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Old 17-01-2021, 21:15   #1
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Antenna splitters and signal loss

I read the thread on 'AIS antenna choice?' until my brain hurt. But I have a different question.
I have a new (to me) boat and exploring antenna setup I noticed I have a Shakespeare AIS antenna on the stern pulpit that is not connected to anything.
I have an Icom MXA-5000 AIS receiver. As far as I can tell, this is connected to the antenna on top of the mast (via a connector that has been in a wet bilge). The mast antenna leads to a splitter in the nav station that splits AM/FM and VHF. AM/FM leads to my stereo. VHF leads to the Icom AIS unit, and hence to my VHF. However, my understanding is that the Icom unit also includes a VHF splitter. So one splitter separates out AM/FM, and the second splitter separates AIS and VHF.

Question: with two antenna splitters inline, am I losing signal strength? Am I better off re-attaching the dedicated Shakespeare rail-mounted antenna to the Icom AIS, and getting rid of the AM/FM/VHF splitter, and leading the mast antenna directly to my VHF?

Note: I'm not really concerned about long-range AM/FM reception. If needed, I'd be happy with a separate dedicated AM/FM antenna.

On one hand, it would seem like a mast-top antenna would get me better/longer range AIS coverage. But am I better off with a dedicated AIS antenna, even if it's mounted lower?

Final note: my current AIS is receive-only, but I intend to upgrade to a transceiver soon, so this may affect the antenna choice.

Appreciate any opinions or advice.
Thanks
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Old 17-01-2021, 21:53   #2
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Re: Antenna splitters and signal loss

The best solution is to have dedicated antennas for whatever service you are using. Combining antennas for multiple services will invite unnecessary attenuation in the signal path. I direct your attention to police cars. Ever notice the antenna farm on the top of the squad car? Sure, they could combine antennas for their different service within the car, but that will just cause problems.

Best to keep AIS antenna to AIS system, VHF to VHF and AM/FM to that entertainment center.

JMHO
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Old 17-01-2021, 22:15   #3
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Re: Antenna splitters and signal loss

There's no reason to have the AM/FM go to a VFH antenna. That's just marketing spin. The standard 34" 1/2 wave VHF masthead whip antenna is the wrong length for the FM frequency.

There's a lot of reason to use a splitter for VHF and AIS on a single masted boat. Both have much better range with height. And you can't put two VHF antennas on the same masthead without interference. If you own a ketch or schooner - you're all set

The better splitters (like the one made by Vesper) have virtually no loss. The Vesper even includes an amplifier. Vesper's newest AIS/Radio called the Cortex comes with the splitter built in.

Here's a fairly lengthy discussion about masthead antenna best practices. As he points out, high quality coax and corrosion free connectors that were properly soldered make all the difference.

http://honeynav.com/wp-content/uploa...ax-testing.pdf
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Old 18-01-2021, 09:08   #4
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Re: Antenna splitters and signal loss

There is a significant difference between splitters and combiners, and one must also consider the point of view from the INPUT to the device.

When receiving, the antenna is the INPUT. Signals from the antenna are divided between two (or more) outputs. There may be frequency-sensitive FILTERS in a splitter that restrict the input signal to being available at a particular output terminal by its frequency range.

The typical "splitter" will impose a loss of signal to at least one-half power from input port to two output ports.

Generally there is no problem in having two RECEIVERS connected to a common antenna. The input circuit of a receiver is an amplifier that will immediately increase the strength of the input signal. This amplification will usually overcome any loss in signal in the splitter itself, up to a point. The "point" will occur with input signals of extremely low level, which, if attenuated by the splitter, may become so weak that they arrive at the receiver input stage below the inherent noise level of the receiver's amplifier, and thus will be lost in the noise. To overcome this, some "splitters" contain their own amplifier that may pre-amplify the signal before it is split. This method avoids losing really weak signals in the noise of the following amplifier due to loss in the splitter.

In a VHF Marine Band radio which as an AIS receiver integrated in the same chassis, the antenna signal from the RECEIVE side of the RECEIVE-TRANSMIT relay is routed to multiple receivers. Typically there will be a VHF Marine Band receiver, a dedicated DSC receiver, and two dedicated AIS receivers.


From the point of view of TRANSMITTING into a COMBINER, there will be two INPUTs and one OUTPUT. An antenna will be connected to the OUTPUT, and two transmitters connected at the two INPUTs.

Note that there is NO simple means for a "combiner" to effectively allow two transmitters to send their signal to a single antenna. While it is POSSIBLE to have two transmitters connected to a combiner and feeding one antenna, this sort of arrangement requires very precise conditions to be met. For example, if the transmitters are transmitting the same signal on the same frequency, the precise phase of the signals must be identical so they will combined in an additive manner. If the signals are on different frequencies, the combiner must be designed with extremely sharp, narrow-band filters so that the input signal from one transmitter does end up appearing at the input port of the other transmitter.

Devices that perform this sort of combining are called duplexers, and are usually limited to two transmitters. A duplexer will also allow a receiver to remain connected to an antenna while a transmitter is also connected to the same antenna, as long as the transmitter frequency is fixed and can be deeply attenuated with a notch filter.

For use with VHF Marine Band radios and with AIS transmitters, the typical device (called by various names) permits only one transmitter to be connected to the antenna at a time. This is accomplished by very rapidly detecting the presence of transmitter power at an input and switching the antenna to only that input for the duration of the transmission. These devices should be called high-speed automatic antenna switches rather than "splitters" or "combiners."

When the antenna is switched to one input for transmission, the antenna is no longer connected to the other input. A dummy load may be connected to the other input so that if that transmitter is keyed up it will see a proper load.

As a general consideration in designing a radio system for highest reliability, the number of connectors, cables, and other devices inserted between the radio and its antenna should be kept as low as possible. For a device like a VHF Marine Band DSC radio which is a primary safety device aboard a ship, there is no good justification for inserting multiple splitters or combiners into the transmission line path between radio and antenna.
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Old 18-01-2021, 09:23   #5
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Re: Antenna splitters and signal loss

Regarding reception of FM Broadcast Band signals in the frequency range 88 to 108-MHz by using a tuned vertical antenna resonant at 156.8-MHz, there are three problems:

--most FM Broadcast Band signals are sent with horizontal polarization, so reception with a vertical polarized antenna results in cross polarization and a loss by a factor of 20 dB or a reduction by a ratio of 1:100;
--some FM Broadcast Band signals MAY have some vertical polarization, but this depends entirely on the station; it is also typical that power in the vertical plane, if any, will be less than the power in the horizontal plane; and
--the impedance of an antenna designed for resonance at 156.8-MHz will not be the same at 88-MHz to 108-MHz, resulting in a poor match to the transmission line, and further attenuation of the signal.

Regarding reception of AM Broadcast Band signals from 0.5 to 1.5-MHz, the same same conditions apply as above, except the cross-polarization; AM Broadcast Band signals are generally always transmitted with vertical polarization.

I expect that is an inexpensive "splitter" designed for using a VHF Marine Band antenna for reception of AM Broadcast Band signals were examined, one would find that the "antenna" routed to the AM band output is really just the outer conductor of the transmission line being used as a random length wire with expected vertical orientation.
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Old 18-01-2021, 09:44   #6
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Re: Antenna splitters and signal loss

This may also cause your head to hurt but part of it is on point:
https://cruisingclub.org/sites/defau...30%20FINAL.pdf
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Old 18-01-2021, 09:49   #7
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Re: Antenna splitters and signal loss

I have a B&G Zeus2 set up with a Splitter that takes VHF, AIS and Radio (FM / AM). VHF and AIS works fine, but the radio reception is very bad. I have changed the splitter (no difference), I have changed the Fusion RA55 (no difference) and I have changed the radio again to a Fusion Apollo RA670 (no difference). Now I have connected the Fusion Radio to a separate vertical antenna (3 feet) and it helped a lot. Maybe the antenna should be horizontal (as I've learned above)
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Old 18-01-2021, 09:56   #8
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Re: Antenna splitters and signal loss

Quote:
Originally Posted by maclysaght View Post
Question: with two antenna splitters inline, am I losing signal strength?

Some. If the splitters are designed for the purpose and working properly, probably not much.



Quote:

Am I better off re-attaching the dedicated Shakespeare rail-mounted antenna to the Icom AIS, and getting rid of the AM/FM/VHF splitter, and leading the mast antenna directly to my VHF?

I think you should get rid of the AM/FM/VHF splitter for the reasons the posters upthread have stated at length.


Moving your AIS to the other antenna and getting rid of the AIS splitter will improve your voice VHF performance slightly because there will be less loss, and degrade your AIS performance considerably because the AIS antenna will not be as high. You will have to decide whether this is a good tradeoff.


There are many boats that have one antenna for both VHF voice and AIS. It works well, is cost effective, minimizes windage, and makes the best use of the top-of-mast antenna position.
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Old 18-01-2021, 10:00   #9
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Re: Antenna splitters and signal loss

Quote:
Originally Posted by Hermia II View Post
I have a B&G Zeus2 set up with a Splitter that takes VHF, AIS and Radio (FM / AM). VHF and AIS works fine, but the radio reception is very bad. I have changed the splitter (no difference), I have changed the Fusion RA55 (no difference) and I have changed the radio again to a Fusion Apollo RA670 (no difference). Now I have connected the Fusion Radio to a separate vertical antenna (3 feet) and it helped a lot. Maybe the antenna should be horizontal (as I've learned above)
Hermia, I have the same setup as you, but with the Zenus3.

I have no vhf reception on my fusion RA205 AM and FM work great. Any ideas? It worked when I bought the boat. Thank you
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Old 18-01-2021, 10:19   #10
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Re: Antenna splitters and signal loss

Quote:
Originally Posted by CarlF View Post
There's no reason to have the AM/FM go to a VFH antenna. That's just marketing spin. The standard 34" 1/2 wave VHF masthead whip antenna is the wrong length for the FM frequency.

There's a lot of reason to use a splitter for VHF and AIS on a single masted boat. Both have much better range with height. And you can't put two VHF antennas on the same masthead without interference. If you own a ketch or schooner - you're all set

The better splitters (like the one made by Vesper) have virtually no loss. The Vesper even includes an amplifier. Vesper's newest AIS/Radio called the Cortex comes with the splitter built in.

Here's a fairly lengthy discussion about masthead antenna best practices. As he points out, high quality coax and corrosion free connectors that were properly soldered make all the difference.

http://honeynav.com/wp-content/uploa...ax-testing.pdf
FWIW, I recently had an email conversation with Vesper/Cortex tech support regarding antenna solutions for their Cortex product. The AIS/VHF antenna (VA159) Vesper offers does not fit the 1” 14 thread on Shakespeare masts. The VA159 comes with a mounting bracket for a flat surface mounting or can be adapted to a rail I imagine.
I don’t have anyplace to mount the VA159 and would like to replace an existing Shakespeare VHF antenna with a combo AIS/VHF antenna. Vesper tech support recommended I use the Shakespeare 5225 antenna. A splitter is not required with this setup.
BTW, I have not read the article mentioned, but am surprised “soldering” is suggested. ABYC poo poos solder anywhere in marine environments due to excessive corrosion. Marine grade stranded wire, tinned or not, along with marine grade ring crimp termination are recommended. Ring terminals with heat shrinkable insulators are best, adhesive inside the heat shrink portion are even better to keep moisture out.
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Old 18-01-2021, 20:04   #11
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Re: Antenna splitters and signal loss

Quote:
Moving your AIS to the other antenna and getting rid of the AIS splitter will improve your voice VHF performance slightly because there will be less loss, and degrade your AIS performance considerably because the AIS antenna will not be as high. You will have to decide whether this is a good tradeoff.

Ok I have to ask: It makes sense to me to have the VHF antenna as high as possible - I might want every advantage to hear and be heard at the greatest range. But to my thinking I probably don't care too much about receiving an AIS signal at a 26 nautical mile range. So I don't see much practical loss in locating a separate AIS antenna lower down - ditching any splitter and attendant issues. Thoughts?
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Old 18-01-2021, 20:25   #12
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Re: Antenna splitters and signal loss

https://www.capitalgazette.com/news/...817-story.html


They were doing 25 knots.

How much warning would you have wanted?
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Old 18-01-2021, 21:24   #13
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Re: Antenna splitters and signal loss

Quote:
Originally Posted by Jammer View Post
https://www.capitalgazette.com/news/...817-story.html


They were doing 25 knots.

How much warning would you have wanted?



Point taken!
However, at some point, no amount of electronics can compensate for stupid. Even if the AIS only reached out 3 Nm that is still 7 minutes warning. But I have no idea what lead to the unfortunate circumstances in the picture so
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Old 18-01-2021, 21:47   #14
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Re: Antenna splitters and signal loss

Ray marine have a powered splitter that has virtually Nill reduction in signal strength. When I upgraded to an Ais transponder a normal splitter was not strong enough so the powered one had to be installed. Great signal
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Old 18-01-2021, 21:55   #15
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Re: Antenna splitters and signal loss

I'm not worried about some idiot in a pleasure boat.

I'm worried about a tired deck officer on a 900ft containership that doing 20 knots who's not watching the AIS display as carefully as he should.

And Class B AIS only transmits every three minutes.
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