| Home |
| Search |
| Today's Posts |
|
#1
|
|||
|
|||
Sharing aerial with DSC VHF and AIS
Hi,
AIS operates on 162MHz which is in the maritime VHF band, I think. Can I split the VHF aerial line to feed both a DSC VHF set and a stand alone AIS receiver??? - this receiver in particular http://www.allgadgets.co.uk/ag/produ...pf%5Fid=AG3933 TVMIA |
|
#3
|
|||
|
|||
|
wrote in message ups.com... Hi, AIS operates on 162MHz which is in the maritime VHF band, I think. Can I split the VHF aerial line to feed both a DSC VHF set and a stand alone AIS receiver??? - this receiver in particular http://www.allgadgets.co.uk/ag/produ...pf%5Fid=AG3933 TVMIA NO!!!! The VHF transmitter will burn out the AIS receiver. If it was an AIS transceiver, it would also burn out the VHF Receiver when the AIS transmitted. AIS requires special wide bandwidth antennas. For receive you might get by with a regular marine band VHF antenna, with reduced reception. An AIS transmitter however will soon fry its output stage on a regular VHF marine antenna as the SWR is too high. There are at least 4 antenna manufacturers now offering AIS versions and they cost a bit more! 73 Doug K7ABX |
|
#4
|
|||
|
|||
|
In article . com,
wrote: Hi, AIS operates on 162MHz which is in the maritime VHF band, I think. Can I split the VHF aerial line to feed both a DSC VHF set and a stand alone AIS receiver??? - this receiver in particular http://www.allgadgets.co.uk/ag/produ...pf%5Fid=AG3933 TVMIA Actually, if one were to do a bit of RF Engineering, it is possible to accomplish the above relatively simply. Motorola built a Modar Triton Bridge to Bridge that had a transceiver and a Ch. 13 Monitor Receiver all in one package, that split the Receive Antenna line after the Tx/Rx Switch. These are still one of the best VHF Radio's ever built and still fond on a whole passel of commercial vessels. What one would have to do is reengineer the receive path, after the Tx/Rx switch, and bring out a Rg-174/u line for the secondary receiver. this would surfice for a Passive AIS Receiver System. for an Active System one would have to do a bit more and discribing it is beyound what I would consider easy, for this forum, but it isn't impossible. The govt. boys would use Larry's idea of Ferrite Ring Circulators cause they have all out TAX dollars to spend. One of the best resouces on AIS is Mark Johnson of ShineMicro near Port Ludlow, Washington. He was one of the design engineers for SEA, and now is BIG into AIS. Last time I chatted with him, he had prototypes out on SeaTrials, and expected to be in production this spring, with some consumer versions. Just another note, if antennas were as narrow as Doug suggests most of the Marine Vhf's would have given up their magic smoke long ago. A Passive AIS Receiver will work very nicely with ANY VHF antenna. Any VHF Transmitter that can't sustain full power into an open antenna port, isn't engineered correctly, and needs more heatsink area on the final Amp transistors. Never had a SEA156 or 157 give up any Magic Smoke when transmitting continously into an open antenna port. That was one of the design criterias for any SEA Radio System. Sure beats paying for all the Warrentee Repairs when the sailing public get it wrong with their fancy new radios. Bruce in alaska -- add a 2 before @ |
|
#5
|
|||
|
|||
|
"Bruce in Alaska" wrote in message ... In article . com, wrote: Hi, AIS operates on 162MHz which is in the maritime VHF band, I think. Can I split the VHF aerial line to feed both a DSC VHF set and a stand alone AIS receiver??? - this receiver in particular http://www.allgadgets.co.uk/ag/produ...pf%5Fid=AG3933 TVMIA Actually, if one were to do a bit of RF Engineering, it is possible to accomplish the above relatively simply. Motorola built a Modar Triton Bridge to Bridge that had a transceiver and a Ch. 13 Monitor Receiver all in one package, that split the Receive Antenna line after the Tx/Rx Switch. These are still one of the best VHF Radio's ever built and still fond on a whole passel of commercial vessels. What one would have to do is reengineer the receive path, after the Tx/Rx switch, and bring out a Rg-174/u line for the secondary receiver. this would surfice for a Passive AIS Receiver System. for an Active System one would have to do a bit more and discribing it is beyound what I would consider easy, for this forum, but it isn't impossible. The govt. boys would use Larry's idea of Ferrite Ring Circulators cause they have all out TAX dollars to spend. One of the best resouces on AIS is Mark Johnson of ShineMicro near Port Ludlow, Washington. He was one of the design engineers for SEA, and now is BIG into AIS. Last time I chatted with him, he had prototypes out on SeaTrials, and expected to be in production this spring, with some consumer versions. Just another note, if antennas were as narrow as Doug suggests most of the Marine Vhf's would have given up their magic smoke long ago. A Passive AIS Receiver will work very nicely with ANY VHF antenna. Any VHF Transmitter that can't sustain full power into an open antenna port, isn't engineered correctly, and needs more heatsink area on the final Amp transistors. Never had a SEA156 or 157 give up any Magic Smoke when transmitting continously into an open antenna port. That was one of the design criterias for any SEA Radio System. Sure beats paying for all the Warrentee Repairs when the sailing public get it wrong with their fancy new radios. Bruce in alaska -- add a 2 before @ Bruce, Regarding the antenna, when I first heard of the requirement at a Furuno service seminar two years ago, I was as skeptical as your comments above. However, the fact of life is, a new AIS antenna is required if a transmitting AIS is used. Still skeptical I used an MFJ259B Antenna analyzer to sweep some regular VHF marine antennas such as Shakespeare, Morad, etc. Guess what! They were 3:1 or higher at AIS frequencies. Furuno even markets ones with a hefty $ 316 price tag for their AIS units, but they are quick to point out there are just as good AIS antennas available from other sources at a cheaper price, such as the Comrad AV7 or Morad 159HD. By the way, the MFJ unit has been a real jewel to sweep those top of the line VHF marine antennas with built in filters for RFI from land mobile public safety frequencies. We have customers who have areas on the Columbia River where police and fire departments just blast through on a lot of the cheaper radios. The SEA 156 was the old solution for them as it had a great receiver that was immune to the IF image/front end over load problems. The customer does not like the SEA 157 front panel operation and we are currently testing other brands now on a few of their tug boats. The antennas with filters started showing SWR problems right after install a year or more ago. I now pull them out of the shipping tubes as soon as they arrive, sweep them with the MFJ and weed out the bad ones. At first the manufacturer did nothing but after our pile of rejects had built up to a dozen or more and manufacturing date codes did not reveal a pattern, they had a technically qualified guy come out and see how I was testing them. Bottom line, we got free replacements and their production testing was changed to include a final test after the antenna was placed inside the fiberglass tube. Before that, their testing was done on the innards only at the factory and then it was placed inside the tube later. Regarding VHF transmitters designed to transmit into an open or shorted port, the cheap VHF marine recreational radios have none or little protection. We replace a lot of the VHF amplifier bricks in these radios due to poor owner installed antenna connectors with no solder or the braid and center wire twisted together at the PL259! Back in the good old days, even ham gear had protection built in and some service manuals such as Kenwood required checks with dummy loads with one half or one third the normal 50 ohm impedance and again at 3 times, etc. I have a set of the slotted push on PL259s with these various mismatch resistors color coded for quick ID when working on ham gear. 73 Doug K7ABX |
|
#6
|
|||
|
|||
|
"Doug" wrote in
nk.net: By the way, the MFJ unit has been a real jewel to sweep those top of the line VHF marine antennas with built in filters for RFI from land mobile public safety frequencies. We have customers who have areas on the Columbia River where police and fire departments just blast through on a lot of the cheaper radios. The SEA 156 was the old solution for them as it had a great receiver that was immune to the IF image/front end over load problems. The customer does not like the SEA 157 front panel operation and we are currently testing other brands now on a few of their tug boats. The antennas I use a single hi-split DB Products cavity with two slugs in it as a bandpass cavity between the VHF marine radio and the antenna. Passing the signal through the cavity has two benefits.... 1 - No paging/fire/cop/taxicab/etc. interference breaking the squelch... 2 - The radio isn't connected to the antenna, physically, and if you ground the cavity to the ship's grounding system it keeps the lightning and static discharges out of the marine VHF.... |
|
#7
|
|||
|
|||
|
"Larry W4CSC" wrote "Doug" wrote in nk.net: By the way, the MFJ unit has been a real jewel to sweep those top of the line VHF marine antennas with built in filters for RFI from land mobile public safety frequencies. We have customers who have areas on the Columbia River where police and fire departments just blast through on a lot of the cheaper radios. The SEA 156 was the old solution for them as it had a great receiver that was immune to the IF image/front end over load problems. The customer does not like the SEA 157 front panel operation and we are currently testing other brands now on a few of their tug boats. The antennas I use a single hi-split DB Products cavity with two slugs in it as a bandpass cavity between the VHF marine radio and the antenna. Passing the signal through the cavity has two benefits.... 1 - No paging/fire/cop/taxicab/etc. interference breaking the squelch... 2 - The radio isn't connected to the antenna, physically, and if you ground the cavity to the ship's grounding system it keeps the lightning and static discharges out of the marine VHF.... Larry, there may be some safety benefit to that arrangement, but I'm interested in whether your personal experience is that Decibel Product's ho-splitter actually has some quieting results as well (besides its ?db insertion loss ;-). As you know the grounding provides only a DC-blocker, and unfortunately the majority of lightning's energy is not DC. In practice most of these devices allow static-noise to pass right through, mainly because the noise has such broad RF bandwidth to itself. Even sophisticated lightning surge protectors that promise continual static drain, nonetheless let most of the atmospheric noise through. With a preselector after the static and DC-blocking devices, the noise level at least noticeably drops, that's my experience. Jack |
|
#8
|
|||
|
|||
|
"Jack Painter" wrote in
news:A1che.4837$It1.499@lakeread02: Larry, there may be some safety benefit to that arrangement, but I'm interested in whether your personal experience is that Decibel Product's ho-splitter actually has some quieting results as well (besides its ?db I don't see how a splitter, that would only drop the interference 3 or 4 dB could reduce the intermod because it would be hitting the front end amp of the receiver still pretty hard. With the single cavity tuned to 157 Mhz around the middle of the marine band, the attenuation of the 152 Mhz paging and cop bands is quite large, dropping the signal hitting the receiver to insignificant. As to the lightning comment, you have to see how far apart the radio's loop and the antenna's loop is inside the metal bandpass cavity. They are on opposite sides of the top plate and the loops go directly to ground after making just the one turn to excite the cavity. Even the plunger rod is directly in any path between the loops. They are, probably 9" apart, physically, and everything is a direct ground. I doubt it would survive a direct hit, but the radio wouldn't be the only thing destroyed in that event. Most radios are destroyed by static discharge (St Elmo's Fire), not direct lightning hits. The cavity idea completely eliminates those. I didn't put the cavity in for lightning protection. I got fed up listening to 10 paging transmitters as we sailed across the harbor. |
|
#9
|
|||
|
|||
|
"Larry W4CSC" wrote "Jack Painter" wrote in news:A1che.4837$It1.499@lakeread02: Larry, there may be some safety benefit to that arrangement, but I'm interested in whether your personal experience is that Decibel Product's ho-splitter actually has some quieting results as well (besides its ?db I don't see how a splitter, that would only drop the interference 3 or 4 dB could reduce the intermod because it would be hitting the front end amp of the receiver still pretty hard. With the single cavity tuned to 157 Mhz around the middle of the marine band, the attenuation of the 152 Mhz paging and cop bands is quite large, dropping the signal hitting the receiver to insignificant. As to the lightning comment, you have to see how far apart the radio's loop and the antenna's loop is inside the metal bandpass cavity. They are on opposite sides of the top plate and the loops go directly to ground after making just the one turn to excite the cavity. Even the plunger rod is directly in any path between the loops. They are, probably 9" apart, physically, and everything is a direct ground. I doubt it would survive a direct hit, but the radio wouldn't be the only thing destroyed in that event. Most radios are destroyed by static discharge (St Elmo's Fire), not direct lightning hits. The cavity idea completely eliminates those. I didn't put the cavity in for lightning protection. I got fed up listening to 10 paging transmitters as we sailed across the harbor. Roger that, thanks for the recommendation Larry. The pager interference is awful in Hampton Roads, one of the many areas identified by the Coast Guard as having serious interference to vhf marine band. The Boston and Cape Cod areas were another area identified with that problem, I didn't know Charleston was also so bad. At least part of the problem will be reduced when the new narrow band radios become prevalent. I can reduce most but not all pager interference just by setting a receiver to FM Narrow, and this works even when active splitters, notorious for amplifying pager interference, are used. Jack Va Beach |
|
#10
|
|||
|
|||
|
On Sun, 15 May 2005 09:13:18 -0400, Larry W4CSC
wrote: "Jack Painter" wrote in news:A1che.4837$It1.499@lakeread02: Larry, there may be some safety benefit to that arrangement, but I'm interested in whether your personal experience is that Decibel Product's ho-splitter actually has some quieting results as well (besides its ?db I don't see how a splitter, that would only drop the interference 3 or 4 dB could reduce the intermod because it would be hitting the front end amp of the receiver still pretty hard. With the single cavity tuned to 157 Mhz around the middle of the marine band, the attenuation of the 152 Mhz paging and cop bands is quite large, dropping the signal hitting the receiver to insignificant. As to the lightning comment, you have to see how far apart the radio's loop and the antenna's loop is inside the metal bandpass cavity. They are on opposite sides of the top plate and the loops go directly to ground after making just the one turn to excite the cavity. Even the plunger rod is directly in any path between the loops. They are, probably 9" apart, physically, and everything is a direct ground. I doubt it would survive a direct hit, but the radio wouldn't be the only thing destroyed in that event. Most radios are destroyed by static discharge (St Elmo's Fire), not direct lightning hits. The cavity idea completely eliminates those. I didn't put the cavity in for lightning protection. I got fed up listening to 10 paging transmitters as we sailed across the harbor. A 10 inch vhf cavity with .5 db insertion loss set gives about 30 db attenuation 1 mhz away from center frequency. At .3 mhz away it gives about 20 db loss. At only 100 khz away the loss is around 6 db. Can't cover much of the marine band with that. Seeing as how channel 6 and 16 are some .5 mhz apart it doesn't look like that would work too well. This also attenuates the transmitter by the same amount. What size cavity did you say you were using? Regards Gary |
Linear Mode
