posted 12-29-2001 09:49 PM ET (US)     

My understanding is that VHF radios are suppose to have a "line-of-sight" range. I presume that this means a maximum of 5–10 miles on open water. Right?

Factors that affect range include the height and quality of the antenna, the number of watts used during transmission (1, 5, or 25 watts), obstacles on land, and weather conditions. The brand of the radio may also be an important factor. Is that correct?

My two 5-watt Shakespeare SE700 handheld radios have a transmission range of three miles (one handheld radio communicating to another, identical handheld radio). That's what I have experienced.

If I purchase a 25-watt, dash-mounted VHF radio and an 8' antenna, will I be able to transmit two or three times as far?

posted 12-30-2001 12:25 AM ET (US)            

The formula is approximately

Range (Miles) = 1.5 x SQRT_of_HEIGHT(Feet)

Compute the range between two stations by calculating their individual ranges and adding them.

For example, I am in my boat with antenna about 9 feet high. I want to listen to weather radio station KEC-63 with antenna 625 feet high.

My range is about 4.5 miles
KEC-63 range is about 37.5 miles

I can hear KEC-63 from 42 miles away.

posted 12-30-2001 12:29 AM ET (US)            

I have a 25 watt console mounted radio
(Standard Spectrum), 6 dB gain 8' antenna,
on my Montauk.

Line of sight is kinda-sorta. I routinely
talk from Monterey Bay to Carmel Bay. It's
about five miles, with a 600' high granite
ridge (Monterey Penninsula) in between. I
also routinely hear traffic from Santa Cruz
(20 nm of open water) but I haven't tried
talking to them because I haven't had a
reason to (I'm diver, they're fishermen,
no diving up there).

Antennas are important. Your handhelds have
little 0 dB gain antennas. Before you
replace the radio, the first thing I would
do is to install a 6 dB gain (usually 8')
antenna and the adapters required to attach
it to your handheld.

I don't think there's a lot of difference
between brands, but there may be some
difference between models. A good
(expensive) Standard may beat a less
sophistocated Icom, or vice-versa.

If you do get a new radio, get one that is
rated "submersible", not "waterproof".
Subermsible means 3', 30 minutes. Waterproof
means kinda sorta splashproof. The console
of a whaler is a very wet place. I've wiped
out a "waterproof" Apelco 5200 on the console
of my Montauk. Diagnosis: "Severe saltwater
intrusion". Apelco (Raytheon) replaced it
on warantee, but the replacement is in my
garage, not on my boat.

And, most submersible radios are DSC (Digital
Selective Calling). Wire them to your GPS.
One button push will send a digital Mayday
and your position. The Coast Guard is only
partly on board with this. Some units
are capable of handing it, the rest are
waiting for a major rework of the CG's VHF
network in a couple of years, but there are
enough DSC radios around that the odds are
real good someone will receive the it.

posted 12-30-2001 11:07 AM ET (US)         

I used an adaptor to connect my new submersible, handheld 5-watt Shakespeare SE700 to an 8' antenna that was mounted on the flybridge of a small yacht. The top of the antenna was 22' above the water line. We put a man in an 11' Whaler and had him head for a channel marker that was 3 miles away. Neither party noticed a significant difference in the quality of reception with the external antenna connected or not connected.

However, we hooked the same external antenna to the yacht's dash-mounted, 25-watt ICOM VHF radio. The man in the dinghy 3 miles away reported that he could hear us much more clearly when the 25-watt radio was used. On the yacht, we heard the dinghy's handheld equally well regardless of which radio we used.

So, from that experiment, i concluded that power was an important factor in determining the distance that a unit can transmit.

posted 12-30-2001 12:44 PM ET (US)            

VHF is FM. As long as there's "enough" power,
FM quality isn't influenced by having more
than "enough". It will either sound as good
as it will get, or sound bad (breaking up).
It won't sound great vs. good, based on
signal strength.

The higher gain antenna effectively gives
your more watts by focusing the energy in
the interesting direction. The 8' antenna
most probably had a 6 dB gain. 3 dB doubles
the effective power, the 6 dB effectively
quadrupled it to 20 watts on your handheld
and 100 watts with the console mount.

Finally, the SE700 is not submersible. It's
only rated JIS-4. Submersible is JIS-7. Yes,
they will replace it if it dies cuz it got
wet, but that doesn't mean it's submersible.

posted 12-30-2001 01:59 PM ET (US)            

Since marine channels are shared among many stations, there is often considerable on-channel interference. More power helps to override such signals.

The Frequency Modulation (FM) technique of tranmission enhances noise-suppresion and rejection of unwanted signals, as long as the desired signal is sufficiently stronger than the noise or interfering signal.

A drawback to the FM techniques is that the reability of the signal when its strength is marginal (compared to noise or interference) is much lower than it would be with Amplitude Modulation (AM) methods.

posted 01-01-2002 09:19 PM ET (US)            

Yes, I've heard a rattle inside the antenna
with my old cheap ($35) Shake. It doesn't
necessarily indicate a problem. The
fiberglass tube isn't part of the antenna,
it's just tube that supports the antenna
wire and sometimes some other components. My
current Shake Galaxy doesn't rattle.

Chuck

Chuck

posted 01-01-2002 10:15 PM ET (US)            

Think of your transmitter as a light bulb and your receiver as an eyeball. Antennae are reflectors and/or telescopes.

In theory, if you don't have line-of sight to the receiver, it doesn't matter what power or antenna you have.

If you do have line of sight, and efficient enough reflectors and telescopes, it doesn't matter how far it is.

In more practical terms, how high your antenna is, the greater your range. The more power, the easier it is to communicate. Think of 25 miles, max, with your antenna 15' above the surface.

posted 01-01-2002 11:56 PM ET (US)            

Except that: VHF is only kinda line-of-sight.
It will wrap, just a little. Witness my
Monterey Bay to Carmel Bay experience. But
only just a little.

Other frequency ranges are more or less
sensitive to line of site. The gigahertz
ranges used by GPS are sensitive in spades.
Low frequencies almost not at all.

posted 01-04-2002 10:40 PM ET (US)            

Where my house in WI is, I am at the top of a hill that seems to be about 300 feet above and .5 miles from the bay in Ephraim WI (those that were at my house for the Door County Rendezvous party this summer know what I am describing). This house is also about 5 miles to the south of the water at Sister Bay.

On the top of my roof, I mounted one of those metal Shakespere whips that rag boaters stick on the top of their lightning rods...er...masts - about 3 feet. It hooks to a VHF I have in my garage ( for any FCC people out there, I am not operating a land based transmitting station without a liscense. This is just a hypothetical essay, and I would never intentionally violate any rules or laws).

Now, I am unable to contact or receive transmissions from Ephraim harbor, but I am able to do both to vessles and land stations in Sister Bay, or points farther. Would I be right in assuming that I may be shooting my transmission over the top of vessles that are nearer to me because of the steeper angle down to them over a shorter distance? Or is there some other force at work here?

posted 01-04-2002 11:13 PM ET (US)            

It is also possible to have some rather deep nulls in the antenna pattern at higher angles away from the horizon. Your antenna could have such a null exactly at the angle needed to receive signals from the harbor. This might be mirrored by similar nulls in the antennas of boats in the harbor, doubling the effect.

In television broadcast antennas, which are typically mounted 1000 feet above terrain, special attention must be paid to providing some radiation at sharp downward angles in order that receivers in the vicinity of the antenna (0-3 miles) will be able to get decent reception.

In some settings, where the antenna is located on a hill top, it is not unusual to incorporate "beam tilt" to angle the signal downward from the horizon to put more energy into where most of the receivers are located, instead of sending it all at the distant horizon.

The higher the "gain" of the antenna, the smaller the lobe of its radiation. This is why I do not favor high-gain antennas on boats; the rolling and pitching of the boat causes the antenna to aim its radiation all over the place. The wider the beam the less fading you will experience in these situations.

posted 01-05-2002 09:22 AM ET (US)            

One of these mounted on an extension mast would be great.

posted 01-07-2002 12:16 AM ET (US)            

Up here in the Great Lakes it is not uncommon for me to hear "Coast Guard Group Detroit" making broadcast announcments on Channel-22A when I am 250 miles due north of them--the signal is getting retransmitted on practically every CG station on the lakes.

posted 01-07-2002 12:18 AM ET (US)            

Having the antenna on the roof should help.
What sort of antenna are do you have?

And who are you hearing? If it's the CG,
remember that they have 9 dB gain antenns
up high and remotely located. As you can
see on http://www.navcen.uscg.gov/marcomms/cgcomms/charts/07stpetn.jpg
CG Group St. Pete has antennas well north
of St. Pete.

And is it EVERY Sunday?

Chuck