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Author Topic: small boat multiple antenna considerations - suggestions  (Read 3599 times)
KJ4WNR
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« on: July 18, 2011, 11:51:18 AM »

I have a rugged seagoing very small fishing boat. It has a very small area for antenna installation. Within this small area on the foredeck and some gunnel space along the cuddy forward I need to install 2 marine VHF and a 2-meter VHF (3 5/8 wavelength verticals) along with a 4’ AIS VHF receive only along with a passive tri-lens and an active radar ‘X’band repeater reflector mast similar to the 5/8 wavelength VHF antennas. Within a few feet of these antennas I need another mast with an anchor/all around light on a mast as high as the VHF transceiver antennas. Now that I have finally figured out how to install a ground plane on this 16 foot fiberglass boat with no integral ground I would someday like to install a HF rig aboard for recreation and DX Marine Mobile. All the others are safety related for offshore fishing and communicating.

Any one care to help me with some information about just how close can you have your VHF Marine style vertical antennas to each other.

Not likely to be much simultaneous TX but possible. The forward area of the small boat in the area of where the antennas will be installed has a stainless steel handrail. I had planned 18” ant extensions at the mount so the base of the actual antennas will be elevated a bit. There is a windshield nearby with a metal frame.

What concerns other than close proximity should I be considering? Discussions with some local non-boating hams has brought up questions about possibility of parasitic oscillation interference or coupling? between antennas. Not concerned with looking goofy or looking like a NASA vessel but fishing and fisherman safety high on the list with efficient radio communications.

Thank you!
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W8JX
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« Reply #1 on: July 18, 2011, 01:35:11 PM »

If you operate a VHF marine radio near a 2m radio or other way around you could overload front end of rig with antenna so close together. It you really want to do this you need a serious band pass filter on each rig then work on antennas.
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W3LK
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« Reply #2 on: July 18, 2011, 03:07:03 PM »

All that on a 16-foot boat?
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K0BG
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« Reply #3 on: July 18, 2011, 03:50:40 PM »

Well, that takes the cake! Both the 2 meter amateur radio band, and the main marine band are VHF (yes I know there is a UHF marine band too). In order to separate them with filters, would take band-reject/band-pass filters the size of 20 pound fire extinguishers! You can do almost as well, by vertically separating the antennas on the mast. Yes, you will have desense, but seldom do you talk on both bands at the same time.

On my 43 foot cat, that is exactly what I do. A Larsen NMO2/70BK is mounted at the very top of the 65 foot mast. Just below it, and upside down (the whip point down) is the VHF marine antenna. Both work well enough to contact local stations up to about 5 to 8 miles a sea depending on the swells at the moment.

But I have the same question Lon has. How do you get all of this on a 16 foot boat?
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KJ4WNR
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« Reply #4 on: July 19, 2011, 06:06:57 AM »

Hi,


Any one care to help me with some information about just how close can you have your VHF Marine style vertical antennas to each other.


Discussions with some local non-boating hams has brought up questions about possibility of parasitic oscillation interference or coupling? between antennas. Not concerned with looking goofy or looking like a NASA vessel but fishing and fisherman safety high on the list with efficient radio communications.


Not only is it a small boat but the antenna farm is in the forward one third as the work area has to be free of obstructions.

Redundant Marine Band VHF networked with redundant GPS and registered automated MMSI distress signal features is a necessary offshore fishing safety measure as well as communications with military in naval gunfire ranges while monitoring coast guard and other fishers. The 2-meter rig allows, on some occasions, a higher wattage reach to an onshore repeater for safety and normal HAM communications. The active and passive radar reflectors/repeaters and AIS are also safety with the commercial shipping, commercial fishing and vigorous U.S. Navy activity in our area.

I can afford some radios and some antennas and can install correctly. Not so with a bigger boat as safe as this little one.

Any small boat fisherman Hams out there?

Aloha


and

Thank you!

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W8JX
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« Reply #5 on: July 19, 2011, 06:35:29 AM »

Any one care to help me with some information about just how close can you have your VHF Marine style vertical antennas to each other.

On that boat without bandpass filters not possible with horizontal separation. As one suggested it you mount them vertically one under the other you will get about 40 db or so of isolation between them due to nulls in their fields which should prevent front end overload.

Discussions with some local non-boating hams has brought up questions about possibility of parasitic oscillation interference or coupling? between antennas. Not concerned with looking goofy or looking like a NASA vessel but fishing and fisherman safety high on the list with efficient radio communications.

Only potential problems I see as front end overload and possible radio damage if you do not follow advise above. If proper band pass filters/cavities are used you could even use just one antenna that is broad enough for both uses. If you use separate antennas I would suggest horizontal spacing off at least 1/2 wave (abt 3 feet) to minimize pattern disruption. (more is better) But again if no band pass filters are used you will be likely repairing or replacing radios.

So in a nut shell, either mount them vertical one under other as another poster suggested or space them 1/2 wave or more apart and use bandpass filters. This two bands are pretty close to one another and a lot of energy will be transfer to other radios receiver unless some form isolation is used. If you were doing VHF marine and then HF Ham it would be a whole new ball game and they are far enough apart in frequency that front end overload damage is not going to be a issue.
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KJ4WNR
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« Reply #6 on: July 19, 2011, 07:21:24 AM »

Thank you W8JX,

so, do I have any concerns with the two marine band radios but need to only monitor the 2-meter and shut down the marine receivers when TX on 146. I can get better than a 1/2 wave separation between antennas.
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WB6BYU
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« Reply #7 on: July 19, 2011, 07:37:46 AM »

Quote from: KJ4WNR
so, do I have any concerns with the two marine band radios but need to only monitor the 2-meter and shut down the marine receivers when TX on 146. I can get better than a 1/2 wave separation between antennas.


The marine radios will probably shut themselves down when you transmit on 2m due to the strong signal.
Same with the 2m radio when you transmit on the marine band.  It just isn't practical to install good
enough filtering to prevent it, though you may find some paging filters that will reduce the strength
of the marine signals arriving at the 2m rig.

When I was with Search and Rescue we had about 9 antennas installed on the roof of our van within a
6' square area:  marine, aircraft, VHF high band and low band, UHF, CB, scanner, VHF telephone, etc. 
All the radios suffered desense, but none of them were damaged:  we just knew that we couldn't hear
on one when transmitting on another.

Stacking the antennas vertically will help, and you may get some interactions that increase the SWR
if they are too close side-to-side, but usually it is a matter of "try it and see".


I would be concerned, however, about your plan to put the 5/8 wave antennas on elevated mounts.
They really are designed to be mounted directly on a flat surface, and the RF performance may suffer
as they are raised above it.  (This is especially true of using sloping radials below a 5/8 wave antenna -
it raises the angle of maximum radiation above the horizon and decreases your effective range.)
I think you will be better of with antennas that don't rely on being mounted to a flat sheet of
metal.
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W8JX
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« Reply #8 on: July 19, 2011, 07:56:01 AM »

The marine radios will probably shut themselves down when you transmit on 2m due to the strong signal.
Same with the 2m radio when you transmit on the marine band.  It just isn't practical to install good
enough filtering to prevent it, though you may find some paging filters that will reduce the strength
of the marine signals arriving at the 2m rig.

I would not bet the farm on them "shutting down" or not being damaged. Never heard of a radio shutting itself off in a strong signal. As far as filtering it is very practical and cost would depend on how much attenuation you would want or need. It is done all the time in commercial apps. (it is done with tuned cavities on repeaters all the time too but you do not need to get that extreme unless you want to be able to listen/use 2m and suffer no desense when VHF marine is in use.)   When crossing bands like UHF to VHF and VHF to CB and so on the is enough selectivity in antenna and radio front end that you will just see some desense at most but in band is is very possible to damage front end of radio. 
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WB6BYU
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« Reply #9 on: July 19, 2011, 08:41:00 AM »

Quote from: W8JX

...Never heard of a radio shutting itself off in a strong signal...
 


It's called "desense".  Shutting off the power won't help to protect the front end.
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W8JX
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« Reply #10 on: July 19, 2011, 09:06:20 AM »

Quote from: W8JX

...Never heard of a radio shutting itself off in a strong signal...
 


It's called "desense".  Shutting off the power won't help to protect the front end.

I know that shutting it off will not save it. It is does that radio will "shut down" in a strong signal field is a poor choice of words. AGC clamping on strong signal/desense is not "shutting down" but rather radio trying to save itself so to speak but it has its limits too. I have seen radios fried by this.   
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AA4HA
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« Reply #11 on: July 19, 2011, 10:36:01 AM »

As far as safe separation, it depends upon polarity as well. Since you are probably using whip type verticals the separation in the vertical plane will need to be much greater than if you had the antennas stacked on top of each other.

For vertical antennas mounted on a side bracket to a mast you could get away with using the same bracket for one antenna pointed upwards, another antenna pointed downwards. (there are antennas that are intended for upside down mounting). If you were to mount both antennas on a flat surface and in the same vertical plane you are going to have a significant problem with not just overload and desense but potentially blowing out the front end RF amplifier on either (or both) radios.

On the other hand, if you ever look at the rear trunk on a police car there are a bunch of antennas, some that are only a few feet apart and sometimes they may be running 50-100 watts of power. The problems are much worse when the frequencies are near each other like how 2 meters is not that far away from the VHF maritime.

If you have a mast of some sort I would go with "one up", "one down". The height would also help you out as VHF is very much a line-of-sight mode of communications. Getting 8-10 feet above the water would do wonders for your range.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KJ4WNR
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Posts: 4




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« Reply #12 on: July 19, 2011, 12:14:25 PM »

Good information to be digested and appreciated. Thank you all.

Here are two links.  One to a picture of a boat like to boat in question except the center part of the foredeck of my boat has a man hatch to handle ground tackle at sea. http://www.arimaboats.com/sch16.htm

The other link is to the digital dot com antenna site with pictures of the VHF antennas.
http://www.digitalantenna.com/prods/vhfantenna_8.html


Omni-directional. Collinear 5/8 wave, 8’ fiberglass.   Internal DC ground. The antenna will read shorted on continuity test


The passive radar reflector was four feet up a radio mast extension with the active 'X' band radar repeater above that. I plan to mount the passive tri-lens reflector about 20" off the foredeck in front of the windshield loosing much altitude over sea level but much less for the VHF antennas to see as there is no good way to get it very far from any antenna. (unless you all have better information)

The AIS 4' antenna is receive only on 161.975Mhz and 162.025Mhz . That is intended to be on and monitoring shipping whenever offshore.

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AA4PB
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« Reply #13 on: July 19, 2011, 01:08:34 PM »

Given that marine radios are limited to a maximum of 25W, you are not likely to damage one set while transmitting on the other. You are not likely to be able to receive very well on the one while transmitting on the other however. I'd recommend that you limit your 2M radio to around 25W as well - don't go putting in a 75 or 100W unit.

Get as much antenna separation as you can reasonably manage.
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