Personal Amplifiers/repeaters

January 16, 2004

Today I thought about making a lantern battery powered gps amplifier and repeater so I could get a GPS signal from under trees and indoors. Is there any laws or regulations that would keep me from doing this?

1.7k · January 16, 2004

Just the laws of physics.

GPS works by measuring the time delay between your receiver and the transmitter on the satellite. With four such measurements, a little math, and a knowledge of where every satellite is in space at any given time, you can compute your location in space.

A repeater would have the effect of making your receiver think it was always at the location of the repeater's own antenna, and of making your system clock be off by a tiny fraction of a second.

Edited January 16, 2004 by parkrrrr

2.2k · January 16, 2004

GPS amplifieres and repeaters can be purchased in many shops selling GPSr. They are used to improove GPS reception inside cars if your GPSr does not have an external antenna connector.

The reception antenna and amplifier are placed on the roof of the car and the transmitter antenne is attached to your GPSr close to its build in antenna.

1.7k · January 16, 2004

But they won't help you know where you are inside your house or under heavy tree cover. They work exactly as I described, and they are only useful because their receiving antennas are rarely very far from your GPS receiver.

January 16, 2004

Just the laws of physics.

[rimshot]

I get unsolicited email all the time for "personal amplifiers" but they don't mention anything about GPS....

Edited January 16, 2004 by SamLowrey

January 16, 2004

I get unsolicited email all the time for "personal amplifiers" but they don't mention anything about GPS....

And note they are often a patch....never a quad-helix.

George

Edited January 16, 2004 by nincehelser

2.7k · January 16, 2004

I believe the 'repeaters' inquestion are actually 'passive' repeaters. This simply consists of an inside antenna and an outside antenna connected together with a transmission line.

The purpose of which is to bring the signal from outside the vehicle inside the vehicle. These are available for a couple bucks for cellphones also.

In a word- GIMMICK

IF you have practically no signal inside the vehicle and

IF you have a very strong signal outside the vehicle and

IF the receiver antenna is very close to the inside antenna and

IF the transmission line connecting the inside and outside antennas is good then

MAYBE you'll be able to tell a difference.

The only way this might work in deep woods with a portable is:

IF you climb a tall tree and place the outside antenna there and

IF you install a very high quality transmission line between the 2 antennas and

IF you stand real still near the inside antenna then

MAYBE you'll get a better location on the tree that the antenna is in.

If you really want to improve reception in heavy woods, buy a GPSr that has an ACTIVE (Quadrifiliar) antenna- NOT a 'patch'

I have a Magellan Meridian Platinum. I can throw it in my floor board in deep woods and it still stays locked on. I have used it for nearly a year and found nearly 200 caches with it and I have NEVER lost signal in the woods. It outperforms my Garmin Street Pilot even with an ACTIVE outside antenna directly connected to the Garmin.

So spend your $350 on a better GPS and forget the gimmicks.

January 17, 2004

I was going to make my own, and I would buy a nicer gps if I $350. As long as I use the active antenna in the recevier I'm ok then?

And about the time dealy. If it's only two feet away in my pocket, It wont make too much of a difference anyway, will it?

So I'm assuming that if they sell them, they must be ok if I follow part 5 of fcc regs?

January 17, 2004

If you really want to improve reception in heavy woods, buy a GPSr that has an ACTIVE (Quadrifiliar) antenna- NOT a 'patch'

Quadrifilar does not = ACTIVE. My Garmin Map76S as well as your Platinum have internal passive quadrifilar antenna's. I get noticeably improved reception with my external ACTIVE patch antenna under tree cover.

4.4k · January 17, 2004

Do not know all the rules but on NASA today they were talking about the GPS system and stated,

That many still do not know the true accuracy,you can add several antenna and get Attitude that is Roll,Pitch and Yaw.

Scientist use it to measure the movement of the Tectonic plates,it's that accurate.

You will always have some trouble in dense leaf cover due to the moisture inside the leaves.

An amplifier is an amplifier it boosts the RF signal,or the (Signal).

The Pre-Amp boosts it before it enters the unit.

Much like the DGPS or beacon reciever.

but unless you are licenced technitian I would not try it because the whole system has to be tuned.

It all has to meet ASCII Standards.

1.2k · January 17, 2004

What you call a repeater we call a reradiating antenna antenna. the link will take you to a page with instructions on how to make one for yourself. It also contains a link to my page that explains how I made mine.

Its cheap, easy and works well.

January 17, 2004

It all has to meet ASCII Standards.

Just be glad is isn't EBCDIC.

George

1.4k · January 17, 2004

Today I thought about making a lantern battery powered gps amplifier and repeater so I could get a GPS signal from under trees and indoors. Is there any laws or regulations that would keep me from doing this?

Amplifier is not a problem, but I'm not sure what you mean by "repeater". If you're just talking about getting the signal into your unit, I don't think there are any laws that would prevent that. If you're planing to broadcast those signals any distance (rather then having it connected to your GPS) then you would likely be in violation.

Regardless of what approach you use, there is some risk with an amplified system that it will break into oscillation (like feedback in a PA system when the mic gets too close to the speaker or someone tries to amplify too much). At that point you could well be a source of interference and that would be against the law.

3.8k · January 17, 2004

.........geeks........(sigh) :unsure: :bad:

January 17, 2004

My supervisor used to work at TxDOT. I think it has already been mentioned, but they would used some sort of triangulation method to get accuraccy down to centimeters. But regarding the topic in this thread, he said they had a device that, I guess, was like an antenna that could be mounted somewhere, like a fence, and you could take the reading under a tree. There as a laser (or something) that would communicate with the other unit and calculate the difference. Not that big of a task, really - all you are doing (in addition to GPS) is measuring the distance and direction between two devices and displaying the difference.

1.4k · January 17, 2004

.........geeks........(sigh)

That's my name, want my number? (although I don't think you're my type....)

Edited January 17, 2004 by GeckoGeek

4.4k · January 17, 2004

OK nincehelser What is EBCDIC

I do not have any troubles and I live in the deep woods.

Mine even works indoors most of the time.

But I do have several dishes that have lead-ins inside the house.

I have found that if you put the GPS near a dish you get better reception as well.

but thats another experiment in the works,I have always thought that a minature dish disc ant. would work great.

Especially if pointed at the satellites trilaterating your position.

1.4k · January 18, 2004

What is EBCDIC

A sign of middle age or far too much information of arcane computer trivia. Either that or you work on IBM mainframes.....(do they still use that?)

I have always thought that a minature dish disc ant. would work great.
Especially if pointed at the satellites trilaterating your position.

Dishes are designed to be directional, but a GPS needs to get a good view of sats in different directions to get a fix. However, the basic idea of using a reflective object to gather more RF energy to focus on the GPS is intriguing.

2.3k · January 18, 2004

What is EBCDIC

A sign of middle age or far too much information of arcane computer trivia. Either that or you work on IBM mainframes.....(do they still use that?)

I have always thought that a minature dish disc ant. would work great.
Especially if pointed at the satellites trilaterating your position.

Dishes are designed to be directional, but a GPS needs to get a good view of sats in different directions to get a fix. However, the basic idea of using a reflective object to gather more RF energy to focus on the GPS is intriguing.

ok so how much of a geek am I. I knew what it was without looking it up

EBCDIC = Extended Binary Coded Decimal Interchange Code

January 18, 2004

A sign of middle age or far too much information of arcane computer trivia. Either that or you work on IBM mainframes.....(do they still use that?)

Not quite middle aged yet, but near its doorstep.

As for arcane computer trivia, I've got tons of it. My first exoposure to EBCDIC was playing "Zork" on a mainframe.

Here's the best definition of EBCDIC I've found:

EBCDIC

/eb's*-dik/, /eb'see`dik/, or /eb'k*-dik/

[NHD] n. [abbr. Extended Binary Coded Decimal Interchange Code] An alleged character set used on IBM dinosaurs. It exists in at least six mutually incompatible versions, all featuring such delights as non-contiguous letter sequences and the absence of several ASCII punctuation characters fairly important for modern computer languages (exactly which characters are absent varies according to which version of EBCDIC you're looking at). IBM adapted EBCDIC from punched open-systems company, but IBM's own description of the EBCDIC variants and how to convert between them is still internally classified top-secret, burn-before-reading. Hackers blanch at the very name of EBCDIC and consider it a manifestation of purest evil. card code in the early 1960s and promulgated it as a customer-control tactic, spurning the already established ASCII standard. Today, IBM claims to be anopen-systems company, but IBM's own description of the EBCDIC variants and how to convert between them is still internally classified top-secret, burn-before-reading. Hackers blanch at the very name of EBCDIC and consider it a manifestation of purest evil.

George

Edited January 18, 2004 by nincehelser

1.3k · January 18, 2004

What is EBCDIC

A sign of middle age or far too much information of arcane computer trivia. Either that or you work on IBM mainframes.....(do they still use that?)

Yes. Part of my job is COBOL on an IBM mainframe (where credit card processing takes place). (The majority is smallTalk on a *nix box where the settlement systems and HTML interface for various settlement departments resides, which means we're converting between ASCII and EBCDIC). Really, it's just another way to represent data on a computer. Anyone who can't handle it needs to learn that it's just ones and zeros anyway.

----

My biggest misunderstanding with reradiator antennas is that it always seems to me that they seem to require the GPS only getting the signals from the reradiator and not being able to get the original signals. Am I completely mistaken in that belief?

Also, is there a site that has a more comprehensive list of GPS and their antenna capabilities? I never really know which ones are 2d (must be held flat?) and which are 3d and that sort of thing. I need a lot more self-education when it comes to that sort of thing.

1.4k · January 18, 2004

My biggest misunderstanding with reradiator antennas is that it always seems to me that they seem to require the GPS only getting the signals from the reradiator and not being able to get the original signals. Am I completely mistaken in that belief?

Oh geez, you trying to get us back on topic? :lol:

I'm not sure myself. It might be that the GPS just goes with the strongest signal and ignores anything weaker. I know FM modulated signals work that way, but I don't know how GPS signals are modulated.

2.7k · January 18, 2004

Not really familiar with GPS signalling either, but a radio professional (field engineering). So as the myopic leading the blind:

Another poster mentioned a slight delay from the amplified antenna to the GPSr. This would occur, and mixed with the direct signal, would introduce some error. The math to determine if that error is significant or even relevant is beyond me. For a short transmission line, I seriously doubt the difference in propogation time would make any difference.

I was misunderstanding the nature of the "repeater" in my previous post.

The antenna system described here is not a "repeater", but simply an external amplified antenna. The only issue here is this discussion is that of connecting an external antenna to a device that does not have an external antenna jack, therefore requiring inductive, capacitive, or very short range RF coupling.

As described in the posts with the pictures, I have no doubt that there will be SOME advantage.

My post referred to the practice of "passive repeaters" which, in my experience, almost never work.

Because the antenna used is amplified, there does exist the possibility of oscillation if the donor antenna is held in close proximity to the GPS coupling loop. This must be avoided. Several feet of separation may be necessary to eliminate this possibility.

A better way to go, for the technically qualified, would be to remove the internal antenna and install a BNC jack to which you could attach a standard Garmin passive antenna (such as shipped with the Street Pilot) or an amplified antenna as you see fit.

The major consideration in this modification would be the supplying of power for the external antenna. This could be quite difficult without using an external battery (which would be very Rube-Goldbergish) especially for a unit that uses only 2 AA cells. (I believe my active antenna uses 5 volts bias)

Anyway, cool experiments, but considering the chances of damage and the money (and time) that could be wasted on failed attempts, buying a better GPSr or one with an external antenna jack still makes more sense.

2.3k · January 18, 2004

Not really familiar with GPS signaling either, but a radio professional (field engineering). So as the myopic leading the blind:

Another poster mentioned a slight delay from the amplified antenna to the GPSr. This would occur, and mixed with the direct signal, would introduce some error. The math to determine if that error is significant or even relevant is beyond me. For a short transmission line, I seriously doubt the difference in propogation time would make any difference.

I was misunderstanding the nature of the "repeater" in my previous post.

The antenna system described here is not a "repeater", but simply an external amplified antenna. The only issue here is this discussion is that of connecting an external antenna to a device that does not have an external antenna jack, therefore requiring inductive, capacitive, or very short range RF coupling.

As described in the posts with the pictures, I have no doubt that there will be SOME advantage.

My post referred to the practice of "passive repeaters" which, in my experience, almost never work.

Because the antenna used is amplified, there does exist the possibility of oscillation if the donor antenna is held in close proximity to the GPS coupling loop. This must be avoided. Several feet of separation may be necessary to eliminate this possibility.

A better way to go, for the technically qualified, would be to remove the internal antenna and install a BNC jack to which you could attach a standard Garmin passive antenna (such as shipped with the Street Pilot) or an amplified antenna as you see fit.

The major consideration in this modification would be the supplying of power for the external antenna. This could be quite difficult without using an external battery (which would be very Rube-Goldbergish) especially for a unit that uses only 2 AA cells. (I believe my active antenna uses 5 volts bias)

Anyway, cool experiments, but considering the chances of damage and the money (and time) that could be wasted on failed attempts, buying a better GPSr or one with an external antenna jack still makes more sense.

Total cost on mine was less than 25 dollars. If I remember correctly. I find it most usefully in a car with the non-gps friendly windshields, I place it on the roof, or with a camper shell over the truck. I did however load it to a cacher who had it on a 10 foot expandable pole for use in a forest. He loved it.

To each his own. this is but one idea.

2.7k · January 18, 2004

Where did you find the active antenna so cheap?

Mine was $75 from TESSCO (a commercial two-way radio wholesaler), not including the mount.

I can't imagine walking around in a forest with a 10' pole. That even beats my attempt to use my mobile active antenna attached to a hardhat with my Garmin Street Pilot.

I got tired of the hassle (and STILL poor signals) and bought my Magellan.

As you say, to each his own. I'm a "technogeek" myself, but as I age, I tend more and more to go with equipment designed for the purpose for which I am using it and leave the Rube Goldberging to the younger and better geeks.

Edited January 18, 2004 by ChurchCampDave

2.3k · January 18, 2004

Where did you find the active antenna so cheap?
Mine was $75 from TESSCO (a commercial two-way radio wholesaler), not including the mount.

I can't imagine walking around in a forest with a 10' pole. That even beats my attempt to use my mobile active antenna attached to a hardhat with my Garmin Street Pilot.

I got tired of the hassle (and STILL poor signals) and bought my Magellan.

As you say, to each his own. I'm a "technogeek" myself, but as I age, I tend more and more to go with equipment designed for the purpose for which I am using it and leave the Rube Goldberging to the younger and better geeks.

Ebay for active antenna's is the only way to go. This model is the one I got. works from 2.5-5 volts.

3.8k · January 18, 2004

GPS Geek has active antennas for cheap too.

PC-mobile.net has a reradiating antenna for hikers.

Edited January 18, 2004 by PDOP's

January 18, 2004

I can't imagine walking around in a forest with a 10' pole.

But a two foot one in a backpack works great.

January 18, 2004

With that pole sticking out of your backpack, you really ARE a Poindexter.

1.4k · January 18, 2004

ok so how much of a geek am I. I knew what it was without looking it up

EBCDIC = Extended Binary Coded Decimal Interchange Code

Well, color me impressed. I know what it is, but never knew what the name stood for. The closest I ever got to it was to be able to choose it as an option in Procomm. (And before anyone asks, that was a popular personal computer program back in the pre-Internet days.)

2.3k · January 18, 2004

Procomm rocked!!! and its still in used today for communicating with certain Fire detection systems, Its just getting harder to find machines with dos to run it on.

2.7k · January 19, 2004

Procomm's windows version is the industry standard for "talking" tomachines such as routers, mux's, controllers, radioand such that have standard rs232 interfaces for programming and setup. Hyperterminal supplied with windows works, but has bugs, so many still use Procomm.

But a two foot one in a backpack works great.

I wouldn't want to go many of the places I've been caching with that thing sticking up out of my backpack. (nor would I actually want to wear a backpack at all for that matter).

I have been on many a bushwhack where I had to practically crawl through the low branches. Reckon that's why they want you to stay on the trail. Whazup with that anyhow?

January 19, 2004

Procomm's windows version is the industry standard for "talking" tomachines such as routers, mux's, controllers, radioand such that have standard rs232 interfaces for programming and setup. Hyperterminal supplied with windows works, but has bugs, so many still use Procomm.

Oh, if you're going to do something stupid like attempt to use a M$ platform to do real things... Heck, use minicom on a linux box for all your real world serial needs!

1.4k · January 19, 2004

Oh, if you're going to do something stupid like attempt to use a M$ platform to do real things... Heck, use minicom on a linux box for all your real world serial needs!

Why would I want to carry two machines around - one that can run useful applications and one to run Linux?

2.7k · January 19, 2004

Big brother tells me which operating system to use. Long live Big Brother!

Personal Amplifiers/repeaters

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