Removing antenna from mast, good move?

[1camper]

My Bristol has an antenna plate about halfway up the mast and a cable going down thru the cabin roof using a gland type fitting. I'm installing a tower on the boat and was considering just removing the mast antenna, filling the hole and locating a new one on the tower as there are no other electronics on the mast. The antenna would be about 6' lower by my approximation. I'm interested in opinions of what the downside of this would be . Thanks!

[Brian.D]

Depending on how high your antenna is now, you will lose about 6' of "line of sight" communications. This doesn't mean much but none the less, you are reducing your "radio horizon".

Radio horizon is defined by

Horizon(miles) = 1.23*SQRT(antenna height in feet)

So assume that your antenna is 40' above sea level. Your theoretical radio horizon is 7.7 miles. Drop your antenna 6' and your horizon reduces to 7.1 miles. Not much. But, there is more at play here.

You say you hear and talk to boats that are more than the theoretical horizon. True. It is caused by ducting or refraction. Atmospheric conditions can cause a tunnel effect allowing your signal to go farther than normal. Plus at the frequencies we use for Marine VHF there is the ability of our signals bending and thus going farther than the theoretical radio horizon.

In summary, you might not see a difference. However, do not be surprised if you lose some communication distance.

[1camper]

Thanks Brian, that's great info! The mast antenna looks to be a stainless 18" variety. There's room for a larger one on the tower, is there a type that would help me gain back some of that loss? ..not really wanting an 8' fiberglass whip tho.

[GordMay]

Sailboat antennas are usually 3' or 4' (end fed, ½ wave, 3 dB gain) stainless/steel whips.

[1camper]

Thanks..I was reading up on gain. Sounds like a 4' ss 3dB is going to be a good choice. Trying to simplify where I can and maybe get rid of one ugly wire.

[stewie]

Antenna gain is not going to take you over the horizon as VHF is line of sight.

[S/V Alchemy]

Originally Posted by stewie

Antenna gain is not going to take you over the horizon as VHF is line of sight.

Correct, but "line of sight" is relative to "height of eye", or, in radio terms, the height of the receiving and transmitting sides of two VHF antennas.

An example would include an AIS-only antenna, which operates in the same band as VHF, which is why you can use the same antenna for both VHF and AIS, although a splitter for such purposes adds a complication/point of failure.

You may be able to "see" a large ship AIS target on a Class B AIS transceiver over the horizon, and it may be able to see, under favourable conditions, you. Voice comms on VHF, however, may be more problematic. This is because the AIS antenna of the ship may be four or even five times the height of a small sailboat's whip antenna, and the AIS transmission, being compact and digital, can be detected much more quickly than even a mic click. So distances of 40 NM or greater can show a ship, well beyond even the best small boat radars will reveal it and certainly beyond the circa 25NM max. of most VHF transmissions outside of weird "skip" conditions.

I noticed this, oddly, from Toronto Harbour, where I could "see" lake freighters "ascending" the Welland Canal over 40 NM away, because they were locking up the side of a hill, in essence, giving their AIS transmissions formidable range. I have my doubts my own weaker AIS signal could be seen by the bridge crew of sailors busy in a lock, but my boat would be of zero interest, anyway.

Moral of the story is that height serves range, both in terms of seeing and being seen. The importance of that to you will depend on a number of factors that bear examination before you move your antennas.

[Dougtiff]

I remember a few year's ago, while sailing down the west coast to Mexico, hearing the U.S. coast Guard on the VHF, clear as a bell, tried to call them, of coarse they didn't hear me.

[1camper]

I understand. Another question, I'll probaby be replacing my vhf with one that has an AIS id feature, are there other considerations regarding transmitting and receiving of that data as opposed to just radio signals?

[1camper]

Thanks Alchemy! I think you answered my question before I was able to formulate it.

[KP44]

When an antenna is rated 3db gain, that can be confusing. Gain is the antenna's ability to direct or concentrate radio frequency energy in a particular pattern. There is no actual gain of signal energy. Our sailboat whip antennas are made to radiate more horizontally and less vertically. So 3db gain means the antenna throws twice as much energy where it is wanted; towards the horizon.

Use only the best connectors and use good marine coaxial cable like RG113. It is a one-time expense and worth doing right.

RG-113 has attenuation (loss) of about 2.5db per 100 feet.
RG-8x is 4.7db and RG-58 is nearly 6db.

[Cheechako]

There have been times on crossings where the VHF needed to read quite a ways away to contact fellow crossers and having the antenna on mast top was a very good thing. But if you are satisfied with it only partially up the mast, I guess the 6 ft wont hurt you much.
Sailing Mexico, I heard VHF transmissions probably 150 miles at night at times. "skip" I guess. I was hearing boats anchored near Cabo and I was nearly at Mazatlan.

[stormalong]

Originally Posted by Brian.D

Depending on how high your antenna is now, you will lose about 6' of "line of sight" communications. This doesn't mean much but none the less, you are reducing your "radio horizon".

Radio horizon is defined by

Horizon(miles) = 1.23*SQRT(antenna height in feet)

So assume that your antenna is 40' above sea level. Your theoretical radio horizon is 7.7 miles. Drop your antenna 6' and your horizon reduces to 7.1 miles. Not much. But, there is more at play here.

You say you hear and talk to boats that are more than the theoretical horizon. True. It is caused by ducting or refraction. Atmospheric conditions can cause a tunnel effect allowing your signal to go farther than normal. Plus at the frequencies we use for Marine VHF there is the ability of our signals bending and thus going farther than the theoretical radio horizon.

In summary, you might not see a difference. However, do not be surprised if you lose some communication distance.

Brian is right about the horizon distance but does not take the height of the other antenna into account. So while the figures presented apply well to someone talking to a handheld radio the true distance is also determined by the height of the other antenna.

[1camper]

We are getting ready to change the rigging so I could put the antenna on top of the mast but its factory plate is about halfway up. The difference between the top of the tower and the top of the mast is more than 20'. I just liked the simplicity of having all my wires where I can easily get to them and in very short runs. On the tower my wire would only have to be about 13-15'. It takes me that much wire to get to the base of the mast.

And ya, I'll be changing out the wire...thanks for that suggestion.

[tankersteve]

So other than the cable going thru the deck, and being a little less visually pleasing, it is fine? From my perspective, I'd keep it, and rig a backup on the tower. But then, I'd also look to mount it at the top of the mast, too, rather than 1/2 way up.

Can one haul an antenna up the mast as a jury-rig in an emergency, trailing a coax cable?

Then again, maybe once I saw it, I'd agree that it doesn't look 'right'. We do a lot for our boats so they look 'right', and I probably shouldn't second guess it. How about a pic?

Tankersteve