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Cellular forums Home > Archive > Garmin GPS > March 2007 > Remote Antenna Question
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Remote Antenna Question
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| ABLE_1 2007-03-18, 10:33 am |
| Hello All,
Question concerning the db rating for a remote antenna.
Is the better db value higher or lower?? What I mean is if two remote
antennas are rated for 28db and 56db which is considered better???
TIA
Les
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| Caveat Lector 2007-03-18, 10:33 pm |
|
"ABLE_1" < royboynospam@somewhe
re.net> wrote in message
news:Dt- dnfWKWI8YxmDYnZ2dnUV
Z_vamnZ2d@comcast.com...
> Hello All,
>
> Question concerning the db rating for a remote antenna.
>
> Is the better db value higher or lower?? What I mean is if two remote
> antennas are rated for 28db and 56db which is considered better???
>
> TIA
>
> Les
>
Les the higher the dB value -- the higher the gain of the antenna. See URL:
http://en.wikipedia.org/wiki/Antenna_gain
However, the higher the gain, in general, the narrower the beamwidth.
Beamwidth may be a factor in selecting a GPS antenna
So which one will work better is questionable.
Actually trying the two antennas in your situation would be ideal.
Others here may have tried out the various antennas and have sage advice.
To go by the db rating alone may or may not be the best solution.
It would seem to me that Garmin has extensively tested their units with
various antenna schemes and have the best solution. Others might comment on
that.
A minor point - for gain, antennas are compared to a reference antenna -
dipole or isotropic -- that is dBd or dBi as explained at wikipedia. The
figures are fairly close either way. Hope this helps
CL - An Amateur Radio Operator.
| |
| ABLE_1 2007-03-18, 10:33 pm |
| CL thanks for the response. Please excuse my ignorance but I always thought
db was the Signal to Noise Ratio factor. So I was thinking that a lower db
would be better. But I guess that it then depends upon the actual
application you are discussing.
So when it comes to antennas the higher the db the narrower the beam thus
being more directional and then not the better for a GPS that is trying to
pick up signals from a wide area of the sky.
Did I say that right??? If that is true, then during a discussion of a GPS
the lower the db would be the better for the application of a remote
antenna??
If the above statement are true then I think that I understand. However if
they are not then I need more education on the subject.
My main reason for asking is because I am considering the purchase of a
remote antenna for my Garmin Quest and was trying to make the right choice.
Thanks to all that reply. Very helpful.
Les
> Les the higher the dB value -- the higher the gain of the antenna. See
> URL:
> http://en.wikipedia.org/wiki/Antenna_gain
>
> However, the higher the gain, in general, the narrower the beamwidth.
> Beamwidth may be a factor in selecting a GPS antenna
>
> So which one will work better is questionable.
> Actually trying the two antennas in your situation would be ideal.
> Others here may have tried out the various antennas and have sage advice.
> To go by the db rating alone may or may not be the best solution.
> It would seem to me that Garmin has extensively tested their units with
> various antenna schemes and have the best solution. Others might comment
> on that.
>
> A minor point - for gain, antennas are compared to a reference antenna -
> dipole or isotropic -- that is dBd or dBi as explained at wikipedia. The
> figures are fairly close either way. Hope this helps
>
> CL - An Amateur Radio Operator.
>
| |
| Caveat Lector 2007-03-18, 10:33 pm |
| Les I found an article that I think will help -- it is a Comparison of four
popular AMPLIFIED GPS ANTENNAS
Gain may be achieved with a low noise amplifier, thus getting around the
problem of narrow bandwidth from a high gain unamplified antenna.
http://gpsinformation.net/main/gpsantrev1.htm
Notice they have maximum gain at 90 deg then less at 45 deg and even less at
10 deg. In other words a clear view of the sky. These antennas are omni
directional since the satellites can be anywhere in azimuth
Another interesting URL:
Gilsson vs. Garmin --external GPS antenna comparison
http://www.gilsson.com/ external_gp..._comparison.htm
From this you can see the omni directionality of the antennas and how an
amplifier (gain) comes into play.
Hope this helps.
CL
| |
| Caveat Lector 2007-03-18, 10:33 pm |
|
More about dB
The decibel (dB) is a dimensionless unit of ratio which is used to express
the relationship between a variable quantity and a known reference quantity.
Since decibels express a relationship between a variable and a known
reference, they are useful for a wide variety of measurements in acoustics,
physics, electronics and other disciplines. The calculation of decibels uses
a logarithm to allow very large or very small relations to be represented
with a conveniently small number (similar to scientific notation). Basically
dB = 10 log (P2/P1) where the log is to base 10. The P's are two
power levels to be compared.
Noise figure (NF) is a measure of degradation of the signal to noise ratio
(SNR), caused by components in the RF signal chain. Noise Factor is a
straight ratio of SNR ratios. Noise Figure is the deciBel equivalent of
Noise Factor. More at URL:
http://en.wikipedia.org/wiki/Noise_figure
In antennas as stated before gain is the logarithm of the ratio of the
intensity of an antenna's radiation pattern in the direction of strongest
radiation to that of a reference antenna. If the reference antenna is an
isotropic antenna, the gain is often expressed in units of dBi (decibels
over isotropic). For example, a dipole antenna has a gain of 2.14 dBi [1].
Sometimes, the dipole antenna is used as the reference (since a perfect
isotropic reference is impossible to produce), in which case the gain of the
antenna in question is measured in dBd (decibels over dipole).
Adding an amplifier to an antenna allows us to have a omni directional
pattern and maintain a high angle of the antenna pattern. This
preamplification gives a higher signal level to the GPS electronics.
In Acoustics, The decibel unit is commonly used in acoustics to quantify
sound levels relative to some 0 dB reference. See Sound#Examples of sound
pressure and sound pressure levels.
CL
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| ABLE_1 2007-03-18, 10:33 pm |
| CL thanks, very helpful. Have a good week.
Les
> Les I found an article that I think will help -- it is a Comparison of
> four popular AMPLIFIED GPS ANTENNAS
>
> Gain may be achieved with a low noise amplifier, thus getting around the
> problem of narrow bandwidth from a high gain unamplified antenna.
>
> http://gpsinformation.net/main/gpsantrev1.htm
>
> Notice they have maximum gain at 90 deg then less at 45 deg and even less
> at 10 deg. In other words a clear view of the sky. These antennas are omni
> directional since the satellites can be anywhere in azimuth
>
>
> Another interesting URL:
> Gilsson vs. Garmin --external GPS antenna comparison
>
> http://www.gilsson.com/ external_gp..._comparison.htm
>
> From this you can see the omni directionality of the antennas and how an
> amplifier (gain) comes into play.
>
>
> Hope this helps.
> CL
>
| |
|
| ABLE_1 wrote:
> Question concerning the db rating for a remote antenna.
> Is the better db value higher or lower?? What I mean is if two remote
> antennas are rated for 28db and 56db which is considered better???
It's an indication of how much the signal from the antenna is
amplified before transmitting it down the cable to your GPS. An
amplification of 28 dB means the power has been increased by a factor
of about 630 times [10^(28/10)] while 56 dB indicates an amplification
by a factor of almost 400000 times.
Which of these is "better" is less easily answered since it depends
partly on your application. Your GPS calculates your position based
on the assumption that all the signals are coming to the antenna on
direct, line-of-sight paths. If you apply too much amplification then
some reflected signals (called multi-path) may well be received as
well - and if the direct signals are blocked (say by buildings) then
your GPS will use the reflected signal time delays in determining a
position and come up with wrong results. OTOH, in some cases you may
have very poor reception and be willing to sacrifice some accuracy in
exchange for getting enough signals to make a position determination
at all. In that case it might be preferred to have sufficient
amplification to use those weak reflected signals even though they
normally degrade your accuracy. High amplification might also be
called for if you want to use a very long cable between the external
antenna and your GPS.
But in most normal situations I'd be inclined to go with the 28 dB
gain antenna instead of the 56 dB to reduce the chance of multi-path
degradation of accuracy and also to avoid the possibility of
overloading the input circuitry of the GPS.
| |
| Caveat Lector 2007-03-19, 10:33 am |
| I agree with Peter. Multipath and front end overload are considerations in
selecting an amplified antenna. More is not always best. As I stated in my
original post, I wouldn't buy either one unless I could test them in my
environment. Garmin knows the characteristics of their GPS units and I would
trust they design amplified antennas to properly match their units. Where an
after market designed antenna must attempt to satisfy a several
manufacturers and models.
Good Luck - Let us know what you decide.
"peter" <prathman@comcast.net> wrote in message
news:1174275973.973305.264980@y80g2000hsf.googlegroups.com...
> ABLE_1 wrote:
>
>
> It's an indication of how much the signal from the antenna is
> amplified before transmitting it down the cable to your GPS. An
> amplification of 28 dB means the power has been increased by a factor
> of about 630 times [10^(28/10)] while 56 dB indicates an amplification
> by a factor of almost 400000 times.
>
> Which of these is "better" is less easily answered since it depends
> partly on your application. Your GPS calculates your position based
> on the assumption that all the signals are coming to the antenna on
> direct, line-of-sight paths. If you apply too much amplification then
> some reflected signals (called multi-path) may well be received as
> well - and if the direct signals are blocked (say by buildings) then
> your GPS will use the reflected signal time delays in determining a
> position and come up with wrong results. OTOH, in some cases you may
> have very poor reception and be willing to sacrifice some accuracy in
> exchange for getting enough signals to make a position determination
> at all. In that case it might be preferred to have sufficient
> amplification to use those weak reflected signals even though they
> normally degrade your accuracy. High amplification might also be
> called for if you want to use a very long cable between the external
> antenna and your GPS.
>
> But in most normal situations I'd be inclined to go with the 28 dB
> gain antenna instead of the 56 dB to reduce the chance of multi-path
> degradation of accuracy and also to avoid the possibility of
> overloading the input circuitry of the GPS.
>
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