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Antenna on a Fiberglass Airboat - Grounding Question

mmckenna

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May be, but I don't recall an issue with having the VHF whips on the arch above the outboards. But then again, that was stuff used on the ocean where it was simplex and we needed range.

My bigger concern would be mooring lines with the antenna mounted right down on the deck like that. Or, having it at that level and someone using it as a grab point. An NMO through fiberglass isn't going to support a human.
Not to mention on the same level as the operators head, and close proximity to the aluminum windshield frame.

It might 'work', but it's not ideal.
 

kb4mdz

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At no point in this discussion have I seen any mention of a forward/reflected wattmeter test on this umm, setup. That will tell you a lot It will tell you:

1) How much power the radio is putting out, whether it's in spec?
2) How much power if any is reflected back from the antenna and/or connectors?
3) If the coax can be disconnected at the antenna end and a thru-line wattmeter can be inserted there, that can tell you a lot; whether you're getting all or most power the length of your coax, and whether you have reflected from the antenna?

This is RADIO. You have to use RADIO appropriate tools and radio appropriate thinking to test & troubleshoot.

BTW, how much RF power is this radio? Think about how much RF power you're radiating, how close to people in the seats. (Once had a customer wanted a 100 Watt VHF Syntor on a boat, and wanted to know if the 'thru-glass antenna ' mounted on the windshield would be OK. I looked at them with the coldest eyes and said "NO" and proceeded to talk about RF heating, eyeballs, brains, etc etc. We came up with a different solution, involved an elevated feedpoint coaxial dipole antennna on a length of stainless pipe. Antenna was a Motorola #, think made by Celwave, similar to a Kreco #CO-151 or CO-155, but not as rugged. Kreco Antennas - High Band Co-Axial Antennas

If radio work was easy, everybody would be doing it.
 

prcguy

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I have one of those sold by Motorola but it was made by Phelps Dodge before it became Celwave.

We came up with a different solution, involved an elevated feedpoint coaxial dipole antennna on a length of stainless pipe. Antenna was a Motorola #, think made by Celwave, similar to a Kreco #CO-151 or CO-155, but not as rugged. Kreco Antennas - High Band Co-Axial Antennas

If radio work was easy, everybody would be doing it.
 

WA0CBW

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I would second kb4mdz suggestion to check the SWR. Ran into the same situation once. SWR was infinite and coax showed a short. After pulling the coax out I found someone had tried to splice the cable using a Scotch-lock wire tap connector. You just never know what you might find.
Bill
 

epatchen

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switch to a direct channel and have someone take a hand held on the same direct channel a few hundred yards away or further, see if they talk to each other
I tried that. I got 4 trucks away (roughly 30 feet) with my 8 watt handheld before I could barely be understood by the person in the airboat.
The antenna cable has been replaced and it makes no difference. The radio was brand new last year, so I don't think that can be the problem. I'm just running out of things to try with it. It has to be either the radio or the power supply.
 

prcguy

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What is the transmit power of the radio in the boat? Are you going through a repeater or direct simplex? If you are going through a repeater its typical that a radio or handheld used in very close proximity to another radio thats transmitting will not work. The handheld would be trying to receive the repeater at a distance on one frequency and if you are transmitting with high power on the repeater input frequency it will blitz out the handheld due to interference. If you are using simplex then there is something completely different going on.


I tried that. I got 4 trucks away (roughly 30 feet) with my 8 watt handheld before I could barely be understood by the person in the airboat.
The antenna cable has been replaced and it makes no difference. The radio was brand new last year, so I don't think that can be the problem. I'm just running out of things to try with it. It has to be either the radio or the power supply.
 

epatchen

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To all the haters complaining about the antenna, it's this Motorola all band antenna, designed for VHF, UHF, 700 and 800. It's not a HAM antenna.
All of our vehicles have this same antenna. All the other vehicles' radios work perfectly.

Just to be clear on what's been done since I originally posted this thread, we've done the following:
  • Tried another antenna
  • Added a ground plane kit
  • Replaced the antenna cable
  • Bent the pin on the antenna to ensure proper contact with the NMO mount
  • Tried attaching a wire between the disc on the ground plane kit and the fan shroud
 

epatchen

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What is the transmit power of the radio in the boat? Are you going through a repeater or direct simplex? If you are going through a repeater its typical that a radio or handheld used in very close proximity to another radio thats transmitting will not work. The handheld would be trying to receive the repeater at a distance on one frequency and if you are transmitting with high power on the repeater input frequency it will blitz out the handheld due to interference. If you are using simplex then there is something completely different going on.
The boat has a 45 watt Motorola APX radio. This was on a simplex VHF frequency. I also tried UHF and it seems to be about the same.
 

kb4mdz

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I have one of those sold by Motorola but it was made by Phelps Dodge before it became Celwave.Been
I've been trying to find out the part number for that for a long time, either Motorola catalog or PD#. I've found the # for the base station version, but not the mobile version. I have one I got for (drum-roll) $0. at a hamfest. Didn't take a lot to get it working well. I've been told it was generally used for the mobile in IMTS service.

When I put it on this boat, the customer also had a machine shop, and we worked up a stainless tube about 6 ft long, passing thru a cupholder hole in the rear corner of the boat, and anchored in a block on the floor of the hull. I cautioned them to make sure nobody ever used it for a tow-point.

2 years had passed and I was at a different contract, they looked me up and asked if I knew where to get another antenna. I asked, they admitted someone had used the pole as - yes - a tow point. Oy vey.
 
Last edited:

kb4mdz

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To all the haters complaining about the antenna, it's this Motorola all band antenna, designed for VHF, UHF, 700 and 800. It's not a HAM antenna.
All of our vehicles have this same antenna. All the other vehicles' radios work perfectly.

Just to be clear on what's been done since I originally posted this thread, we've done the following:
  • Tried another antenna
  • Added a ground plane kit
  • Replaced the antenna cable
  • Bent the pin on the antenna to ensure proper contact with the NMO mount
  • Tried attaching a wire between the disc on the ground plane kit and the fan shroud
,

First, I apologize if I come across as cross, it's not my intent. But there seem to be several things missing that have been overlooked but are vital. So, look at my message for trying to help you, not for trying to chastise you.

Second, you mentioned this antenna is used on all your vehicles; I assume you mean 4 wheel vehicles, right? If that's the case, it is unique to the boat installation. I also assume you've swapped an antenna from a vehicle to the boat & vice versa and made comparisons?

Third, was this installation done by a real radio shop? If so, and it's under some sort of warranty, they should be handling it. And if they can't straighten things out, I'm gonna get really harsh and say they are not a real radio shop. A real service oriented shop should not leave a customer hanging like this, esp. that you're doing emergency services.

Fourth, have you used a simple multimeter to check for DC ohms/continuity of the antenna cable, end to end and center to shield? (which should be OK if you've already replaced it, but still...)

Last, I hope you've got a firehouse or garage to do this work in. I looked up your location; Pretty area, but cold & blowy & snowy right now. - I spent enough years up in Potsdam, after growing up an hour south of Buffalo. ;-)
 

mmckenna

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To all the haters complaining about the antenna, it's this Motorola all band antenna, designed for VHF, UHF, 700 and 800. It's not a HAM antenna.
Not haters. It's just that we've seen a lot of people cut corners when it comes to the antenna. There are some Chinese manufacturers that make 'look alike' antennas. If it's the official Laird/Motorola antenna, then that's good.

All of our vehicles have this same antenna. All the other vehicles' radios work perfectly.
Got it. My question would be about the boat install. Since the fiberglass is going to be thicker than the roof tops of most vehicles, the NMO mount type should be checked.

Just to be clear on what's been done since I originally posted this thread, we've done the following:
  • Tried another antenna
  • Added a ground plane kit
  • Replaced the antenna cable
  • Bent the pin on the antenna to ensure proper contact with the NMO mount
  • Tried attaching a wire between the disc on the ground plane kit and the fan shroud
OK, I think you are at the point where you need some test equipment. If you do not have the right test equipment, enlisting someone who does would be worthwhile. If that involves money, or a 6 pack of cold beer, either would be a wise investment. Trying to troubleshoot this stuff on a hobby website without hands on access and proper test equipment is going to take a lot of time and effort. An experienced tech with the right tools should be able to solve this in a short time.

  • Tried another antenna
OK, good step. That didn't fix it, so you know that likely the original antenna is good.

  • Added a ground plane kit
Good. That was needed anyway.

  • Replaced the antenna cable
Question:
Was the connector pre-installed?
How much excess cable is coiled up somewhere?
Do you have the tools/skills to replace the coaxial connector?
Are you using any adapters?

  • Bent the pin on the antenna to ensure proper contact with the NMO mount
OK, but are you 100% sure it's making contact? Close doesn't count. If it's a standard NMO mount, then there's the chance the center pin is sitting too low to make contact. Photos would be helpful. We only have the one photo you've shared.

  • Tried attaching a wire between the disc on the ground plane kit and the fan shroud
Won't fix anything. DC ground and RF ground are two different things.
 

KevinC

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Somewhere other than home :(
I believe it's an APX 4500, but I'll check next time I'm at the station.
Ok, that's a single band radio so how are you checking both VHF and UHF on it?

The reason I asked about 7500 vs 8500 is if it's a 7500 I wondered how you had the 2 outputs of the radio diplexed. And if a 8500 I would triple check the QMA adapter/connector as those are tricky to deal with.
 

davidgcet

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as others have said with this being in fiberglass a standard NMO is not thick enough to fit properly, you need a thick mount to ensure the mount sits right.

question, so 30 feet away talking on the handheld the mobile in the boat could barely pick it up? i'm starting to lean towards a bad connector on the coax, either the center conductor is not making contact end to end or more likely if an inexperienced person installed the connector then it is shorted center to shield. the only other thing that would kill tx and rx that bad is a bad radio, even though it is new it does happen. even with no coax attached to the radio it should pick up a handheld easily, but a shorted line or a shielded line with an open center would give similar results.


agree with those above about ohming out the line. check center to shield( connector body), should be open. then check center of the connector to the center pad on the NMO, should be shorted. next check shield of the connector to the brass of the NMO, again should be shorted. anything more than 1 ohm at most and you have a bad cable.
 

epatchen

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Ok, that's a single band radio so how are you checking both VHF and UHF on it?

The reason I asked about 7500 vs 8500 is if it's a 7500 I wondered how you had the 2 outputs of the radio diplexed. And if a 8500 I would triple check the QMA adapter/connector as those are tricky to deal with.
I checked earlier. Those are APX 8500 radios.
 

prcguy

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The potential recessed NMO connection from mounting on a thick surface is a real possibility. Testing some of my tri-band antennas from Larsen and Laird I see there is no short from the center pin to ground and the whip has nearly 0 ohm continuity to the center pin of the NMO interface. Therefore a simple test would be check across the antenna connector shield and center pin at the radio end with an ohm meter for an open then check the center pin of the coax connector to the whip for continuity, should be under a few ohms including coax. That will test for basic shorts from a bad connector install or damaged coax and also rule out the antenna not making full contact with the NMO mount.

Anything further will require an antenna analyzer or SWR/wattmeter and if the antenna checks out ok then a service monitor to verify radio operation.
 

kb2ztx

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On our air boat we installed a NMO mount on an aluminum plate on the fan shroud. VHF 1/4 wave works pretty darn good there and keeps it up in the air away from people. Might be an option. As other said Id definitely get a watt meter on it to start with.
 
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