Two VHF Radio's and Two antennea

[hardee]

I am considering putting in a second VHF radio. Several good reasons: 1. high power backup, 2. Continuously monitoring 16, and VTS channel and still having a working channel (68 etc), 3. Hailer and 4. More important, the advantage of an automated Fog Signal.

My concerns are any problems with two radios in that close of a proximity, (or more correctly two antennas that close together), as far as feedback from one radio to the other, bleed over from one to the other on same or different channels, and power requirement (3-5 amps), which is one of the least of the issues.

Would like to hear from some of you that are running 2 now and see if you would do it again, if you think it was worth the upgrade, if there have been any unforseen problems.

Thank you in advance for your input.

Harvey
SleepyC

[thataway]

I have been using two VHF radios, plus often a separate SSB and Ham radio, and in many boats a VHF ham radio for over 30 years. These require multiple antennas.

Generally it is advisable to keep the antennas 6 feet apart, but in the C Dory, alternate sides of the cabin work fine. The Tom Cat has the 4 antennas separated by about 5 feet (square pattern).

I would alternate VST and 68 on the scan or second radio--and 16 on the single radio--since that is the channel to be monitering for emergencies.

If done properly, there are no problems with quality radios. Heck, look at your average police car--in our area, some have as many as 6 antennas.

[Grumpy]

I agree with Bob.

Before fitting AIS I had (temporarily) 2 VHF on the "tower" where they were about 3 ft apart and there was definite front end overload on one radio when transmitting on the other.(some channels)

Now I have the AIS on top of the tower and the 2 VHF on the rear cabin roof about 7 ft apart and 3 ft below the AIS and everything works just fine.

Merv

[mcc272]

I agree with Bob. I have had 2 VHF radios aboard all my boats for probably 15 years or more. Where I normally boat, VTS is not in use but the commercial traffic announces its intentions and communicates with one another on VHF13. You can learn a lot from their communications and are much better able to anticipate vessel movements. Plus, their comments about events going on in their vicinity can be very useful and instructive. In my area, VHF9 is our major hailing channel left over from the days when, in the Northeast, there was an legal effort to force traffic off of VHF16. One of my radios is set to VHF9. The other scans VHF16 and VHF13 and will stop on WX if there is a WX alert. My wife used to always tease me about having too many radios until many years ago at Trawlerfest in Solomons, MD when we went to a talk by a pilot who could move anything on the Cheasapeake. As I recall, his name was Bill Band. He gave a great and eye opening talk about us small boaters (including boats many times the size of our C-Dorys) from the perspective of the big boys. He had a lot of slides so you could clearly see how inconsequential we looked, how poor his visibility often was, and how we disappear from view long before we would think that we would. Part way through his talk he said that if you can afford a another piece of electronics, in his view, the most important thing was to have a second VHF radio and to tune it to the commercial channel. I turned to my wife with an "I told you so" look but before I could say a word, she asked, "How much did you pay him?"

Antenna spacing is very important but just as important is spacing of the radios at the helm. The greater the separation between the radios, the easier it is to tell which one sqawked. On my CD-22, I am planning to put the radios adjacent to one another above the helm (ease of access to the mic and the VHF controls) but the port radio will have an extension speaker that will be located to the left.

[Dreamer]

Harvey,

The best advice I ever got from a boating friend was "put in 2 VHF radios". After following that advice on our Krogen and the TC, I couldn't imagine boating in our area without at least 2 radios. Janet monitors the club channel and ch. 16 communication, I have vessel traffic and Coast Guard comm on my Icom 602. The fog signals and loud hailer are a plus.

Dual Shakespeare 8' antennae are on opposite sides of the Radar arch and don't interfere with each other. Go for it!

[hardee]

Merv said:

[Grumpy]

Harvey,

Best I can suggest is to turn both radio's on, pick two different but frequently used channels such as 16 and 68 and set them both so the squelch just makes them silent. Then key each one alternately and see if you get a burst of noise out of the other one. You might try a variety of channels but it will usually be obvious very quickly. Hold the key about 1/2 second or so.

BTW make sure you transmit on full power.

Merv

[hardee]

Merv, (et al)

Thanks for the "How to" and when I get them in I will be sure and try that. I'm looking at placing both antenna's at the opposite forward corners of the cabin roof. They will be ~4ft apart. The antennae will be on the primary radio, an 8ft Shakespeare 6db gain, (already on the boat, forward port corner. The secondary will be a 3ft steel, base loaded (sail boat) antenna, mounted on top of a 24" extension post that is currently carrying the RayMarine GPS puck (forward Stabrd corner), (which will be moved off the extension and onto the cabin roof.) There is a radar between the two antenna, up on a 6-8" post.

Anybody before we start dismantling and moving parts?

I would appreciate not messing up up right from the start.

Harvey
SleepyC

[edwardf]

Hello all,

Two antenna's on two radios will work but keep them as far apart as possible. The theory is as they say "beyond the scope" of this web page but a couple things will happen.

One is the two antennas will couple to a varying degree. Antennas are tuned circuits so the transmitting radio will see a near field resonant circuit provided by the second antenna. Significant RF voltage will appear at the input of the second radio's receiver when the first transmits (especially at full power). While it's possible this could fry the active device on the input of the receiving radio, a good quality radio (even a consumer radio) should be OK. That's why you will hear the squelch open on the receiver (as Grumpy's suggesting) and hear either noise or your own voice(!) on the second radio. Especially on nearby channels. Even if the first stage active device isn't fried, over time this overvoltage can degrade the performance of that stage. But there is something called the inverse square law that will protect you some here and a low transmit duty cycle too. By the way I'd key up on low power first then high power.

Second is that these two coupled antennas will affect the transmitted radiation pattern. Two antennas resonant at 158mhz and 6.2 feet apart are one wavelength apart. You may get some directivity changes from a "normal" single antenna radiation pattern. You surely will at lesser spacings. This is often used as a method of producing gain in a particular direction even though you won't be using a single "phased" coaxial feedline. I like to be 5 or more wavelengths apart if possible to prevent coupling but I know you can't on a small boat. Think of your TV antenna on the roof. Even though only one element actually has a feedline (coax or ribbon) going to it, the other elements act as passive reflectors and directors of RF energy to give your TV antenna directivity.

One last comment. Even though police cars have many antennas on them, they are usually for very different frequency bands; UHF, VHF-Hi, VHF-Lo, Cellular, even in some places CB. There may be hundreds of megahertz difference in their resonant frequencies. Even if there are two VHF-Hi antennas, they are typically a couple megahertz apart. You're talking here of being only 25 kilo hertz (ch 9 and 68) apart. Then too these are commercial radios, typically with helical resonator front ends that attenuate adjacent channels greatly. That's partly why police car radios cost a couple thousand and boat radios a couple hundred.

Hope this was informative. Bottom line is you should be OK, most likely. But you may see some directivity, long term degradation in performance, or possibly get some howls and whooshes from one radio when the other transmits. Of course this antenna stuff is really all magic.

Ed.

[journey on]

Ed certainly makes a good case for not having two identical radios each with its own antenna on board. The initial discussion was to allow CH 16, VTA and a working channel all at the same time, as well as having fog signals, etc.

The Standard radio I have provides monitoring of several channels, with Ch 16 priority, which would solve the first desire, and having a handheld for use when the electronic fog horn is in use would appear to be viable for that function. After all, emergencies and fog don't happen too often. At least now that my kids are grown up. And indeed I have a fixed mount VHF and a handheld VHF. Served well and is cheaper and more flexible. Judy can take the handheld on shore and then call me to pick her up.

As Bob indicates other radios, such as ham or marine SSB require their own antennas because they operate at different frequencies, and as Ed mentions that's not a problem.

Boris

[thataway]

Ed makes some excellent points, but I would not use Ed's theoretical objections to avoid having two fixed VHF radios if they increase your safety. The reasons I run two radios is because if you have one radio on scan--even as 16 priority, there is a chance of missing a security call on one of the other frequencies, as happened to me on a narrow section of the ICW a number of years ago--where both the tug and I gave security broad casts on opposing channels (16/13) and thus missing each others call. True a scanner or hand held will do the same monitoring, but with far less range.

The front ends on the marine VHF radios which I have used seem pretty good at rejection. In fact I am constantly amazed at how good they are. I don't have any problem with my Standard Horizon (which makes many of the public safetyl radios as Vertex Standard). When making directors or reflectors on a multielement antenna, the passive elements are a shorter or longer than the driven element. Most of the Marine VHF antennas are 5/8 or 1/2 wave antennas.

I have found much more directivity by the sailboat rig--than by other antennas in the same frequency range. There the rig does act as a reflector and there can be noticable difference as the boat changes direction. I have owned several large multi element ham antennas, and there is a definite effect of directors and reflectors in a very finely tuned antenna.

Ideally it would be nice to have the antennas some distance apart, but that is not practical, until you get way up in boat size.

[edwardf]

All,

I think two radios are are a great idea. I remember going under the Tacoma Narrows bridge a few years ago and I had a handheld so I could talk to the guy in the skiff that was directing us on where to cross under while I never missed normal traffic with the main radio.

I know you guys don't want a discussion on antenna design but I was just pointing out some gotchas that don't seem obvious. I don't know much about boats but I know a little about antennas and I saw this as a chance to contribute.

Maybe another thing I should have said which is important tho is to try and keep ANY metal objects more than a wavelength away from the antenna if you can. Even an aluminum radar arch can be an antenna!! The mast on a mast mounted antenna can form part of the antenna if you or the manufacturer aren't careful.


Absolpositively a good idea.

[hardee]

Dr Bob, Ed, Boris and all, I have really appreciated your input here. Yes, a bunch of it is more technical than I follow, but you have done a good job of sharing and making it understandable.

I have had two experiences where it would have been good to have a second fixed vhf. I have always carried a second portable handheld but it lacks range that may be neccessary in some instances. On our trip up to PLI last fall, there was a boat that was within our range of offering support, but we were on 68 with a scan to 16. Since we were traveling with another boat we were on 68 quit a bit, discussing the trip, and the 16 scan was pretty intermittent. The handheld vhf did not pick up the call. We heard about it from the cruise boat captain at Egmont when we got in there.

Here's hoping that the RayMarine radios are of a high enough quality that they can survive this application.

Harvey
SleepyC