Amazing Power of Good Antennas

[colemj]

Originally Posted by rickusc

I also went back and reviewed Practical Sailors article comparing VHF Antennas, and they gave High Marks to Digital Antennas - being very well made:

Marine VHF Antenna Test - Practical Sailor Article

They have a 4 foot - 4.5dB

4 ft AIS Marine Antennas - 4.5dB Gain

Does anyone have any knowledge or experience with Digital?

We have one - works well.

Mark

[Goudurix]

This and other threads have debated the effects of a 1dB insertion loss from an AIS splitter. 1dB loss is not significant compared to 14dB in line of sight VHF over open water. It is much more important that connections are well made, water is kept out and the cable and connectors are high quality and the right impedance. Also, receiver sensitivity can be a significant factor but most good radios are sensitive enough

First I would like to reply to Transmitterdean's reply. I have tried to evaluate AIS splitter loss in my Raymarine AIS250. Its manual is quite unclear about actual loss but advises "to use a dedicated AIS RX antenna for optimal results".
I did an A/B test using my Yaesu FT897 as receiver since it has an S-meter.(although a rather uncalibrated one so results are only relative). A/B switching with masttop antenna going through AIS splitter to receiver or directly to the receiver.
As transceiver I had a friend moving 1 km away (test 1) then 2 km away (test 2) with an icom handheld VHF set to low power.
Each time I asked him to put the handheld on the ground, antenna pointing in my direction, the operator behind the VHF, only pushing the PTT without any other movements.
Test 1 at 1 km: S9 through splitter - S9+20db directly
Test 2 at 2 km: S7 through splitter - S9 directly.

I know this S-meter does not give absolute & standardised results. I have no microV readings etc.

But I'm sure it DOES show that I have substantial loss in the splitter and the difference between receiving an S3 signal or an S1 signal lost in the noise makes a difference.

As for the last post, all these marine VHF type antennas will advocate 3 to 6 dB gain as is normal for a 1/2 wave or 5/8 WL type of VHF antenna. It is usually expressed as dBi so as to compare it with a theoretical isotropic dipole "in free space".
In theory they all have comparable results.
What I want to know is what type of antenna it is (dipole? 5/8 WL? End-fed half wave) and how it is built.

Jan

[transmitterdan]

If you have 20dB loss either the splitter is defective, connectors are bad or there is another installation error. Splitters require DC power. Are you sure it is getting the right power and is otherwise installed correctly?

Raymarine has an excellent web forum for technical help. They usually respond within few hours during normal business hours in the Americas.

[Goudurix]

Dan,

I'm sure the loss isn't 20dB I think this test was far from standardised and it is difficult to interpret readings from the FT897 LCD S-meter display.

I rely more on the "so called S7 to S9" difference where the TX signal was already enough weakened to avoid strong signal effects.

But I do think the loss in the Raymarine AIS250 is more than 1dB more in the 3dB range - I think I did read or hear that figure somewhere but can't recall where.

Jan

[Goudurix]

Dan,

it was in a thread I had started.

SSCA Forum • View topic - Interference from AIS on marine VHF

3 to 4 dB loss is to be expected?

Jan

[transmitterdan]

Originally Posted by Goudurix

Dan,

it was in a thread I had started.

SSCA Forum • View topic - Interference from AIS on marine VHF

3 to 4 dB loss is to be expected?

Jan

It depends on the design. It's possible to design a splitter that has 3-4 dB loss on the receive side (not transmit). But that would qualify as a cheap design in my opinion. When I get to a real high speed location I'll try to find how Raymarine do the splitting.

[Goudurix]

Dan,

I did not find a schematic diagram for the Raymarine AIS250 (yet).

My previous one was indeed faulty and having a problem with AIS reception.

It was replaced by Raymarine by a brand new one, and I did my splitter loss test with the new one.

Anyhow I will get back to using a dedicated AIS antenna on the pushpitbut will switch off the AIS box whenever I use my HAM SSB since I suspect the proximity of the sloping end-fed wire to have damaged the receiver of my previous Raymarine AIS250.

So I won't be bothered by splitter losses.


Jan

[transmitterdan]

Be careful as you can damage a receiver even if it is powered off.

[Goudurix]

Transmitterdan,

Hmm....the AIS dedicated antenna is about 3 metres away from the base of the ssb sloping wire antenna.
Following your warning, to really protect the receiver I would have to include a caxial switch that switches to input to ground....
Given the fact that I'm talking SSB (40m - 15m HAM bands) versus VHF and mainly a risk for harmonics, do you really think there is a risk?

Jan

[transmitterdan]

I don't know if there is a filter inside the AIS or not. If you already had one receiver fail due to SSB overload then I would be tempted to try a high pass filter at the input to the AIS receiver. This might work (http://www.minicircuits.com/pdfs/NHP-50+.pdf). Note this would only be for an AIS receiver. I would not try it with a transponder. I could not find one with UHF connectors so some adapters would be required. I admit I did not look very hard. A Ham shop might have a high pass with the right connectors.

[ka4wja]

Jan,
I don't have a print of the Raymarine AIS250, nor do I have any specs on it....
But, I doubt the AIS antenna's proximity to your SSB transmit antenna has anything at all to do with your AIS250's problems....

Originally Posted by Goudurix

...the AIS dedicated antenna is about 3 metres away from the base of the ssb sloping wire antenna.
Following your warning, to really protect the receiver I would have to include a caxial switch that switches to input to ground....
Given the fact that I'm talking SSB (40m - 15m HAM bands) versus VHF and mainly a risk for harmonics, do you really think there is a risk?

NO, there is no risk...

1) 3 meters (~ 10 feet) is more than enough separation....
Even if they were both VHF radio systems, this would still probably be adequate separation to prevent damage, but with one being HF and one being VHF, you have no worries...
I have run >1000 watts on HF, with VHF antennas within just a few feet (approx. 1 meter away), for many years and have never had an issue...


2) Next, whatever the problem with your AIS250 is/was, it was NOT caused by your HF antenna's radiation...
And, if you care to read over some stats/facts/calculations regarding VHF antenna-to-AIS antenna isolation/separation, please have a look at this thread here...
http://www.cruisersforum.com/forums/...ml#post1286802
While, not specifically dealing with HF-to-VHF isolation/separation, it should help...

Also, if you think about it, most sailors with backstay antennas have the end of their backstay antenna only a few feet (about one meter) away from their masthead VHF antenna...(this is how my backstay antenna is rigged)..
No issues ever...

Further, there are some sailors who simply feed their grounded stay and mast, and use this as their HF antenna...with the VHF antenna mounted/screwed directly to the top of the alum mast, with the VHF antenna base/ground in direct contact with the HF antenna energy, and the actual VHF whip only an inch (2cm) away from the HF antenna....
And, here again, no issues ever...

Sorry to be so blunt, but anyone that tells you your HF antenna is too close to your AIS antenna, either doesn't really understand any of this every well at all, or is just BS'ing you!!!



3) as you see, there is no need for you to worry about a "switch" to ground the AIS250's input....
Nor any need for a "hi-pass filter"!!!

Heck you could use the same coax for your AIS transponder and to your remote HF tuner, without either radio causing any problems for the other, if you used a "diplexer" with as little as 30db of isolation....but commercial ones will have as much as 45-50db of iso between HF and VHF....


4) Bottom line:
Save your $$$$ (or Euros)...
You are chasing fake dragons looking for solutions to problems that don't exist!!!

Now if you want to talk about scraping that Raymarine AIS250, and buying a real AIS unit....that would be a great idea...
But, please try to forget much of the hyperbole and BS in this thread!!!


~~~~~~~~~~~~~~~~~

5) Regarding your testing of the AIS250's built-in "splitter"/relay losses....
Your results are as expected....
A passive splitter WILL have approx. 3.5db loss...(although I've seen some crappy ones that were close to -6db loss!!)
You are SPLITTING the received signal in half, which -3db..(plus some losses in the splitter design itself)....

On transmit, these "splitters" are NOT splitters!!!
They are RF-sensed relays....where they SWITCH the antenna to the transmitter (AIS or VHF) that requires it at that time...(usually the "power-off default" is the VHF radio is connected to the antenna)
In the transmit path there is NO "splitting" of the signal, just the loss thru the relay and associated circuitry, which should be less than 1 db, but can be as high as a few db...

If you have an "active splitter"/relay, there would be a low-noise high-input-level receive pre-amp built-in, that would not only compensate for the normal splitter losses, but could also reduce your receive system noise level AND actually improve your reception!!
The only "active splitters"/relays that I'm aware of, only place this receive pre-amp inline with the AIS receive line and not the main VHF receive line, as they desire complete fail-safe full-operation of the VHF radio in the event of power failure to the "splitter" or a "splitter" failure itself...

The ONLY AIS-VHF "splitter"/relay that I recommend is the Vesper SP-160, which is an "active splitter"/relay, with excellent specs!!!
Antenna Splitter for Class B AIS Transponders
Yes, it does sell for about $250 USD....but it IS worth it!!!

Specifically to your AIS250's "splitter"....it is crap! and I suspect its loss is about 4db, on receive....(no better than 3.5db), and I don't know what on transmit (but should be much lower loss)...
And, your tests results are fine....while some untrained people may think that "2 S-units" or "a 20db difference" on an S-meter is too much to be true, they should understand that in FM mode, most S-meters are VERY VERY inaccurate, and are not really measuring signal strength at all, as they would in amplitude-modulated mode (SSB, AM, or CW), as FM doesn't use what we'd call an AGC (which is where the S-meter gets its data/signal from), but rather a limiter and discriminator where there is little change once the receiver is above threshold or quieting....
So, use of the S-meter in FM-mode will not be very accurate...
BUT...
But, in your situation where you were just trying to compare two signal levels, your s-meter DID a good job proving your suspicion was correct, that the AIS250 has significant loss thru its "splitter"!!!



~~~~~~
Sorry, I don't have the time to go into further details tonight....but I think you get the gist...


I hope this helps...


John
s/v Annie Laurie

[Goudurix]

Thanks, John.
I will forever remain in doubt why my AIS250 developed a problem maybe just bad luck some diode or SMD capacitor died....
By means of precaution I will however shut down the AIS when using the SSB...one never knows...
Thanks,
Jan

[transmitterdan]

Originally Posted by Goudurix

Thanks, John.
I will forever remain in doubt why my AIS250 developed a problem maybe just bad luck some diode or SMD capacitor died....
By means of precaution I will however shut down the AIS when using the SSB...one never knows...
Thanks,
Jan

Gourdrix,

Take a close look at the spacing of the VHF antenna to the back stay. The distance that is important isn't from the base of the backstay to the VHF but rather the shortest distance as the crow flies from wire to wire. If the VHF is on the pushpit it's hard to visualize 3M separation unless the boat has 6M beam.

Since you are not planning to use the splitter function of the AIS250 no need to worry about that. If you want to use an external splitter so the mast head antenna can service AIS then Raymarine (AIS100) and Vesper (SP160) both make active splitters with low loss. Note that having the AIS antenna on the pushpit and thus much lower to the water gives up a significant signal gain over a mast head antenna. So even though you save the loss of the splitter you give up about the same or maybe more in reduced antenna height. There is no perfect solution. Often different solutions turn out to be about the same in performance.

Infant mortality is definitely a possibility. But if it happens again then something else should be suspected.

P.S. I have pretty much ignored the name calling in the previous post because it doesn't add much to the discussion. It is the Internet after all.

[Goudurix]

Hi Transmitterdan,

Well my ssb antenna is a sloping wire led towards the starboard side of the pushpit, from there going down with standoff isolators to the through-deck isolated lead and the ATU belowdecks.

I do not use a (real) backstay antenna.

The to-be-dedicated AIS antenna is on the opposite, port side of the pushpit and indeed 3 m away.


I do think the problem occured after (-2 years ago and after using the AIS, marine VHF and splitter only on the masttop VHF antenna) I detected interference in the marine VHF coming from the AIS onto channels adjacent to the AIS frequencies (161.975 and 162.025 if I'm right). Being on a number of duplex channels used by coastal stations for securite or weather forecasts. Switching off the AIS stopped the interference.
For this reason I decided to install a dedicated AIS RX antenna on the pushpit, knowing very well that I would decrease the reception range as compared to the 14m high masttop antenna.

I suspect the problems started then, maybe because of the Yaesu FT897D overloading the AIS entry stages maybe some harmonics. It is only a guess since my dealer claimed never to have seen this kingd of problem.

After 2 service returns they gave me a banad new replacement which has been working oK for several weeks, but still using splitter and masttop antenna.

Jan

[transmitterdan]

Jan,

Probably the AIS was not damaged by the SSB antenna. Powering the AIS off won't make it less susceptible to damage.

Good luck with the replacement.