Ed:

If you use a Comet Diplexer such as the CF160 that I have, it can split things out to two antennas and then you can use a preamp on the receive side. I have used that approach when I have worked sats low to the horizon or when I had time to find a good parked position with a clear view of the sky.

Ron KA4KYI

Edward Cole wrote:

Jeff,

You are partly correct, but the main job of the preamp is to lower the noise figure of the overall receiving system. The gain in the preamp is primarly to provide a strong enough signal for the following receiver that its noise figure is not significant. Having enough gain is important so that there is enough extra too overcome coax line loss and still have sufficient gain left over at the receiver (approx. 15-dB is needed).

So, a preamp can help a mobile satellite station if there is a way to install it. If your radio only has one antenna connector for both 2m and 70cm, then it will not be possible to use a preamp for a full duplex radio. If it is not able to operate duplex then a switching preamp would work OK.

73 Ed - KL7UW

At 09:52 AM 3/31/2008, Jeff Mock wrote:

...

It's not clear that a preamp will improve your reception. The 2720H has a pretty sensitive receiver. The preamp will help you if the noise figure of the preamp is better than the noise figure of your radio. It's not clear to me that you can make much of an improvement here.

I have a preamp on the masthead at home, but I have a long feedline from my receiver to the antenna, I have about 7db of loss from the antenna to the radio at 70cm and I get a noticeable improvement with a preamp. The main job of my preamp is to overcome the loss of the feedline, otherwise the preamp is not that much better than the radio.

jeff AD6EO

Kent R. Frazier wrote:

...

Hi all,

I am really enjoying the satellite aspect of the hobby and have enjoyed the contacts I have made with my Icom IC-2720H with a 19" whip antenna from my vehicle. I have seen talk recently of using a preamp to improve the reception. From what I've read and understand, it appears that a

"switching"

...

preamp is needed for a setup using only one antenna for Tx and Rx such as mine. Is this correct?

It also appears that most of the preamps are mast mounted. Since this would not be an option for me can the preamp be put near the transceiver? Are there any issues that result of doing so?

Lastly, what preamp(s) would you recommend for an application such as mine?

Thanks,

Kent

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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