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Re: Preamp for a Mobile Application MM




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,  
for example.

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
> installation.


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...

http://www.repeater-builder.com/antenna/3db.html

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  
idea, anyway.

(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@natetech.com
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