VHF mystery

[ka4wja]

jmaja,
If you read the links I posted above, I think you'll find the answers you seek...
http://www.cruisersforum.com/forums/...ge-149499.html


I don't know your antenna heights, nor exactly where and how tall any land mass / hills are, between your two locations, so I have no idea what your exact path loss is...
But, if you read the above link, you'll be able to estimate / calculate it within a few db...

Originally Posted by jmaja

I just tested to send from the boat to the handheld the same 3 nm distance with different squelch levels.
So there seems to be quite a lot of marging. Any idea what that level might be in dbm? For a free space the path loss would be 91.5 db, but it's not a free space. There are at leas plenty of trees maybe even the small hill in between blocks the line of sight.

Need to test a much longer distance at sea, but it is not that easy to find someone to test with.

But, my guess is that at 3nm, even with some land obstructions, you're likely to have path loss of only slightly more than the direct-wave free-space path loss....

And, FYI, as you'll see in that above link....the "margins" are quite large with typical VHF Marine direct-wave (line-of-sight") comms....
Sailboat-to-sailboat VHF comms, even at 20nm line-of-sight, the margins are typically 40 - 50db above threshold!!

With our 25 watt transmitters and fairly sensitive receivers (typically -117dbm for 12db SINAD, -110 to -113dbm for 20db SINAD, which is a fairly "noise-free" signal....and -107 to -109dbm for full noise-free signals and low BER for data), our line-of-sight ranges are not only 100% reliable but also incorporate significant margins...(typically 50db above the "20db SINAD" spec, at 15nm - 20nm range, with two vessels w/ antennas at 65' above the water...)

Regarding reliability and margins above receiver thresholds....
FYI, the Free Space Path Loss at 156.8mhz being 107.6db at 20nm (22.8 statute miles / 36.5km), which is the approx. line-of-sight range between two vessels with VHF antennas at 65' above the water....

With a transmit power of 25 watts (+44dbm), minus the coaxial feedline loss (3-4db typical), plus the antenna gain (3-4dbi typical), each of yours' signal at the other vessel should be very strong, at approx. -63.6dbm.....which is about 50db above the typical marine VHF's receiver sensitivity for "20-db of quieting" (20db SINAD) and approx. 55db above the typical marine VHF's 12db SINAD ("12db of quieting") spec....(and 44db above the typical AIS unit's receive spec for low Bit-Error-Rate BER, but remember a Class A AIS is only 12.5watts / +41dbm....and Class B AIS is only 2watts / +33dbm...so AIS has a bit less margin than the 25-watt Marine-VHF-FM-Voice or Marine-VHF-DSC...)

So, as you see, there is plenty of margin built into the design of the Marine VHF system....which is good, 'cause you need to account for lots of factors other than mother nature, such as vessel heel angle (and subsequent negative gain of the antenna), antenna whipping around from both wind and sea-motion, lossy cable and connectors, poor radio operators that don't speak clearly / loudly enough (as well as those who shout and over-deviate), poor receiver, extraneous noise / RFI on-board, etc. etc....

I hope this helps...

John

[jmaja]

Thanks John,

I have already read your excellent post about the range and I had done some range calculations several years ago. I was just wondering, if squelch can be used as a "signal meter", but I didn't find any db levels for it. Neither radio has a real signal meter.

So 20 nm should work even with 1 mW power between two sailboat mast top antennas (17 m height, like mine) and very well with 1 W. Is this the case in real life? If there is so much margin, why do people care about a few db in the cables etc.?

The calculations I did a few years ago were based on this http://arundale.com/docs/ais/AppNote_UHF_VHF_Calc.pdf
It gives much higher (wrong?) path loss. About 145 db for two antennas at 17 m height and 20 nm distance. This formula for path loss has antenna height in it and also a clear difference is that the multiplier for distance (log10(D)) is 40 instead of 20 used here continuousWave: Whaler: Reference: Marine VHF Radio Communications

So it seems the latter has the correct formula for PL for a line of sight case. Is there a formula also for the beyond line of sight case? I guess that must include antenna height?

Joakim

[ka4wja]

jmaja,
Well, don't want to get too deep in the woods here....

1) But, FM has a capture ratio, and a C/N threshold, which do AM does not, FM receivers use a discriminator and limiter, rather than a product detector, etc...and when using squelch in a VHF FM circuit, you are simply shutting off the receiver output until signals reach a certain C/N....
SO...
So, technically there would be a way to very roughly use the squelch settings when various signals are received, and derive some rough idea of signal strength from that....
BUT...
But, there is no way I can correlate that to even an approx. dbm number...
SO...
So, I doubt you'll find this exercise to be of much use....


2) As for beyond line-of-sight....that's "troposcatter", and yes there are formulas....but don't have 'em at my fingertips at the moment (200 miles away)....so, I'll need to do it from memory...

Originally Posted by jmaja

. Is there a formula also for the beyond line of sight case? I guess that must include antenna height?

Using your example of the path loss of a 3nm, NON-line-of-sight (obstructed) path....well, again not knowing where the obstructions are, nor there height, density, composition, etc....it's impossible for me to give you anything but an approximation...

{FYI, even if I had all that data from you, my text books are 200 miles away....and there's damn little vhf troposcatter path info on-line....so, I'm going off my memory!!!!}

But, if that's what you want:

In round numbers...
--- Figure about 140db - 145db path loss....
And, yes that is do-able with our 25-watt transmitters, 3dbi gain antennas, at masthead heights....

--- Transmitter:
+44dbm transmit power, -3db feedline loss, +3dbi antenna gain = +44dbm from antenna

--- Receiver:
-117dbm for 12db SINAD, -3dbi feedline loss, +3dbi antenna gain = -117dbm SINAD threshold at antenna

--- Link budget= +44dbm - 144db = -100dbm...
This should allow clear, noise free comms (> 20db SINAD), with about 10db margin...



3) And, as for using 1mW (0dbm), for comms out 20nm....yes, technically it would work...assuming direct line-of-sight, no obstructions, no waves rocking the boats, no extra feedline / connector loss, etc. etc. etc...
But, there are reasons that there is this much margin built-in to the VHF Marine radio system!!!!
Not the least of which is that it must be reliable and it was designed to work beyond line-of-sight for most users...


Hope this help...'cause I'm about out-of-steam today!

John

[jmaja]

OK, so squelch is pretty useless for this test, but I guess it still shows that 1 W was nowhere near the limit, which doesn't really tell much since that sould be the case at 20 nm distance also.

The path loss formula in continuousWave: Whaler: Reference: Marine VHF Radio Communications assumes no loss of signal while travelling through air. 20log10(D) comes just from increasing area of the sphere around the antenna. Is that a good assumption? With that assumption one should get 5 times the range with 25 W compared to 1 W, if both are inside LOS.

The other formula has 40log10(D) and thus assumes much higher loss in the air. 1->25 W inside LOS should give only 2.2 times the range.

[continuouswave]

Originally Posted by jmaja

OK, so squelch is pretty useless for this test, but I guess it still shows that 1 W was nowhere near the limit, which doesn't really tell much since that sould be the case at 20 nm distance also.

The path loss formula in continuousWave: Whaler: Reference: Marine VHF Radio Communications assumes no loss of signal while travelling through air. 20log10(D) comes just from increasing area of the sphere around the antenna. Is that a good assumption? With that assumption one should get 5 times the range with 25 W compared to 1 W, if both are inside LOS.

The other formula has 40log10(D) and thus assumes much higher loss in the air. 1->25 W inside LOS should give only 2.2 times the range.

For more estimates of path loss not using the simple 20log(d) free space figure, see

Estimating Path Loss on Marine Non-Line-of-Sight Paths
continuousWave: Whaler: Reference: Estimating Path Loss on Marine Non-Line-of-Sight Paths

Even when the path is a line-of-sight path, the free space conditions to not really apply because in real-world paths the signal from the transmit antenna will reach the receiver antenna by more than one path. Typically there is a direct path, and then there are slightly longer paths that include reflected signals, usually from the ground (or the sea). The existence of more than one path, and the interference of signals arriving via many paths of slightly difference length, will tend to make the path loss higher than would occur in free space, where there is nothing at all between the receive antenna and transmit antenna except empty space and there are no reflected signals.

I have not found any really great studies of VHF propagation over sea, particularly over saltwater, but there are plenty of studies over land. Most of these studies find propagation paths have much higher attenuation than the free space formulas calculate.

A good study done with actual observations, not with just calculations, assumptions, and models, is the EGLI'S MODEL, which was developed back in the 1950's. I discuss this in an article at

Radio Propagation Over Water - Moderated Discussion Areas

The first article I link to above also gives some observations of reception of NOAA signal.

As for using the setting of the SQUELCH to provide an indicator of received signal strength, there should be some correlation. SQUELCH circuits operate in various was, either based on signal strength directly or based on recovered modulation signal-to-noise, called noise-operated squelch.

If the squelch circuit is operated directly by signal strength, then when the squelch opens marks some sort of threshold of signal strength.

If the squelch circuit operates on the basis of signal-to-noise of the recovered audio, then this also correlates with signal strength, because the signal-to-noise of the recovered audio will increase with increasing signal strength.

Thus, we can say with some certainty, that when a receiver has its squelch threshold set for the highest level, the signal strength necessary to open the squelch will be greater. That means squelch setting can be used as a rough metric of signal strength.

In your particular receiver it appears the squelch is set by software, so we assume it is quite repeatable. If the squelch were just set by a knob, it may be harder to gauge signal strength from knob position.

[continuouswave]

Further on estimating path loss, I want to point out an important finding in EGLI'S model: the increase in antenna height above ground has a direct and very favorable effect on path loss. According to EGLI's model, there is decrease in path loss with antenna height at a rate of 6dB per octave, that is, doubling the height improves the received signal by a factor of four.

This correlates very well with real-world observations of VHF marine band radio installations: increasing the antenna height is the most effective way to improve range.

The reason antenna height helps so much is that it reduces the effect of the most common reflected path, the slightly longer one-bounce off the ground (or sea) path between antennas. An antenna site with higher ground clearance really helps improve coverage, not just because of improved distance to the radio horizon, but by reduction of that indirect path signal affecting the main signal.

P.S. I am also a licensed radio amateur, more than 50-years. Callsign is K8SS. I have also worked professionally as a Broadcast Engineer for over 40-years. And been a boater for about 60-years.

[jmaja]

Egli model seems to be the one I used a few years ago: http://arundale.com/docs/ais/AppNote_UHF_VHF_Calc.pdf

According to it the path loss between two antennas at 17 m height is 145 db for 20 nm distance. My radio should have 20 db SINAD at 113 dbm (Manual: "< 0.5μV emf for 20 dB SINAD"). So with -3 db cables and +3 db antennas it should be quite close to 20 db SINAD at 1 W power and well above at 25 W. At 25 W 20 db SINAD should be at 39 nm according to that formula.

How well that supports reality at sea?

[jmaja]

My friend bought a new VHF and now everything works fine again. So it really was a broken receiver in the old one (8 years old Raymarine, 55?). Everything else worked OK except reception. Setting squelch low he could here the noise, but no real signal. How often do VHF fail like that? What is needed to break the receiver? Could a bad VHF splitter do that by letting the B class AIS transmission into the VHF? Or a lightning? Or is it just a bad unit?

[jmaja]

Finally I was able to do some real tests yesterday. I could send and receive loud and clear to 32.7 nm at 25 W and at 1 W nothing could be heard (both ways). Squelch needed to be almost off, just above the noice level. I was able to receive a DSC call, but my friend did not here my DSC call. Later he was at about 29 nm and could here my DSC call as well. His antenna is at 18.5 m and there were no or only few very low and small islands between us. So I'm happy with the performance of my VHF!

[chuckr]

I should not respond to this but will

First listen to John - KA4WJA -- he has forgotten more about radios and stuff like that than most on this board know. and he is good at it.

But second why the splitter?? who really cares if you can see and AIS target 70nm away - wow impressive. We are simple old folks who believe a radio antenna should be dedicated to the radio and the AIS should have it's own antenna. We put our AIS antenna on the top of our bimini between our solar panels and we can see a long way off - enough to plan for as the most we run our chartplotter is 24nm so 12nm fore and aft and find that is more than enough to make sure we are safe

just our thoughts for what they are worth.

[jmaja]

I don't have any AIS splitter I have a separate antenna for it below deck. Works well enough, I see ships 5-30 nm and pleasure boats 1-5 nm which is enough.

My friend has a splitter, but his AIS is B class thus sending as well. Then it makes sense to have something better than I have as an AIS antenna.