The VHF radio horizon calculation is wrong

[hzcruiser]

Originally Posted by Jammer

Marine VHF occupies a narrow band segment, but amateur radio operators would have access to 50 MHz/6 meters and 28 MHz/10 meters, and many boaters can and do use CB radio operating at 27 MHz. There are also Marine HF allocations at 25 MHz that effectively operate as somewhat longer-range VHF frequencies except during band openings at the peak of the sunspot cycle.

As briefly mentioned above, VHF is using FM whereas 27 Meg CB is AM. Not easy to compare the two in terms of usable range.

Also, keep in mind that on FM the squelch is your friend or foe; if the far station has theirs a bit too high you won't get through. My squelch is usually higher when sailing coastal to reduce the constant chatter on 16. One needs to remember to turn it down when off shore.



Those rules of thumb are just that, and one shouldn't rely on his setup to achieve much greater distances when he really need them, as in an emergency.

[er9]

slightly off topic but may i ask why they dont gimbal VHF antennas so the line of sight remains more horrizontal? id gladly give up my windex to allow my VHF to gimbal.

[rslifkin]

Originally Posted by er9

slightly off topic but may i ask why they dont gimbal VHF antennas so the line of sight remains more horrizontal? id gladly give up my windex to allow my VHF to gimbal.

With lower gain antennas (such as a 3db), the radiation pattern has enough vertical beamwidth that it's not really an issue. You'd have to be heeled quite far before there's significant signal degradation. Now, if you want to run a big, tall, high gain antenna (which typically wouldn't be on the masthead), then the vertical beamwidth gets narrower, so you'd have far more need to keep the antenna upright.

[er9]

Originally Posted by rslifkin

With lower gain antennas (such as a 3db), the radiation pattern has enough vertical beamwidth that it's not really an issue. You'd have to be heeled quite far before there's significant signal degradation. Now, if you want to run a big, tall, high gain antenna (which typically wouldn't be on the masthead), then the vertical beamwidth gets narrower, so you'd have far more need to keep the antenna upright.

ah right thats true. those 6db+ antenna's are not top of mast mount.

[Bill_Giles]

While it is pretty clear from both the discussion and my experience that VHF range varies and that generally the commonly used 1.4x sqr root etc can easily be exceeded. Also “line of sight” is often incorrect as well and the radio wave can “bend” around hills etc. But if we take the 1.4 etc as a general limit I feel we won’t be disappointed. If one can get more than that in ideal conditions so what? We don’t always work in ideal conditions.

Also AIS which also works in the VHF range seems to give much more range. But I wonder if the technology is such that the sets act as repeaters.

[betwys1]

Originally Posted by Jammer

/snip/

I did some searching to try to better understand what happens, the theory behind it, and the actual factors that go into useful range.


I offer this article, which summarizes three models (that do not agree with each other): continuousWave: Whaler: Reference: VHF Radio Propagation Over Water


There is some discussion of the article here: Effect of Antenna Height on Signal Strength - CONTINUOUSWAVE

/snip/
.

Sad to see a respectable survey which takes in references to statute miles, feet, power referred to 1 milliwatt = dBm, power referred to 1 microvolt in 75 ohms = 1 dBu (that is a reference of 0.133 picowatt), kilometers, meters.
I have not exhaustively checked but you can imagine how unsurprised I would be to find unit conversion errors in one or more of those respectable sources.

[GrowleyMonster]

VHF is limited to line of sight when it is limited to line of sight. It is limited to a lot less than that if you have a crappy lossy install. It doesn't really have a hard and fast limit with a good install and good conditions for tropospheric ducting. It isn't limited strictly to line of sight with a good install including good antlers and reasonable power levels, due to refraction.

Like they said, rule of thumb. But at full power if you cannot communicate with another station when your antlers are in line of sight of each other, you know something is probably wrong with your antenna, feedline, radio, ground, or most likely, connections. Or the other guy's station. So it is a good idea to be aware of just what your horizon distance is from the antenna. And if you and another station are not in line of sight and you can't communicate, be patient and wait until you are closer rather than tying up the channel uselessly and draining your batteries for no good reason.

And yeah don't forget about your ground. A good RF ground is essential for making those "impossible" ex line of sight contacts on VHF.

[Jammer]

Originally Posted by GrowleyMonster

And yeah don't forget about your ground. A good RF ground is essential for making those "impossible" ex line of sight contacts on VHF.

Depends on the antenna.


Most of the fiberglass antennas don't require (or benefit from) a ground. They're either sleeve dipoles or phased co-linear designs.

[GrowleyMonster]

Originally Posted by Jammer

Depends on the antenna.


Most of the fiberglass antennas don't require (or benefit from) a ground. They're either sleeve dipoles or phased co-linear designs.

Arrrrr. you got me. You are right, at least mostly right on that point.

[st0nes]

I've heard Portishead radio (UK) loud and clear on VHF from Cape Town, 6,000NM away! The atmosphere can do weird things. they couldn't hear me, though.

[RUSTYNAIL]

Originally Posted by Brian.D

Yeah, it is a rule of thumb. A Catalina 36 was headed to Hawaii when it lost its rudder. They called for help on VHF and the San Diego Coast Guard heard them and dispatched a helicopter. The Catalina was over 200 NM away from San Diego when the call was placed. So like a lot of 'formulas', there is always X, an unknown factor that is involved.

For the new users trying to figure out deck mount or mast mount, use the formula and maybe the X factor will be in your favor.

Brian, That sounds like an incredible distance for a vhf radio. I remember repairing a VHF radio for Station Yerba Buena Island near San Francisco when I was in the Coast Guard many moons ago. The actual VHF radio and antenna was remotely mounted with a telephone line connection on the top of a high mountain called Mt. Diablo. Maybe the same situation in San Diego.

[st0nes]

Originally Posted by Brian.D

You want 'line of sight' communications? Talk to the moon on VHF. 270,000 miles, 540,000 miles round trip.

I've done that. It doesn't reply.

[Brian.D]

Originally Posted by st0nes

I've done that. It doesn't reply.

LOL. Hate it when that happens. LOL

[clarkj]

I fed in the parameters of this discussion thread into the propagation program Splat! and did a Longley-Rice analysis using the following between two boats on Lake Ontario with 10dBi antennas 20m above the water:


Pt = 25W = 25000mW = +44dBm
Freq = Marine Mid Band = 156 - 162MHz = 159MHz
Gt = Gr = 10dBi @ avg height = 20m
Lt = Lr = 25m RG213/U = 2.7dB/100ft(30.5m) @ 200MHz = 2.2dB/25m
EIRP=44+10-2.2=51.8dBm
ERP=51.8dBm-2.2dB=49.6dBm=91W
Prx=-13dBu for 12dB SINAD=-120dBm
Prx at Antenna=-120dBm-10+2.2=-127.8dBm


This resulted in a range of 55Nmiles for -130dBm receive level.
Blog Post:
https://jeremyclark.ca/wp/telecom/sp...f-propagation/

[GrowleyMonster]

Originally Posted by clarkj

I fed in the parameters of this discussion thread into the propagation program Splat! and did a Longley-Rice analysis using the following between two boats on Lake Ontario with 10dBi antennas 20m above the water:


Pt = 25W = 25000mW = +44dBm
Freq = Marine Mid Band = 156 - 162MHz = 159MHz
Gt = Gr = 10dBi @ avg height = 20m
Lt = Lr = 25m RG213/U = 2.7dB/100ft(30.5m) @ 200MHz = 2.2dB/25m
EIRP=44+10-2.2=51.8dBm
ERP=51.8dBm-2.2dB=49.6dBm=91W
Prx=-13dBu for 12dB SINAD=-120dBm
Prx at Antenna=-120dBm-10+2.2=-127.8dBm


This resulted in a range of 55Nmiles for -130dBm receive level.
Blog Post:
https://jeremyclark.ca/wp/telecom/sp...f-propagation/

That sounda about right for two boats with good VHF installations at full power and normal conditions. With shorter masts In real world conditions and typical installations, one learns to expect and get used to about half that.