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Old 17-06-2003, 05:15   #1
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VHF RADIO RANGE

I'd appreciate & welcome any comment/criticism of the following first draft article. You could post your comments here, email or PM to me, or merely rate the thread.
Thanks in advance.

BASIC VHF RADIO-WAVE PROPAGATION ©
by
Gord May
Copyright 2003 by Gordon A. May - All Rights Reserved

LINE of SITE:

VHF radio communication (and UHF) is by “line-of-site” propagation - that is, the corresponding radio antennaes must be able to “see” one another. The higher up that one or both antennaes are mounted, the greater the communications range (distance) they will enjoy. As the radio antenna is elevated, the distance to the radio horizon becomes greater and greater.

The distance to the visible horizon can be calculated (visible dip):

D = 0.97 x √Hf -or- D = 1.76 x √Hm
Where:
D = Distance in Nautical Miles
√Hf = Square root of Height of eye in Feet
√Hm = Square root of Height of Eye in meters.

Because a radio signal is slightly refracted, the Radio horizon (range - or radio dip) is calculated:

D = 1.22 x √Hf -or- D = 2.21 x √m

Note: The above radio range calculation also applies to Radar, which explains why Radome mounting height is more important than Power (Watts) in determining maximum Radar range.

To calculate radio range, we add the two (D1 transmit & D2 receive) antennae ranges together, thus:

Range = D1 + D2
or
Range = (1.22 x √Hf Transmit) + (1.22 x √Hf Receive)

Example 1:
A handheld VHF radio with antenna height 6 Ft ,transmitting to with a receiving base station with a mast-top antenna mounted at 45 Ft.
Range = (1.22 x √6) + (1.22 x √45) = (1.22 x 2.449) + (1.22 x 6.708) = 1.768 + 8.184
Range = 9.952 Nautical Miles

Example 2:
Lets raise the transmitting antenna from 6 Ft to 45 Feet (two mast-top installations communicating).
Range = (1.22 x √45) + (1.22 x √45) = 8.184 + 8.184 = 16.368 NM
Quite an improvement!


TROPOSPHERIC DUCTING:

Tropospheric Ducting is the phenomenon responsible for those happy occasions when VHF radios communicate over hundreds (or even a thousand) miles. The “Tropo” effect can occur at any time in any season, but late spring and early fall sunrise and sunsets are the most likely times.

Tropo’ Ducting occurs when humidity and temperature combine to create a wall of ionized particles in the troposphere, typically during a wide-spread temperature inversion. At sunrise, the air aloft is warmed more rapidly than the air below. As the sun begins to set, later in the day, the upper air is kept warm, while the ground cools. This creates a duct, or channel, between the ionized layer. Because sunspot activity increases ionization, certain times within the 11 year sunspot cycle are more likely to produce ducting. The current cycle is increasing through 2004, then will subside until 2009, when the cycle begins increasing again. The next 2 - 3 years should produce some interesting opportunities for VHF’ers to communicate over long distances.

The radio signal is bounced back & forth between these ionized layers, until they find a path back to earth. The result is that your signal may seem stronger 300 - 500 miles away, than it does a mere 15 miles away. As long as both stations are within the duct zone, extremely long range may be experience. The phenomenon is more likely at frequencies above 144 MHz (VHF is 156-157MHz), and most likely between 432MHz & 1296MHz.

So, watch the weather for temperature inversions - you may get the opportunity to do some “Ducting” in the next few years.

There is an on-line “Tropospheric Ducting Forecast” available at:
<http://www.iprimus.ca/~!hepburn/tropo/_nwe/html>

Happy ducting!

E. & O. E.

Gord May
Gord@BoatPro.zzn.com

“Basic VHF Radio Wave Propagation” First Draft Copy - June 17/03
Copyright 2003 by Gordon A. May - All Rights Reserved
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Old 21-02-2007, 09:56   #2
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This is an excellent little thesis Gord with very valuable information.
In fact I've saved it to have it to hand.
Is it OK if it goes on my website one day with due credit of course?
How come nobody else noticed in over three years?
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Old 21-02-2007, 10:38   #3
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Pretty good article... Obviously it is written for the HAM/Cruiser. Here are a couple of things you may want to take a look at.

"Note: The above radio range calculation also applies to Radar, which explains why Radome mounting height is more important than Power (Watts) in determining maximum Radar range."

Do not forget that Radars have a BEAM WIDTH of around 1.0 to 2.0 degrees. The range is affected by the height as well as the beamwidth. Mounting a radar as high as possible is not always the best thing to do as it could negate the close in range...

"The phenomenon is more likely at frequencies above 144 MHz (VHF is 156-157MHz), and most likely between 432MHz & 1296MHz."

MARINE VHF is between 156-157MHz. VHF in general is 30MHZ to 300MHz.

Very high frequency - Wikipedia, the free encyclopedia

The general services in the VHF band are:
  • 30–46 MHz: Licensed 2-way land mobile communication
  • 30–88 MHz: Military VHF-FM, including SINCGARS
  • 43–50 MHz: Cordless telephones, "49 MHz" FM walkie-talkies, and mixed 2-way mobile communication
  • 50–54 MHz: Amateur radio 6 meter band
  • 54–72 MHz: TV channels 2-4
  • 72–76 MHz: Remote Control devices
  • 76–82 MHz: TV channel 5
  • 82–88 MHz: TV channel 6
  • 88–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial)
  • 108–118 MHz: Air navigation beacons VOR
  • 118–132 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency
  • 132–144 MHz: Auxiliary civil services, satellite, space research, and other miscellaneous services
  • 144–148 MHz: Amateur band 2 Meters
  • 148–156 MHz: "VHF Business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
  • 156–174 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency. The 160 and 161 areas are AAR 99 channel railroad radios issued to the railroad (Sample, AAR 21 is 160.425 and that is issued to TVRM and other railroads that want AAR 21)
  • 162.40–162.55: NOAA Weather Stations, narrowband FM
  • 174–216 MHz: TV channels 7 through 13, and professional wireless microphones (low power, certain exact frequencies only)
  • 216–222 MHz: reserved for future use
  • 222–225 MHz: Amateur "1&#188; Meter" band (really closer to 1.33M)
  • above 225 MHz: Federal services, notably military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104
Also keep in mind that range is affected by more then just height. You need to take into account the power output, antenna gain, receiver sensitivity. Mounting the antenna higher without improvements to the coax will result in less power getting to the antenna thus less power being radiated. (less range)

Line of Sight Formula

VHF transmission range is a function of transmitter power, receiver sensitivity, and distance to the horizon, since VHF signals propagate under normal conditions as a line-of-sight phenomenon.
An approximation to calculate the line-of-sight horizon distance is:
  • distance in miles = where Af is the height of the antenna in feet
  • distance in kilometres = where Am is the height of the antenna in metres
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Old 21-02-2007, 11:54   #4
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Good post Gord,

Here is what I saw;

The paragraph about sunspot activity says;

Quote:
The current cycle is increasing through 2004
Might change that to something like;

Quote:
The current cycle reached it's peak in 2004, and will decrease through 2009
Just a thought. Good material for anyone to read, especally if they plan on using that handheld to call for help when they are in the water.
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Old 21-02-2007, 14:47   #5
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Good ol' Tropospheric Ducting!

Back in the '90's in Mexico after a session on the Manana Net (14.250 mhz), I was contacted by a HAM friend of mine via the HAM radio. He said the TD was running quite heavy and asked me if I'd give him a call via the VHF. (He was about 470 miles northwest of me in Baja, and I was way down the Mexican coast.)

We made an easy contact! He then asked me to try it on low power (1 watt). When I did,he said "Go back to high power", so I did. He said he could hear me, but he couldn't quite understand my words, but he could recognize my voice. We chatted a few more minutes with great reception.
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Old 21-02-2007, 20:54   #6
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Must be something in the air in Mexico - I had a very clear VHF (ch 16) conversation with a USN warship that was in the ranges off San Diego, well over 300 miles away. Another lucky fluke that happens at sea.

Kevin
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Old 22-02-2007, 03:11   #7
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Maby so, but I have had my hand held calling a bridge, within 100 feet and did not get a response. It all depends what the receive does and how much traffic there is.
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Old 22-02-2007, 08:11   #8
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Did north of Lizard Island to the $hitsunday's once, quite a few miles 3-400 maybe, clear as a bell, yet couldnt get Cairns or Cooktown 50-100miles south.

Dave
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Old 22-02-2007, 11:09   #9
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I have talked to someone in Australia on ch 16. I was on the East coast of NZ and I don't know exactly where in Oz the guy was. But it was so clear, it was like he was only 100m away from me.
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Old 22-02-2007, 11:33   #10
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Guys....

Lets make sure that people who are reading this understand that Ducting is the EXCEPTION rather then the RULE. I would not want someone new to boating or communications get the impression that they will be able to talk to the Coast Guard or another boat that is 100's of miles away.

The "typical" range for a fixed mount VHF is roughly 25 - 30 miles line of site. For a hand held VHF using the existing rubber duck is roughly 4 - 10 miles line of site.

Everything discussed above must be factored in but those are the general guidlines...
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Old 22-02-2007, 12:01   #11
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James,

Absolutely right! Ducting is the EXCEPTION to the rule: VHF communications are basically LINE OF SIGHT, plus about 20% due to bending and refraction. If you've got a tall mast and the other station is on a mountain, or has a very tall mast, or is an SAR airplane operating on the marine VHF bands, then you can communicate over pretty good distances.

Still, it's great to remember the ducting episodes. When I was operating in Morocco as CN8CW, I had a great location with a great antenna system at 65' height, near to the ocean. With some regularity, I was able to communicate on 2 meters (146mHz) with stations in Spain and Portugal....many miles away. Seems from that location ducting often took place along the seaside in a north-south direction. Super fun!

Bill
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Old 22-02-2007, 17:07   #12
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James,

I don't believe any of us need to have it explained that ducting is atypical - Gord's original post was clear in that regard, and the other experiences listed seem to corroborate the rarity of ducting events.

Kevin
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Old 22-02-2007, 19:54   #13
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If I'd recorded my conversation that day, I guess it would be a duct tape.


(groan)

Steve B.
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Old 24-02-2007, 00:43   #14
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Gord,

Ii don't know whether anyone else has picked up on it, but you mentiontioned "ionized particles in the troposphere". Ducting is usually the result of variations in the refractive index of the air in the troposhere (much lower than the ionosphere) causing radio signals to 'bend' in such a way that they are trapped between two layers. There are some ionospheric effects, one is "Sporadic-E" reflection from the E-layer, which can affect 156 MHz, but that's a different phenomenon.

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Old 24-02-2007, 02:53   #15
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Quote:
The "typical" range for a fixed mount VHF is roughly 25 - 30 miles line of site. For a hand held VHF using the existing rubber duck is roughly 4 - 10 miles line of site.
And to add to that, it also depends on the Tx power being set to 25W output for 25-30miles. Low power will be similar to the H/H distances of 4miles.
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