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Old 18-01-2018, 15:16   #1
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VHS and AIS Antennas ? Difference ?

I had my rig down on the ground and decided to install a shinny new VHS and a new AIS Antenna and coaxial cable each ..... I was preparing for purchasing the a new radio

This is the issue hopefully someone will be able to help me with, I would have bet money that I labelled the ends of the coaxial cable so I could tell them apart .... but it didn't work

Is there a way to check the Ohms/Impedance of the antenna's to tell which is which ?

Thanks
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Old 18-01-2018, 17:29   #2
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Re: VHS and AIS Antennas ? Difference ?

Not an expert, but maybe find a Ham with an antenna analyzer and they should be able to tell you the resonant frequency. http://www.mfjenterprises.com/Produc...ductid=MFJ-223
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Old 18-01-2018, 18:35   #3
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Re: VHS and AIS Antennas ? Difference ?

An ohm meter will work, but you need a bosun's chair too.
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Old 18-01-2018, 18:47   #4
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Re: VHS and AIS Antennas ? Difference ?

Is the AIS antenna a generic VHF antenna or one that is specifically tuned to AIS channels? A generic VHF can be used for AIS.

How close a part did you mount these?
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Old 18-01-2018, 18:57   #5
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Re: VHS and AIS Antennas ? Difference ?

I don't think any reasonably found people or equipment will be able to identify which coax goes to which antenna without someone going up the mast.

VHF and AIS antennas are electrically the same, except an AIS antenna MIGHT be more optimized for the AIS channels, but should still work for the rest of the VHF band. And a VHF antenna will work fine for AIS.

It is probably beyond what a typical HAM would have to be able to tell the difference.
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Old 18-01-2018, 19:19   #6
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Re: VHS and AIS Antennas ? Difference ?

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Originally Posted by wholybee View Post
I don't think any reasonably found people or equipment will be able to identify which coax goes to which antenna without someone going up the mast.



VHF and AIS antennas are electrically the same, except an AIS antenna MIGHT be more optimized for the AIS channels, but should still work for the rest of the VHF band. And a VHF antenna will work fine for AIS.



It is probably beyond what a typical HAM would have to be able to tell the difference.

Many Hams now have antenna analyzers that will sweep a frequency range and graph the SWR. In this way, you could see the frequency of lowest SWR, which might tell you which antenna was which. That said, since the AIS frequencies ( 162 MHz ) are so close to the Marine VHF channels (around 156-157 MHz ) it isnít really going to matter much.
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Old 18-01-2018, 19:22   #7
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VHS and AIS Antennas ? Difference ?

Vesper markets an antenna meant for both VHF and AIS.
I bought and installed it, and can tell no difference at all in either the VHF or AIS performance.
Iíd suspect that they are so close in frequency that the difference in antennas may be the sticker?
If your in the US and if there is a seatow automated radio check where you are at, you could try both antennas and listen to the playback and see if you notice a difference, or climb the mast.
However it sounds like they may be mounted close to each other as I assume the Co-ax is the same length? If so that may be problematic
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Old 18-01-2018, 19:50   #8
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Re: VHS and AIS Antennas ? Difference ?

Try them both into your ais. And look at the swr on your laptop with ais software. The ais ant will be lower. (Assuming the cables and connections are good)

The difference between them will be minor. Both will work for the ais or vhf.

For best results climb mast. Disconnect ant. And check cable ohms
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Old 18-01-2018, 19:52   #9
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Re: VHS and AIS Antennas ? Difference ?

Actually, the long "gain" VHF antennas are seriously mis-tuned at the AIS frequencies (around 162 MHz). These antennas are usually tuned for Chan 16 (156.8 MHz), and this is enough of a difference to matter. The shorter antennas, as usually used on sailboats, are usually more broadly tuned but I can still easily measure the mismatch at the AIS frequencies. I slightly shortened a short whip antenna to get an optimum match for use with my AIS transponder.

However, because of loss in the coax cable (which hides the mismatch somewhat), and other factors, even the long antennas usually work well enough. But the "AIS" antenna is more than a label difference.

You do need some specialized equipment to measure this. I have it.
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Old 18-01-2018, 19:55   #10
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Re: VHS and AIS Antennas ? Difference ?

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Originally Posted by smac999 View Post
Try them both into your ais. And look at the swr on your laptop with ais software. The ais ant will be lower. (Assuming the cables and connections are good)
Yes! If your AIS transponder let's you see the SWR, that's a good way to compare the two antennas.
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Old 18-01-2018, 20:26   #11
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Re: VHS and AIS Antennas ? Difference ?

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Try them both into your ais. And look at the swr on your laptop with ais software. The ais ant will be lower. (Assuming the cables and connections are good)

The difference between them will be minor. Both will work for the ais or vhf.

For best results climb mast. Disconnect ant. And check cable ohms
That would be easy to try. Not sure if the repeatibility of the measurement is good enough. It takes a fair amount of time for my VSWR reading to stabilize and it can be off a few tenths in day to day testing.
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Old 18-01-2018, 21:44   #12
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Re: VHS and AIS Antennas ? Difference ?

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Many Hams now have antenna analyzers that will sweep a frequency range and graph the SWR. In this way, you could see the frequency of lowest SWR, which might tell you which antenna was which. That said, since the AIS frequencies ( 162 MHz ) are so close to the Marine VHF channels (around 156-157 MHz ) it isnít really going to matter much.
I stand by my statement. I have a nice Agilent Network Analyzer, and I think it would see the difference. I can see a kink in the cable with it, and tell you how far that kink is from the unit. Or how long the cable is to the antenna, as well as a VSWR plot.

I don't know that the lesser expensive gear would be reliable or accurate enough to be sure it saw the differences. And while some HAMs might have profesional equipment like I do, I don't think it is the norm. Hence my wording "readily available."

Anyway, in less time than it would take to set up and calibrate my equipment, I could climb the mast, install a load or short at that end, and then test for it at the bottom with an ohm meter. And the HAMs I know that have pro gear also climb towers for a living. I think they would jump at the chance to climb a mast and choose that route also.
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Old 19-01-2018, 06:02   #13
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Re: VHS and AIS Antennas ? Difference ?

Bigger issue than which antenna is which is, did you mount them side by side at the top of your mast? If so, they are going to interfere with each other. VHF and AIS antennas need to be separated either by a few feet vertically, or by a whole lot more horizontally.
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Old 19-01-2018, 08:15   #14
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Re: VHS and AIS Antennas ? Difference ?

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Bigger issue than which antenna is which is, did you mount them side by side at the top of your mast? If so, they are going to interfere with each other. VHF and AIS antennas need to be separated either by a few feet vertically, or by a whole lot more horizontally.
This reply identifies the real problem you have.

The difference between a VHF antenna (not VHS, an obsolete tape recorder format) and an AIS antenna is simply the tuning of the antenna for resonant frequency and best match to a 50-Ohm transmission line.

For ship station transmitters in the VHF Marine Band, the transmitter frequencies are all below 157.850 MHz.

For AIS transmitters, the frequency is 161.975-MHz or 162.025-MHz.

Because most VHF Marine Band antennas are made with elements of small diameter, the VSWR bandwidth is not particularly wide. This means the antenna sold as a "VHF" antenna will be tuned for the very low end of the band; an antenna sold as an AIS antenna will be tuned for the very high end of the band.

If you connect the wrong antenna to the VHF radio and AIS transponder, you will have a much higher VSWR than if the proper antennas were used.

Many AIS transponders will complain or even stop transmitting if the see the VSWR on their output is greater than 2.0:1.

However, since the both antennas are at the end of long runs of transmission line, there will be significant loss in the transmission line. Transmission line loss tends to mask the true VSWR at the antenna when the VSWR is being read at the transmitter end of the line.

As for mounting the two antennas atop the same mast, that is not a good arrangement. The presence of a second and closely-resonant antenna to the first antenna will cause two problems:

--the radiation pattern will become distorted

--the antenna impedance will change

Neither of those effects are likely to be beneficial.

As suggested, you could assess which antenna is which by plotting their VSWR response. The antenna that is tuned for the low-end of the band will have better VSWR on those channels. You can use your VHF Marine Band ship transmitter as a signal source for the low-end of the band. You cannot use it for the high-end of the band to see the VSWR at the AIS channels; the ship station transmitter does not transmit on those frequencies.
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Old 19-01-2018, 08:40   #15
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Re: VHS and AIS Antennas ? Difference ?

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Originally Posted by wholybee View Post
I stand by my statement. I have a nice Agilent Network Analyzer, and I think it would see the difference. I can see a kink in the cable with it, and tell you how far that kink is from the unit. Or how long the cable is to the antenna, as well as a VSWR plot.

I don't know that the lesser expensive gear would be reliable or accurate enough to be sure it saw the differences. And while some HAMs might have profesional equipment like I do, I don't think it is the norm. Hence my wording "readily available."

Anyway, in less time than it would take to set up and calibrate my equipment, I could climb the mast, install a load or short at that end, and then test for it at the bottom with an ohm meter. And the HAMs I know that have pro gear also climb towers for a living. I think they would jump at the chance to climb a mast and choose that route also.
Sure, your analyzer would deal with this in seconds. But a simple SWR meter (or even the one built into most AIS transceivers) will be perfectly adequate. You'll see the difference immediately, I think, as the tuning really is different, as Paul Elliot pointed out.

The difference in the tuning COULD be masked by some big difference in the feedline -- say the VHF antenna is well installed with a better and/or shorter run of coax and/or better connectors, and the AIS antenna has a problem with the installation. But you can eliminate that by checking SWR on Channel 16 on the VHF as well. If you don't have an SWR meter, they are cheap and useful. I use a Daiwa one which cost less than $100 -- crude and cheap, but perfectly adequate for this job.

Pay attention also to what someone said about location -- don't put the two antennas next to each other. You can find guidelines if you google around. My AIS antenna is on a spreader; VHF at the masthead truck. Both installed with RG-213 cable and a minimum number of quality Molex connectors.
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