A question concerning VHF antennas...

[jmschmidt]

A short metal masthead antenna, due to the narrow beam that it transmits, cannot connect with a receiving antenna on shore or on another vessel if the sending vessel is rocking and rolling in heavy seas.

This is why I have a standard 6' Shakespeare antenna mounted at deck level which transmits a beam several degrees wider than a small masthead unit. A simple and cheap antenna selector switch allows selection between the two mounted antennae. With a short patch-cable and a simple connector, by removing the antenna on my hand-held and installing the connector and patch-cable, I can directly connect the hand-held to either mounted antenna.

[Lake-Effect]

Originally Posted by S/V Illusion

Actually, connecting a radio to two separate antennas is a workable solution if fed in phase (in simple terms) meaning same antenna, same length/type coax, same impedance, etc... They will also work out of phase or dissimilar polarization and the power each 'sees' would be approx. half what the radio transmits with no negative effect on receive.

I can't see any practical benefit to such an arrangement for marine VHF. Any time you have two or more driven and physically separated antenna elements, you no longer have an omnidirectional transmit/receive pattern. And if one element is damaged or broken, the resulting overall performance would be worse than having just one antenna.

[Adelie]

Originally Posted by jmschmidt

A short metal masthead antenna, due to the narrow beam that it transmits, cannot connect with a receiving antenna on shore or on another vessel if the sending vessel is rocking and rolling in heavy seas.

This is why I have a standard 6' Shakespeare antenna mounted at deck level which transmits a beam several degrees wider than a small masthead unit. A simple and cheap antenna selector switch allows selection between the two mounted antennae. With a short patch-cable and a simple connector, by removing the antenna on my hand-held and installing the connector and patch-cable, I can directly connect the hand-held to either mounted antenna.

Installing an antenna at the masthead increases range by increasing the distance to the horizon for the radio.

I suspect you mean a 6db antenna which is probably 8' long. The problem with such antennae is they don't like to be heeled, the vertical radiation spread is narrower not wider. For a larger power boat that doesn't heel as much they are fine but you will loose reception more rapidly as the boat heels towards or away from intended recipient. The increased gain of the 6db antenna is quickly lost to heel.

In general on a sailboat or small power boat a 3db antenna will give better performance than a 6db antenna.

Check out the Shakespeare system selection guide. Great graphic.
Antenna Selector Guide | Shakespeare Marine Antennas

If the boat has lost it's mast and is not subject to rolling the 6db antenna will do better.

[Adelie]

Originally Posted by S/V Illusion

Actually, connecting a radio to two separate antennas is a workable solution if fed in phase (in simple terms) meaning same antenna, same length/type coax, same impedance, etc... They will also work out of phase or dissimilar polarization and the power each 'sees' would be approx. half what the radio transmits with no negative effect on receive.


The problem arises when one antenna is damaged, shorted, etc... which will effect the entire antenna system including both antennas so it's not a good idea in a dismasting to rely on that as a redundancy.

Unless the antennae are coaxial and fairly close you are going to have significant phase differences in certain directions.

[Richv]

Originally Posted by Apollo366

To connect two antennas to the same radio would take a fancy electronics box of some kind, my only thought is a Ham Radio shop might know of one and be able to advise you.

I also have a second VHF antenna, but it's because I have little faith in the coax in my mast, it's very corroded down at the foot of the mast in the bilge.

I just bought a very good quality antenna and base mount from West and mounted it on the solar panel arch and ran the cable down to the back of my electrical panel where the other cable has a joint from moving the VHF to the cockpit where I can use it and hear it much easier.

PS, most VHF radios can be removed from the front, usually a pop off bezel around it, ask a radio guy if you can't get it to go.

No; do not attempt to connect both antennae at once. Even if you used a power splitter, the way the two signals interact would be very complex. If your boat has an AIS as well as a VHF transceiver, I'd suggest the rig I'm going to install on my son's boat. The main VHF antenna is on the masthead, and I'll mount the backup antenna on the stern railing. A crossover switch will connect the two antennas to the VHF transceiver and the AIS transponder. Under normal conditions, the VHF rig will be connected to the masthead antenna for range, while the AIS transceiver is connected to the railing antenna. When longer range is desirable for the AIS, we can switch the antennas. If either antenna is lost, the other can be switched into circuit. In extreme emergencies, I've used a magnetic mount antenna stuck to a cookie sheet for Amateur communications, so it wouldn't hurt to have that in your emergency locker. If you already own a handheld transceiver (HT), I'd suggest getting the adapters that allow you to connect the HT to the main antennas. and to an emergency antenna. I prefer the "belt, suspenders, and ten-penny nails" school of emergency communications.

For those of you who may be operating outside Line-Of-Sight range, I would suggest investing in an MF/HF marine rig.

[Waveguide]

Originally Posted by Jim Cate

Chris, most folks, myself included, that have a backup VHF antenna simply leave it disconnected until needed, or perhaps use it for their AIS or even the FM receiver. Ours sits on the solar arch, off to the side to minimize shading, and has been tested for both VHF and AIS use. The end of the cable sits next to the VHF unit and the splitter for the AIS. A very quick switchover should it be needed.

Jim

Yep. That's what I do. I have a VHF antenna on its own mount down near deck level. Normally, it's my AIS antenna, but I have a Vesper Marine antenna splitter wired and ready to go to accept my voice communications transceiver and AIS transponder to share that antenna together should the mast ever go away.

Normally, the Vesper splitter is powered up but not connected to any radios.

It only takes a minute to swap around PL-259 connectors to reconfigure.

About the posts above: the only way to share one antenna full time between two transceivers is with an antenna splitter. Good ones aren't cheap, but what they do isn't simple. If you try it any other way, you'll smoke one or both radios.

Getting adapters to connect the handheld to an external antenna is also prudent. If lightning takes out your radio, you'll need that handheld as a backup. Put it in a metal box (microwave oven or a steel ammo box) if you are penetrating an area with lightning. Put a handheld GPS receiver in there too. You can get ammo cases cheap at surplus stores.

[S/V Illusion]

Originally Posted by Adelie

Unless the antennae are coaxial and fairly close you are going to have significant phase differences in certain directions.

Phasing can only occur when there is a parasitic interaction between the two. As we don’t know in the hypothetical given, the proximity, I mentioned it only to cover that possibility.

[rondelais]

Don’t connect two antennas together. First, this will really screw up the VSWR seen at the radio. Second, even if you did it though a 3 dB splitter or directional coupler you would end up with an antenna pattern that would be a nightmare. If the two antennas were within a half-wavelength of each other you would have two main beams and two nulls. If they were further apart you would have a scalloped beam pattern with multiple nulls. The only way to combine two antennas is vertically, (meaning one over the other) which if they are carefully phased can generate more gain toward the horizon but at the expense of a narrow beamwidth and nulls on elevation. Not a good idea.

[Richv]

Originally Posted by Waveguide

Yep. That's what I do. I have a VHF antenna on its own mount down near deck level. Normally, it's my AIS antenna, but I have a Vesper Marine antenna splitter wired and ready to go to accept my voice communications transceiver and AIS transponder to share that antenna together should the mast ever go away.

Normally, the Vesper splitter is powered up but not connected to any radios.

It only takes a minute to swap around PL-259 connectors to reconfigure.

About the posts above: the only way to share one antenna full time between two transceivers is with an antenna splitter. Good ones aren't cheap, but what they do isn't simple. If you try it any other way, you'll smoke one or both radios.

Getting adapters to connect the handheld to an external antenna is also prudent. If lightning takes out your radio, you'll need that handheld as a backup. Put it in a metal box (microwave oven or a steel ammo box) if you are penetrating an area with lightning. Put a handheld GPS receiver in there too. You can get ammo cases cheap at surplus stores.

An antenna splitter which is intended to allow an AIS transponder and a VHF transceiver to share an antenna is a complex device. Most devices contain an electronic switch, usually based on a PIN diode. Only one transmitter can use the antenna at once in this situation. The assumption is that your VHF broadcast is more important, so the VHF transmitter is routed to the antenna, while the AIS transmitter is connected to a dummy load. If your VHF transmitter is not in use, the AIS transmitter gets access to the antenna. That's how AIS/VHF splitters work. If you already have such a device in circuit, and a masthead antenna for VHF (our boat has a 6', 3db vertical antenna on the mast), a simple VHF switch can connect your splitter's output to either antenna. Keep in mind that without power, a splitter will default to one transmitter, usually the VHF transceiver as this is the one you do not want to fail in an emergency.

[Reefmagnet]

Install the second antenna at the arch and hook an AIS to it. Ensure your VHF leads are set up so they can be swapped if the need arises. I use a small racing vhf whip above the radar and it has about half to two thirds the range of the mast mounted antenna.



Having said that, a handheld doesn't lose much in distance and has the advantage that it can be used from almost anywhere on the boat. Do you really want to be forced to sit by the fixed radio when the SHTF? If things get really grim, it's probably time to pull the pin on the EPIRB anyway.

[Waveguide]

Originally Posted by Richv

An antenna splitter which is intended to allow an AIS transponder and a VHF transceiver to share an antenna is a complex device. Most devices contain an electronic switch, usually based on a PIN diode. Only one transmitter can use the antenna at once in this situation. The assumption is that your VHF broadcast is more important, so the VHF transmitter is routed to the antenna, while the AIS transmitter is connected to a dummy load. If your VHF transmitter is not in use, the AIS transmitter gets access to the antenna. That's how AIS/VHF splitters work. If you already have such a device in circuit, and a masthead antenna for VHF (our boat has a 6', 3db vertical antenna on the mast), a simple VHF switch can connect your splitter's output to either antenna. Keep in mind that without power, a splitter will default to one transmitter, usually the VHF transceiver as this is the one you do not want to fail in an emergency.

All very correct except:

>If your VHF transmitter is not in use, the AIS transmitter gets access to the antenna.

Actually, BOTH the VHF transceiver AND the AIS receiver are able to receive simultaneously from the single antenna when using any quality splitter that I have seen. With an A/B switch (what you call a "a simple VHF switch"), only one receiver will be connected to the antenna at any given time. Since I (and I assume most others) want both receivers to function continually on a single antenna, an A/B switch won't allow that.

Which receiver, AIS or VHF comms, do you prefer to disable with your switch? Or do you intend to manually and continuously switch back and forth every few seconds so you don't miss receiving something?

I am not a proponent of using a splitter in place of two antennas for VHF comms and AIS. It's unnecessary complexity and a good splitters costs more than a sexcI would only recommend sharing an antenna with a splitter in an emergency, which I believe was the OP's topic.

[chrisr]

thanks all. heaps of good ideas and advice

fyi we presently use the masthead antenna for the AIS & VHF, via a splitter. this gives heaps more range for the AIS when compared to a deck level antenna - we have picked up AIS targets at 100'

i very much like Riddle's suggestion of a coaxial cable antenna selector switch

if i can't find such, guess i'll just run a cable (from a seperate antenna) and leave this unconnected near the existing vhf...to be connected if / when needed

thanks again

cheers,

[Jim Cate]

Chris, you could (at a small expense of cash and signal loss) run a short cable from the back of the VHF to a convenient spot, one that you can actually reach, and put a barrel connector there... and lead the spare antenna lead to the same spot. This would address your concerns about being able to make the changeover under duress.

Jim

[Mark Thurlow]

Originally Posted by DefinitelyMe

We are also proponents of having a second antenna, in our case mounted on the stern rail. We use it for AIS transmission and can easily plug it into the radio if necessary. It's also cheaper to buy a second antenna than a splitter, so it's win-win.

Before the days of AIS we kept a second antenna down below with about 20' of cable and the PL259 connector already installed. Had to use it once when the masthead antenna failed coming into Bermuda. Two minutes had us up and running again, with the antenna zip-tied to the binnacle.

I don't see any reason to have a second one installed permanently unless you're actively using it. Just one more thing to wear out and need replacement.

One of these days I'll get around to buying the connectors to plug the hand-held into the masthead antenna, so that in the event of a complete power failure or whatever we can significantly improve range on the handheld.

Interesting thought about h/h to main antenna: I was told that would not work and that I was better off with the H/h stubby. Thoughts?

[Lake-Effect]

Originally Posted by Mark Thurlow

Interesting thought about h/h to main antenna: I was told that would not work and that I was better off with the H/h stubby. Thoughts?

If your handheld supports use with 50 ohm cabled antenna (confirm in manual or from dealer/manufacturer) and you can get the proper connectors/adaptors, the main antenna should work great with the handheld.