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Old 24-07-2013, 11:23   #61
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Great thread.

Makes me think I should reconsider having both antennas on my masthead. What I worry about with any lower position is blind spots caused by the mast, rigging, etc., but I'm no expert. For example if you put an antenna on your aft rail don't you get a pretty good blind spot exactly forward of the boat, right where you don't want it? How should bind spot considerations weigh in on this discussion?

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Old 24-07-2013, 11:28   #62
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Re: AIS transponder and VHF antennas on the masthead ?

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Originally Posted by jr_spyder View Post
Great thread.

Makes me think I should reconsider having both antennas on my masthead. What I worry about with any lower position is blind spots caused by the mast, rigging, etc., but I'm no expert. For example if you put an antenna on your aft rail don't you get a pretty good blind spot exactly forward of the boat, right where you don't want it? How should bind spot considerations weigh in on this discussion?

Thanks,

JR
A stern-rail mounted antenna will see little or no blind spot from the mast. The wavelength of the AIS / VHF signal is long enough that the mast will be essentially invisible. If the antenna were much closer to the mast you might have a problem. For what it's worth, my AIS antenna is on the upper spreader and is pretty close to the mast and the shrouds. I've seen no obvious blind spot, but without careful tests it's hard to say -- there could be some effects.
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Old 24-07-2013, 12:33   #63
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#1 on masthead -the works, di-pole like the Little Giant discussed before, LMR400 cable
#2 on upper spreader -also the works like above but now the AIS variant, with LMR400 cable
#3 on lower spreader -dual band 2m/70cm HAM antenna with LMR400 cable
#4 on bimini/arch or one of those emergency stubbies stowed in a drawer.

A ketch can put #2 at the mizzen masthead and then use main and mizzen upper spreaders for 3&4. Leaves two pairs of lower spreaders for more goodies
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Old 24-07-2013, 12:49   #64
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Re: AIS transponder and VHF antennas on the masthead ?

OP " I am thinking of replacing all lights with LED's but I am also thinking of adding an AIS transponder antenna to the masthead."

Some LED nav lights produce RF noise near the AIS frequency, and can reduce both your transmit and detection range if placed close to the AIS antenna.

Other, better, LED nav lights produce essentially no noise. So be a little careful.
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Old 24-07-2013, 12:52   #65
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Re: AIS transponder and VHF antennas on the masthead ?

This is a sailing website....sometimes I forget...
(I purposely avoided the erroneous "facts" in another discussion of vhf propagation, as I figured no casual marine vhf user cares!!....
But here, I thought that the technical details were important to understand the situation....)

[BTW, I'm working at a client's office today, and all below is just off the top of my head...so if a make a minor error, forgive me, as I don't have the time to proof-read, etc...]

So, my apologies for drifting the thread away from the primary question of antenna spacing.....while I was also trying to show what "db's" mean to the users of vhf marine radio, I suppose I added too much "stuff"....
Sorry about that!!!


Back to the original point of the discussion...
Do NOT place your vhf and AIS antennas too close, or you'll have issues/problems (see earlier postings for details)





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


As for Dan's point about a db being a db....yeah, literally true...but...
But. when figuring db's in voltage (or current), rather than power (watts, dbm, etc.) the equation (and answers) are different....
And, I do not wish some (non-tech) folks here to go away with the wrong information....

So, as I wrote earlier, Db vs. Power gain is of course is a logarithmic equation...
Db = 10log (p1 / p2)

But for voltage gains....
Db = 20log (v1 / v2)

Just wanted everyone to understand that I was referring to power loss/gain, and using the power formula....
No big deal, just wanted to be clear...





Now, as for path loss...
Here again, I probably shouldn't have even mentioned it.... Sorry about that...but, since I did, it's up to me to clarify things....
(Sorry to ramble on and on here...but I think it best to clear things up!!!)

As, the issue is not what a 1 to 2 db change in signal level does to the theoretical path / signal levels /received C/N, but rather what this does to the received S/N ratio in an FM radio system...especially with marginal signals...

A one to two db increase in rf C/N, can make the difference between actually being able to hear someone or not....
This is NOT to say that anyone can tell the difference of a one db change in an AM or SSB signal, but rather how that one db change effects FM reception with marginal signals (those at, or near, threshold)

FM has a nice characteristic in that it has excellent noise quieting...
A change of RF s/n (usually referred to as C/N, so as to not be confused with receiver S/N), of only 1 to 2 db can have a drastic effect on receiver S/N...
When at or near the FM receiver threshold (typically 6-10db) a increase of 1 to 2 db in the C/N (rf S/N), will result in a massive increase in received S/N, of typically 15-20db, or more...
(note that this is NOT the case for AM, or SSB, signals which have a rather linear C/N vs. S/N curve....)
Further FM receivers use "limiters" which remove most of the natural and man-made RF noise (which is AM) from the path, which adds to the improvement of rec s/n above threshold.....(and many FM transmissions and receivers use pre-emphasis/de-emphasis which even further increase the received s/n, once above rec threshold)
FM does require more bandwidth than AM (for the same baseband signals), but once threshold is reached, the received s/n is very high....
Although modern two-way radio's audio amps aren't great, and do limit this final s/n, you can easily see receiver s/n's 20 - 30db (or more) above the received c/n, especially with marginal signals...

Please understand that when signals are strong, you usually will not notice any variation between other boats with signals varying +/- 20 - 30db (or more), BUT...
But, when signals are weak/marginal (at, or near, threshold), and you hear lost of noise, a 1 to 2db increase in the other boat's signal can make a massive improvement in your received S/N.....
I believe this is what Bill was trying to convey in his post....

Okay, enough rambling about equations, db's, path loss, etc...
This is a sailing site!!!


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




Back to the original query...

Fair winds.

John
s/v Annie Laurie
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Old 24-07-2013, 13:03   #66
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Re: AIS transponder and VHF antennas on the masthead ?

jr,
Paul has given the straight scoop!!
No worries about masts, rigging, etc. causing blind spots when mounting a VHF / AIS antenna on the stern rail...

And, I wouldn't worry about reflections / nulls ("blind spots"), unless your antenna was really close, < 1/2 wavelength (> 3'), away from something big like your mast....(and even then the yawing, rolling, etc. at sea will certainly move any "blind spots" around...)




And, Evans' point about some LED tri-color nav lights is a great point to be aware of....
I don't have any problems with my OGM tri-color/anchor combo, but I do know some that have had an issue....
(you can check your own lights/radio, using your vhf radio and squelch level.....does the squelch level need to turned higher, when you switch on the LED tri-color???, if not, you're probably fine!!)



Gotta' go...
fair winds..

John
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Old 24-07-2013, 14:43   #67
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Re: AIS transponder and VHF antennas on the masthead ?

My AIS is currenlt on my stern rail and today had one ship at 37nms that I received through the length of the island of Grenada i.e through mountans (800 meters high and many at 600 meters) and there wern't no ducks nowhere!

I curently have 2 at 20 NMS, neither in direct line of sight.

Dunno why it works but it does
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Old 24-07-2013, 14:48   #68
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My AIS is currenlt on my stern rail and today had one ship at 37nms that I received through the length of the island of Grenada i.e through mountans (800 meters high and many at 600 meters) and there wern't no ducks nowhere!

I curently have 2 at 20 NMS, neither in direct line of sight.

Dunno why it works but it does
I bet you're on the rum punch at Hog Island
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Old 24-07-2013, 14:48   #69
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Re: AIS transponder and VHF antennas on the masthead ?

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Originally Posted by estarzinger View Post

Some LED nav lights produce RF noise near the AIS frequency, and can reduce both your transmit and detection range if placed close to the AIS antenna.

.
Yep, I put the new series of Aqua Isgnal LED nav lights at mast top and they cause a distinct drop in AIS reception. So I dont use those nav lights at all
They arnt the tricolour but red and green nav lights.

I put a ferrite on the antenna supplied by a Raymarine agent but it doesnt work.


Mark
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Old 24-07-2013, 14:49   #70
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Re: AIS transponder and VHF antennas on the masthead ?

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I bet you're on the rum punch at Hog Island
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Old 24-07-2013, 16:51   #71
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Re: AIS transponder and VHF antennas on the masthead ?

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Yep, I put the new series of Aqua Isgnal LED nav lights at mast top and they cause a distinct drop in AIS reception. So I dont use those nav lights at all
They arnt the tricolour but red and green nav lights.

I put a ferrite on the antenna supplied by a Raymarine agent but it doesnt work.


Mark
The reason the ferrite didn't help is that the switching regulator in the light is in the light, not at the other end of the cable. Additionally the switching regulator circuit is poorly designed allowing RF radiation, and lastly it is placed very close to the antenna.
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Old 24-07-2013, 17:15   #72
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Re: AIS transponder and VHF antennas on the masthead ?

OK, I hear the logic about spacing ---- but what sort of boat do you have?? I simple have no options unless I am going to dedicate my boat to VHF and AIS antennae (note the correct plural!!). As stated previously we have 3 antennae positioned about 8 inches apart in a triangular arrangement at the mast top (26ft). We get excellent VHF reception/transmit and so far as I am aware excellent AIS reception (no transmit). With the VHF radios off, there is no obvious change in AIDS reception. And frankly, if we lose a few miles, so what?? We have used this system very effectively for the past 4 years and it works more than adequately for our needs. We also have a 65 mile radar but we never use it at that range!!
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Old 24-07-2013, 18:55   #73
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Re: AIS transponder and VHF antennas on the masthead ?

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......and AIS antennae (note the correct plural!!).
Don't be so sure.

antennae - definition of antennae by the Free Online Dictionary, Thesaurus and Encyclopedia.

and

Antennae vs. antennas - Grammarist

Common usage differs according to what part of the world you are in. So, when referring to insects it is always antennae, but for radios ... it depends where you are.
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Old 25-07-2013, 20:18   #74
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Re: AIS Transponder and VHF Antennas on the Masthead?

Hey, love this topic. I've been lurking here and following this thread. I'd like to suggest a couple of possibilities regarding the positioning of antennas and coax. One possibility for location, if practical, is to mount upside down on the bottom of a spreader directly below one on the top. There is pretty good isolation in this area and while not the ideal location, it could be a good trade-off when there are 2-4 antennas competing for prime real estate. You could still get some decent height while not being in the same plane.

As far as coax cable, I recommend LMR240 on VHF runs<100'. The difference between it and LMR400 is minimal and practically undetectable without test gear. For example, at150MHz, LMR400 specs indicate 1.2dB loss per 100, while LMR240 is 3dB per 100'. The difference boils down to 1.8dB which, from my experience, is negligible and undetectable in most real world operating situations. In other words, most people wouldn't be able to distinguish between the two.

The advantage of LMR240, besides cheaper price, is size. It's 1/4 inch in diameter and easily flexible. Two or three runs of LMR400 up a mast can get tricky making it fit in some cases. Connectors are slightly cheaper too.
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Old 26-07-2013, 08:05   #75
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Practical Sailor tested a number of masthead tricolor LED modules for RF interference production. There was a wide variation. IIRC the most RF quiet was a module from LunaSea Lighting. I installed this in my masthead tricolor, which was adjacent to the VHF antenna. There was even with this module a noticeable amount of RFI.

While best practices will go far to reduce RFI coming out of a switching current regulating circuit supplying LEDs, placing it immediately adjacent to your antenna is going to be a severe test of this that few will pass. Even a foot or two of separation can make a huge difference.

I would keep an AIS antenna well away from an LED module. I can temporarily switch off the tricolor (and use the deck level running lights instead) if I need best performance from the voice VHF, but that is not a practical solution for potential interference with an AIS receiver. My AIS antenna is on a radar arch. No complaints about AIS range with this position.
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