Okay, before I address (hopefully for the final time), the original issue of close-spaced, shared-band/freq antennas.....perhaps a few RF and mathematical facts would be helpful here...
And, while I'm more of a lemonade or iced tea guy, I understand the technical complications
Mt. Gay rum
I agree here with Bill...
Originally Posted by btrayfors
While I agree that small gains or losses in signal strength may not be noticed in many instances, larger ones may certainly cause problems.
And, the small losses DO add up!!
And, for the rest of you, some facts for clarification here, might help...
1) When loss (or gain) figures, in decibels (db), are used in regards to antennas, coaxial cables
, splitters, amplifiers, etc. these are power ratio figures, NOT voltage (nor current) ratio figures...
(the only exception is when measuring rf field strengths in uV/M, etc....but then it is known that you are using voltages....all other db figures used here abouts are assumed to be power ratios, and any that are not, will always be designated as such....)
Mixing these things up will cause you serious problems!!
2) Db vs. Power is of course is a logarithmic equation...
Db = 10log (p1 / p2)
dB = 10·log10(P
1 / P
In common usage (out to 2 decimal places) we use 3db as doubling (or halving) power, with 6db as 4 times (or 1/4), and 9db as 8 times (or 1/8), etc. etc....and unless I'm measuring multiple items in the lab, then that is good enough....
So, while some may quibble about 1/1000 error, in this discussion let's just round things off...
3) The Vesper SP160 specs state:
1.0db loss thru VHF radio Transmit path.. (21% loss)
1.0db loss thru AIS Transmit path... (21% loss)
1.5db loss thru VHF radio Receive path.. (29% loss)
(and a 12db gain in AIS Receive path, although that cannot be taken to literally quadruple its AIS receive range, I suspect that it does provide a slight receive improvement, of a few db, in AIS receive...)
Antenna Splitter for Class B AIS Transponders
If I get a chance to actually measure these figures in the real world, I'll let you know.....but I suspect that the transmit losses will be a bit less (better)....
4) For some common db figures, here are the approx. power values...
0.5 db = 11% loss
1.0 db = 21% loss
1.5db = 29% loss
2.0db = 37% loss
2.5db = 44% loss
3.0db = 50% loss
4db = 60% loss
5db = 68% loss
6db = 75% loss
7db = 80% loss
8db = 84% loss
9db = 87.5% loss
10db = 90% loss (1/10 of the power you started with)
13db = 95% loss
16db = 97.5% loss
20db = 99% loss
30db = 99.9% loss
40db = 99.99% loss
50db = 99.999% loss
60db = 99.9999% loss
70db = 99.99999% loss
80db = 99.999999% loss
Just in case anyone wonders why I've ran these figures up so high, the reason is the free-space path loss (line of sight, direct-wave) on VHF-hi-band might surprise some...
At 156mhz, at 1/4 mile the loss is 68.5db....and at 1 mile it is 12db higher at 80.5db!!!
Not to worry though, as our vhf-marine radios can receive signals that are VERY weak....as much as 140db weaker than what the transmitter produces....
FM SINAD specs of the receivers, FM capture ratios, FM signal-quieting once threshold is reached, etc. all enter into how things work....
But, unless we have some tropo enhancement, ducting, etc. typically our systems are pretty limited to line-of-sight plus 10%-15%....
So, when Bill writes that signal strengths make a difference, up to a point,
he is VERY correct!!!
Originally Posted by btrayfors
Does FM signal strength make at difference at VHF frequencies? You betcha it does. Up to a point, anyway.
In any case, I'm not willing to compromise on VHF signal strength -- on transmission
or reception -- since I believe the VHF to be a critical communications
device on a boat.
So, here again Bill points out (without the math and $$$) something quite helpful....
You should all read what he wrote, and take it to heart!!
5) Now, back to the original issue, close spaced antennas on the same band/frequency...
a) None of us know the absolute "do NOT exceed" receiver input level of our particular receivers (AIS or VHF-marine)...
b) In years past, many assumed that +10dbm was a maximum, and while I still think this is a good target, in actual practice most rf receive and test equipment
My own analyzers spec +15dbm to +20dbm as their "max" do not exceed input....and while I have accidentally only subjected one of them to a signal a bit higher, once.....I have some friends who accidentally hit a VERY expensive (~ $75,000) R & S analyzer with a signal about +26 to +30dbm, and ended up with a bill of about $10,000 for repair and recalibration!!!
So, a lesson here...don't play with fire, if you can't afford it....afford both the $$$$ and the loss of use of the radio!!!
c) If you use +10dbm as a max receiver input, you'll be fine!!!
If you use +20dbm, you'll probably be okay, but you must pay attention to antenna spacing, cable losses, radio specs, etc....
d) Those with antennas in close proximity (as my old AIS rec-only antenna and my primary VHF antenna were), are playing with fire....and you need to understand that!!!
6) Bottom line:
Close-spacing (< 6', on VHF-marine / AIS freqs) of same-band / same-freq, is NOT good for AIS transponders and VHF-Marine Radios....and should NOT be done!!!
(although, IF you are willing to accept possible failure of an AIS receive-only system, then closer spacing can be done, but is not recommended....)
Do NOT place your AIS transmit/receive antenna near your VHF-Marine transmit/receive antenna....
Use the spacings and placements I mentioned in earlier posts....
Those who choose to discount these recommendations (provided by myself, Bill, and others of knowledge/experience), and the factual, mathematical information, etc. please take note and understand that the AIS transponder manufacturers, Vesper, Furuno
, etc. (and marine vhf radio manufacturers) also give the same recommendations....
Just saying, if you were new to diesel
engines, etc. and you got recommendations out of your Yanmar
manual, and from an experienced diesel mechanic
, and many who have used, maintained, and repaired their own Yanmar's for years....and they all gave you the same recommendations....
But someone down-the-dock says, "hey, no worries....I know someone who does things completely different, and he's never told me of any troubles."....
Which recommendation are you going to use???
To sum-up, I will not argue what is "right"....just providing the facts, and some learned advice (which was asked for)!!
I do hope this helps....
s/v Annie Laurie