Hello to all,
A couple years ago, I upgraded from an
AIS receiver-only (a stand-alone SiTex "Ais Radar", that I'd been using since 2006), to a Class B
AIS transponder and Vesper Watchmate 650 AIS Display. I installed an aft-rail-mounted, bimini-height,
VHF whip for my new AIS transponder, as I had been using a separate masthead-mounted VHF/UHF ham
radio whip for my old AIS receiver.
This all worked well, and I wrote an article (w/ photos) describing my decisions,
installation, and results.
http://www.c470.jerodisys.com/470pix/47148.htm
But, I also considered using a Vesper SP-160 "splitter / relay", that would allow both my primary
VHF (M-602) and my new AIS transponder to share the masthead VHF-Marine
antenna, as my "range" to/from other Class B AIS targets was limited with the lower
antenna.
Since the SP-160's "fail-safe" mode (and powered-down mode) allows full use of the primary VHF
radio, and I'd have a PL-258 barrel connector nearby to bypass it completely if ever needed, and since its specs showed a VHF thru loss of < 1.5db (transmit and receive), and its 12 db gain pre-amp for the AIS unit was spec'd to improve AIS
reception / received S/N by approx. 6db, this seemed like an excellent piece of
gear....
So last year, I went ahead and added the SP-160, and found (no surprise) excellent results....much improved AIS range (as expected), and NO noticeable degradation in primary VHF radio performance, both transmit and receive worked as well as ever. And, I now have the aft-rail-mounted antenna (about 10' off the water), fed with 35' if LMR-400uf, as my spare/back-up VHF antenna...
I attributed the good results to the Vesper SP-160's excellent specs, which say < 1.5db thru loss on primary VHF (transmit and receive) and a 12db gain amplifier on the AIS output (which is supposed to increase received AIS target S/N by approx. 6db)...
I made various transmit / receive tests, over multiple days, using SeaTow's "automated radio check"
system (which I can almost always use two of their systems, right from my
dock, one close-by and one 25+ miles away...and many times also use a third one, ~ 30 miles away on a lower antenna), as well as using NOAA
Weather Radio stations (of which I can always get at least 3 - 4, and many days I get 2 more NOAA stations that are weak, but readable....
Bypassed vs. Thru the SP-160 showed NO difference at all in any of the "automated radio checks", NOR in any of the regular NOAA stations, except for the very weakest (which at times was unreadable)....but even though there was some slight difference noticed by me, it was not enough to change the readability / intelligibility of the signal, and unless you were a trained/experienced radio op, it is doubtful that anyone would ever be able to notice this ever so slight difference...
(This is not surprising to me, as decades of radio experience has taught me that while I try to squeeze every last 1/10 of a db out of something in the lab / on the bench, etc....in the real world, using real signals, a "one db difference" is about the absolute minimum that can be noticed, the one exception was when I'm bouncing my VHF radio signal off the Moon, where there is a path loss of ~ 245db, and some of the stations S/N was zero or close to it (on CW of course), so if I can get an extra few tenths of a db out, and/or improve my antenna, feedline, rec
noise floor, etc. a couple tenths of a db, I will find a slight noticeable improvement....But, VHF-FM
Marine radio is NOT that!!
And of course when dealing with VHF-FM signals, the radio's rec bandwidth,
noise floor, and the terrestrial noise, etc. mean that until signal levels are right at the C/N "threshold", there will be little advantage/notice with even larger changes...)
I also verified the VHF thru loss as < 1db, using my M-602 as transmitter and showing 24.5 watts on my Bird 43 wattmeter INTO the SP-160, and 20.4 watts coming OUT of the SP-160....giving a loss of 0.8db....pretty good!
(and even figuring some slight mis-cal of the Brid slug, I was happy to say that "it was < 1db"...)
Some purists will say that "every db counts!", and I actually agree!!!
But, with 25 watts of transmit
power, on VHF-FM, and typical antenna heights of 65' HAAT, there is a LOT of margin in the
Marine VHF-FM system.....and with the original Marine VHF-FM antenna systems design spec of 3db of feedline loss, I figured I was still good.....as I was very close to that design spec at 3.3db...
I use lower loss cable than most (70' of LMR-240uf @ 2.1db loss and 25' of LMR-400uf @ .4db loss = 2.5db + the SP-160's loss of 0.8db = 3.3db of total thru loss, from antenna to the M-602, transmit and receive...)
Note that someone using 95' of RG-213 / RG-214 (2.8db loss per 100') and using the SP-160 would place them only 0.2db lower (more loss), and that is hardly even going to be measurable with 1000's of dollars of test
gear.....but I didn't have the room in my
mast conduit for the thicker cable, hence the reason I went with the LMR cable...
{Also take note that even using the SP-160 "splitter / relay" and its added loss, and these lower loss
cables mentioned here, still places us far ahead of those using "RG-8x" / "mini-8" coax, which has 4.5db of loss per 100', and NO "splitter / relay" at all!!!}
---- Now fast forward to this spring, when a friend asked my
advice about installing a "splitter" for his new AIS transponder....
I made sure he understood that it was not just a "splitter", and said the only one I would recommend would be the Vesper SP-160...
And, when he asked "how well did it
work?", I answered with the info above....but I always thought that if I was to post this info, someone would argue (rightly) that my
single "one-user" anecdotal data is useless, and questioning the accuracy of my Bird 43 (whose VHF slugs have never been re-cal'd)....
So...
So, I went ahead and made some quality measurements using some of my lab
equipment, which I have re-cal'd every year, and do a quick
cal and normalize before each test....(although I did use my existing
cables on-board, as i didn't want to introduce any possible variables for argument...)
And, what do 'ya know....everything checked out just as my earlier tests showed...
Here are the spectral scans of the Vesper SP-160 swept from 156mhz thru 162mhz, for everyone to see for themselves...
Note that my
tracking gen was set at -15dbm, and cables swept/normalized...
This first image is the reference, SP-160 bypasses, cables connected together...(note the scale is 1 db / div)
This second image shows 0.6db to 0.9db loss from 156 thru 162mhz, on the main VHF line (transmit and receive on the main VHF radio) (note the scale is 1 db / div.
These next two
images, show the 12db pre-amp gain thru the AIS (note the scale is 2 db / div, in order show this 12db change on the
screen...)
But, please note that this does NOT improve your
reception by 12db, nor your rec S/N by 12db....but rather approx. 6db....(I did not store any further test data, but if you could take my word for it, when looking at a narrow bandwidth approximating the marine VHF-fm rec bandwidth, I actually did see a rise in S/N....It was starting to storm and I wanted to disconnect things before my test equip got struck by
lightning !)
---- Since I was there, I thought some may also want to see a sweep of my Shakespeare masthead 3' SS VHF antenna...and this is my usual first test (verify the antenna is working!), and I thought you may be curious what the VSWR is across the whole band, from 156mhz - 162mhz...
---- And, for those of you that wish to know a bit about RF loss thru connectors, it is effectively Nil / Zero!!!
I know some will disagree, but unless you're in a lab with 10's of thousands of dollars worth of
equipment (and a few trained techs/engineers to set up the tests), you will NOT be able to measure the infinitesimal differences!!!
If you want to SEE what I mean, have a look at this
photo.....the transmitter is outputting exactly 100 watts (measured with that same meter/slug), and after 23 connectors/adapter, etc. N's, UHF's, BNC's, including "T's" and "elbows", a short RG-58 jumper, and even including some "F" connectors and "phono" plugs/adapters (neither of which were ever designed to handle RF), the results are only marginally measurable....showing 95 watts out (a 5%
power loss), which is a loss of 0.2db...
So, next time someone wants to tell 'ya that using an "N" connector will have less loss than a PL-259 {"UHF" connector), you can see for yourself that this will have no effect at all (at least until the freq gets past 400mhz)....take note that in my professional comm
work, connector losses ARE figured into system design, link budgets, etc. but these losses are very slight, typically 0.01 to 0.04db, depending on connector and freq...
Enjoy...
I hope some find this useful....
Fair winds...
John
s/v Annie Laurie
Linear Mode
Recent Discussions