[amsat-bb] Re: Preamp for a Mobile Application MM
nate at natetech.com
Fri Apr 4 23:39:14 PST 2008
On Apr 3, 2008, at 1:34 PM, MM wrote:
> I use a 2/440 splitter into 2 Mono band NMO magmounts.
> This way I get Taller antennas and more gain than with
> a Dual Bander (2/440).
> 2 meters is a 5/8 wave mono bander (Salesman gain = 4,
> Dipole gain = 1).
> 440 is a stacked mono band collinear 1/2 wave stacked
> over a 1/2 wave (Salesman gain = 6, Dipole gain = 3).
A small thought here... "gain" in mobile omni-directional antennas is
referenced to gain toward the horizon. The vertical component isn't
referenced, but a 1/4 wave has a higher take-off angle, and it falls
somewhere around the angles that Bob Bruniga has calculated work for
large percentages of passes... in his articles about a fixed elevation
beam and a cheap rotor as a very good solution for most LEO's.
Have you compared a 1/4 wave to the higher gain antennas on higher
passes? It would seem to me that your setup will give you better low
passes than high ones, but of course... when they're overhead they're
a bit closer to you and no ground "clutter" to get in the way if
you're in an urban area and don't have clear views of the horizon from
where you usually operate.
A quarter-wave ground-plane antenna on a vehicle produces maximum gain
at roughly what... um... 25 degrees above the plane of the roof, and a
5/8 wave at roughly 14 degrees, correct? Something like that. So
you'd have to experiment and keep close track of the horizon angle of
the satellite with tracking software and listen for the "peak signal"
from each over a number of passes to see what angle your antennas are
best at, really.
The other difference here is the "capture area" of the larger
antennas. In theory you should be seeing a virtually identical
increase in gain on receive as on transmit, for purposes of this
discussion -- if the angles are similar and the satellite is in the
main VERTICAL lobe of the respective antennas.
Of course, these angles are all screwed up and multiple lobes appear
as you raise the antenna above ground... and on a vehicle you'd think
the antenna would see the vehicle roof as "ground" but, it's not 100%
that way... so the angles end up all over the place.
http://www.w8ji.com/VHF%20mobile%20vertical.htm <- Has a number of
theoretical examples at 147.000 MHz from EzNEC for 5/8 wave antennas,
So in the long run... we hams always "use whatever works best", but
it's a neat exercise to try to reverse engineer it and figure out why
it's working so well, sometimes! (GRIN)
> Here are some number I put together on coax loss for
> the typical mobile
Here's where the rubber supposedly hits the road (pun intended)
according to Tony and a couple other folks on the list this week, but
I'm going to refute that special cable is needed for mobile
installations... at least not ones with reasonable cable lengths.
[snipped table to save space]...
Nice table of info -- I'll assume it's right for the moment for a
comment... I think it looks generally "sane" at first glance, so...
You're saying that for short distances, in your worst-case scenario
(15' of RG-58U versus LMR240UF @ 900 MHz) the difference is only an
additional 1.5 dB of signal. It's even less at UHF. And would be
even less at VHF.
To keep this in perspective, a calibrated/standard S-unit (which few
radios really do correctly) is 6dB... so at short cable runs, even at
900 MHz -- you've "gained" 1/4 of an S-unit, and paid just about
double the price per foot for the cable?
This is what I meant by dollar-per-dB as my joking way to measure
performance increases on a ham radio/hobby budget... changing cable
types on short runs is definitely NOT worth it.
For the (ridiculously long @ 100' for a mobile installation, but it's
GREAT for demonstrating how this multiplies out) longer cable runs...
your worst case at 900 MHz is just under 10dB cable loss. Just under
2 S-units. Probably worth the upgrade, in that case.
But I don't know anyone with 100' cable runs in a mobile
installation. Even my LMR400 for the 12' mast for the weak-signal VHF
+ rover station in the Jeep is really only about 20'-25' long!
If you're trying to hide the coax and everything under the carpet and
run it through door frames and things, I typically see 20'-25' in most
automobiles, but not much more. If you're running it out the window
to a hand-held or tri-pod/mast mounted yagi for pointing, even less.
I'm just going by your numbers here, and this doesn't factor in
connector losses which also add up a little more loss... but
seriously, I don't think swapping RG8X for LMR240UF is worth doing for
mobile installs -- unless you're installing in a large Motor-home
along half or more of the entire length front to back, or you're going
to run from the mobile to a push-up mast or something like that. Even
then, 100' is crazy for most applications we're talking about here.
So with that said, I might start to buy into this idea for higher than
900 MHz... if you're doing that.
I'm far more curious about what would you see if you had a way to
switch between your "high" gain verticals and a couple of cheap
quarter-waves as-close-to center-mounted on the roof as you could get
while monitoring the received signal from the satellite over multiple
passes at different elevations.
If you're talking about the FM birds, most of the information in this
article from Repeater-Builder would also apply to your receiver in the
mobile setup... you're shooting for maximum performance in very
similar ways to the ways FM repeater building folks (who try hard and
do it well) also try to hear their mobile users better...
The article contends that FM receivers aren't very linear in their
sensitivity response (it's a curve), so maybe a 3dB increase in power
level at a repeater transmitter is worth it, maybe it's not... but
even at 3dB it's a "toss-up" in the article, and you're not even
gaining 3dB back by buying the better cabling in your examples over
short cable runs... long runs... yeah, upgrade probably... but an
additional 6dB or more gain from a long yagi and some way to point it
(bring a friend!) is a LOT more gain for the buck!
Maybe the coolest idea for serious mobile satellite hamming would be
to mount a dual band yagi or set of yagis (let's not talk about
polarization here... well, anyway... maybe later in the thread...) and
Bob B's 30 degree fixed "up-tilt" and an "armstrong" or even a cheap
Rat Shack/whoever rotor... on a solid tripod sitting just out of boom
reach of the vehicle.
Tthen you need AC to power it, or to find a way to convert it to DC...
(most AC inverters for car use throw WAY too much RF noise to even
mess with for this application unless you're going to power them up,
make a course-adjustment on beam heading, and power it off,
constantly). Yadda, yadda... it'd be more complex than your
verticals, but it would make contacts a breeze.
One of the hams out here completed a set of patch feeds on a 6'
surplus (read: free) C-band dish and his own mechanical design for
what he called an "rotatable almost horizon to horizon" system and
mounted on a super-cheap trailer from Harbor Freight. That was the
"ultimate" I've ever seen for mobile satellite work. I saw it during
construction and he was "upset" that it couldn't get below 1 or 2
degrees at full-travel on his design. Heck, I'd take it! (GRIN) He
was also looking for ways to use PDA's for the pass prediction and get
similar output as other satellite tracking software on a serial cable
to his controller, but hadn't gotten that started yet, at that
point... a PDA and your trailer that had a couple of deep-cycle
batteries, a custom aiming controller and away it goes... run some
coax to the rigs and have fun hearing darn near everything... was the
(It was Dr. Robert Sudding W0LMD who many of you know -- a couple of
people have asked me if since I'm local, I've heard or talked to him
-- I saw him at a hamfest where he related that he hasn't been doing a
lot of radio, since he is now playing with aiming solar arrays at the
Sun in similar computer-controlled fashion as his satellite trackers.
He's always working on new projects and tinkering with things to try
to make them work better -- an inspiration to us all around here --
but he can also wear you out just keeping up with all his ideas, let
alone the actual work and experiments that unlike most of us, he
actually gets done!)
Nate Duehr, WY0X
nate at natetech.com
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