 1.
This is below the reliable precision of the above calculators, clearly the VHF antenna works just fine for AIS...
 
I would call a 0.8dB improvement "marginal" ... hpwever since it is effectively a "free" improvment, no reason not to take it. If you use a masthead aerial and feed it with good low-loss coax, that will stack up too. It all "helps" but these are small improvements, and not "significant" as such.
 
rszemeti said:
I would call a 0.8dB improvement "marginal" ... hpwever since it is effectively "free" improvment, no reason not to take it
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The problem is, that there *isn't* a 0.8dB improvement from going from a regular VHF to an "AIS-tuned" antenna.
A 0.8dB loss would mean the VSWR is > 2.4 at 162MHz for the VHF antenna and the antennas really are nowhere near that bad.
Of cource, there can be even bigger differences between two random *installations* of any antenna, so drawing any conclusions about effects of antenna length from comparing two dissimilar installations is pretty pointless.

Edit: Of course there's no harm in going with the "AIS-tuned" antenna if installing a new antenna for primarily AIS use. They cost the same, look the same and work the same. But if the VSWR is too high, it might be the antenna's fault because the antenna is bad (as in broken), not because of the "wrong type".
 
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I have a bog standard VHF antenna dedicated to my AIS transponder. The antenna is located at the top of my mizzen mast. I had reasonable reception but poor transmission. I ended up swapping the antenna and now we transmit over 10nm and we have excellent reception. I cut the old antenna up to see what was wrong and it was corroded internally. Seems like a standard vhf dedicated antenna is perfect for AIS in my experience but it needs to be in good condition and mounted as high as possible.
 
Many thanks for all the responses. In the end I chopped 12mm off the end of the antenna and it has definitely improved my transmit range but not a lot maybe a mile.

I’ll probably fit a new antenna and coax.
 
trbt said:
The problem is, that there *isn't* a 0.8dB improvement from going from a regular VHF to an "AIS-tuned" antenna.
A 0.8dB loss would mean the VSWR is > 2.4 at 162MHz for the VHF antenna and the antennas really are nowhere near that bad.
Of cource, there can be even bigger differences between two random *installations* of any antenna, so drawing any conclusions about effects of antenna length from comparing two dissimilar installations is pretty pointless.

Edit: Of course there's no harm in going with the "AIS-tuned" antenna if installing a new antenna for primarily AIS use. They cost the same, look the same and work the same. But if the VSWR is too high, it might be the antenna's fault because the antenna is bad (as in broken), not because of the "wrong type".
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There are examples for marine VHF antennas tuned below(!) 156MHz, reaching an SWR value of 2 to 3 at 162MHz, one can be unlucky. If you want your signal picked up by AIS satellites, you might need the very last bit of efficiency. I am also unsure if a 2+ SWR value without dampening (i.e. using a very good coax and connectors) wouldn't cause any further calamity?
 
GTom said:
Reviving an aging thread, anyone seen a VSWR plot for the Metz Manta antenna?
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Well part of the trouble with that is that it'll vary a bit with each individual installation. I could do one for my installation but it might be different to another installation. I did do a full plot when I installed both antennas and they both would have been as expected otherwise I would have done something about it. I did write the numbers in the log somewhere.
Was there something specific you wanted to know like the bandwidth below 2:1?
 
G12 said:
Well part of the trouble with that is that it'll vary a bit with each individual installation. I could do one for my installation but it might be different to another installation. I did do a full plot when I installed both antennas and they both would have been as expected otherwise I would have done something about it. I did write the numbers in the log somewhere.
Was there something specific you wanted to know like the bandwidth below 2:1?
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Basically the values would be interesting at AIS and VHF frequencies, 162 and 156MHz.
 
GTom said:
Reviving an aging thread, anyone seen a VSWR plot for the Metz Manta antenna?
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I recently fitted new Metz antenna, did put my analyser on it, will look to see if I’ve saved the trace, if not can do another this weekend if you really want one. As said each install will be differ
 
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