VHF antennas height vs DB

[psneeld]

It's obvious to me that very close and far comms require different tools for the job.

The far or weak comms can use the highest db you can get.

Close comms don't need high db and too high can be a detriment..but I am talking yards, not miles....

I have used 9db antennas for assistance tow use and rarely had issues....usually only in about the same distance as I could shout. Now if the other boat had a 9 db antenna and we were within say 1/2 mile, transmissions occasionally had issues....overpowering may be the factor but I would just switch radios if the other boater complained.

To me, I would never have a cruising boat without both aboard. Not a 3db though, the 6 db is just fine .

A side note, because of bad installs on mast top 3 db antennas on sailboats, I think I had better luck with sailboats that had 6 db antennas on stern rails or radar arches/towers.

And a portable aboard is handy for many reasons.

 

[rslifkin]

Yes a 9db antenna will perform better lower in a roll because it will be closer to the fulcrum hence travel less distance side to side.

Bud

The distance the antenna travels has a minimal effect on performance, however. It's mostly an issue of angle relative to vertical. So mounting it lower doesn't really help.

 

[psneeld]

This is one explanation of why line sight on VHF radios may be operationally varied....

https://www.amateurradio.com/the-myth-of-vhf-line-of-sight/

"
When we teach our Technician License class, we normally differentiate between HF and VHF propagation by saying that HF often exhibits skywave propagation but VHF is normally line-of-sight. For the beginner to ham radio, this is a reasonable model for understanding the basics of radio propagation. As George E. P. Box said, “All models are wrong, but some are useful.”

In recent years, I’ve come to realize the limitations of this model and how it causes radio hams to miss out on what’s possible on the VHF and higher bands."

 

[diver dave]

U know I will make things more complicated. [emoji15]
The planet Jupiter is in our line of site about half the time. But our vhf’s wont do the job.
1/r2 field strength reduction with distance.
Need lots of power and gain for that one.

 

[psneeld]

All true, but at least this guy explains why in marginal line of sight situations, more db can be benificial.

Also why in inland areas with obstructions, db also helps.

And yes, I believe in having both kinds over 2 of one for this very reason.

 

[Benthic2]

Forgive my ignorance as I have no experience with AIS. Will your chart plotter give you an estimate as to who you could receive a transmission from ? ie: If you see a boat that is 10 miles away on your screen, does that mean that boat could also reach you with their radio ?

 

[diver dave]

I am still in early stages of collecting real world data on that question. Tend to think that if ur AIS is sharing ur VHF comm antenna, it should be a tossup on range.
Although AIS is around 10% of the voice comm power level, there are likely other items in its favor, such as forward error correction, and maybe a reduction in bandwidth.
My AIS antenna is only 8’ off waterline, voice antenna is 55’ off waterline, so I have likely much further voice range. Neglecting the fact that AIS also works with satellite systems and ultimately, the internet.

 

[MYTraveler]

Forgive my ignorance as I have no experience with AIS. Will your chart plotter give you an estimate as to who you could receive a transmission from ? ie: If you see a boat that is 10 miles away on your screen, does that mean that boat could also reach you with their radio ?

My AIS routinely shows numerous vessels at 99nm+. So much for the only line of sight theory.

 

[STB]

My AIS routinely shows numerous vessels at 99nm+. So much for the only line of sight theory.

Vessels? That seems doublebwhat I'd expect, even on the far outside. I have a feeling something different is going on.

What type of AIS system do you have? Might it be integrated with an Internet or satellite provider?

But, to answer the questioners question, without thinking too hard about the encoding differences and repetition w/AIS, VHF/DSC radio transmission is 25W vs 12.5W for Class A AIS or 1.5-2W for class B AIS. So, I'd expect to be able to call VHF/DSC to AIS contacts. My intuition is that the difference in transmission power woukd cover any potential efficiency differences.

In practice, I've always been able to call any AIS contact of interest to me. Of course, I've only done so to aid in eventual navigational interactions, not to reach out as far as I can in an emergency. In qn emergency, I hope anyone would wake up for a mayday.

 

[rslifkin]

In some places, there are AIS repeaters. So AIS distance doesn't necessarily equate to usable VHF voice distance.

 

[Hippocampus]

Very good point rs. The issue of repeaters and stations is very important with AIS in my experience. Also think some stations take a weak signal and amplify it. Within your receiving and transmitting distance to the repeating stations it’s a whole different world than when out side it. We tried to assess our distances but initially without avail. Found true assessment could be best judged when outside the range of the stations.
It’s common for cruiser voice nets on VHF to use repeaters as well. Think even for vhf if your comparing different variations in your set up it’s helpful to be far enough out in open ocean where inverse square and earth curvature are the fixed variables and repeaters or shore amplification doesn’t exist. Also need to take into account if it’s a A or B or some idea of signal strength and antenna set up of the other vessel for VHF. We all have heard the CG land station VHF at distances far beyond our usual coverage distances.