When I started this study, I expected that there would be wide variations in antenna quality and performance. I found this was not so. In the end, I had to enlist the aid of Ben Zieg, retired director of the Lockheed Georgia Antenna Laboratory. With his help I was able to make repeatable measurements of "Tangential Sensitivity*" of each antenna RELATIVE to a standard. We were able to use the facilities of an antenna laboratory for some of the measurements, but time constraints did not permit us to set up a calibrated antenna range so as to be able to measure absolute antenna gain. Thus the RELATIVE measurements which are much easier to make are what we report here.
*Tangential Sensitivity> My definition for the purposes of this review is the minimum RF SIGNAL LEVEL which causes our test GPS (a G-II+) to be able to lock and maintain lock on approximately 2/3 of the satellites that are locked using the G-II+'s built in antenna. We made this determination by a) building a box with sheets and blocks of AN-74/75 RF absorber and b) placing the GPS antennas on a steel ground plane inside the absorber box with spacing about 1 wavelength minimum, and c) bringing the cables out of the shallow absorber box at widely dispersed points (to reduce the chance of RF coupling via the coax) and d) adding sheets of absorber on the open top of the absorber box until the signal on a particular antenna was reduced to the tangential level (see above). We were able to switch rapidly between the several antennas so we could make comparisons without losing lock. The RESOLUTION of this method was about 1db (SIGNAL TO NOISE RATIO as measured by the GPS receiver signal display which was calibrated using a precision attenuator). We did not take the time to calibrate the system to measure absolute gain as this would have been a rather major effort and (we thought) immaterial to the relative comparisons which were the objective of this experiment. Note: This method attempts to measure the relative EFFECTIVE GAIN of the antennas (which is related to antenna aperature and losses within the matching network and lumps in the noise figure of the electronic amplifier as well). This is NOT the same as measureing the gain of the antenna element PLUS the gain of the electronic amplifier. Within limits, the overall electronic amplifier gain is of little consequence once the gain of the <antenna+amplifier+cable> is perhaps 8 to 10 db more than the gain of the GPS's built in antenna. That is: Once the signal level input to the GPS is substantially more than the noise floor of the GPS receiver, more signal level does nothing to improve overall system performance.
One thing to be aware of is that when it comes to how an amplified GPS antenna will perform, the GAIN of the amplifier inside the antenna has *almost nothing* to do with how the system (GPS plus antenna) will perform as long as the gain is sufficient to overcome the antenna's cable loss plus perhaps 8 to 10 db to insure that the noise figure of the antenna "swamps" the noise figure of the GPS receiver's front end. Thus, unless you are driving an extra long cable, anything between about 10 and 25db of electronic gain is quite satisfactory.
External Amplified Antenna Advantages
When thinking about the purchase of an external amplified antenna,
you might consider the following problems which can be solved (or alleviated
to some extent) by an external amplified GPS antenna.
1) Does your GPS sometimes lose lock when operating in your automobile?
2) Do you need to maintain lock under heavy tree cover or in
city canyons or other difficult area?
3) Do you want to locate your GPS in a place where it does not
have a clear view of the sky?
4) Do you want maximum freedom from momentary loss of signal
when in your vehicle in motion?
5) Do you want maximum available accuracy which is available
when the largest number of SVs are in view?
6) Do you want much improved operation when looking out the porthole
of a passenger aircraft?
An external amplified antenna will help each of the above to *some*
extent. In MANY cases, the improvement is not worth the cost,
reduced battery life and hassle of dealing with the external antenna.
In other cases, it can make the difference between satisfactory operation
and no operation at all.
SPECIFICATIONS
The following specifications were furnished by the respective manufacturers of each antenna listed.
Garmin's ACTIVE POWERED GPS ANTENNAS:
Manufacturer
GARMIN GARMIN
GARMIN
Antenna
GA 26
GA 27
GA 28
Dimensions (mm) 50x71x15
50x71x17 56x96x59
Mount
Magnetic / Magnetic /
1" x 14 tpi
Suction cup Suction cup
marine mount
Connector
BNC
MCX
BNC
Power
5Vdc@30mAmax 5Vdc@30mAmax 5Vdc@30mA max
Gain (db)
15
15
15
Cable length (m) 2.5
2.5
10
Cable type
RG-174 RG-174
LL RG-58
Cable loss
4-5 db 4-5
db 4-5 db
Noise (db)
2
2
2
Gain Pattern
+1.5db@90 deg. +1.5db@90 deg. +1.5db@90 deg.
-1db@45deg -1db@45deg
-1db@45deg
-6db@10deg -6db@10deg
-6db@10deg
Axial Ratio
3db@90 deg. 3db@90 deg.
3db@90 deg.
Rejection
15db@50MHz 15db@50MHz
15db@50MHz
25db@100MHz 25db@100MHz
25db@100MHz
Op. Temp.
-55/+85 -55/+85
-55/+85
Price (US$ retail) 99.98
99.98
129.98
Tri-M Engineering's ACTIVE POWERED GPS ANTENNA SPECIFICATIONS
Manufacturer
Tri-M Engineering / Tri-M Systems Inc.
Antenna
Mighty-Mouse
Dimensions (mm) 48 x 58 x 15
Mount
Magnetic(2 x magnets), and/or taped screw holes and/or permanent mounting
plate incl.
Connector
Tri-M universal FME to BNC, MCX, SMA, SMB, TNC
Power
5Vdc @ 12 mA max, typical 10.5ma
Gain (db)
25 - cable loss included
Cable length (m) 5
Cable type
RG-174
Cable loss
2 - 3 db
Noise figure(db) 1.5dB typical 2.5dB
max
Gain Pattern
+5db@90 deg.
+3db@45 deg.
-1db@10 deg.
Axial Ratio
3db max>Rejection
20dB min @ Fo +/- 50Mhz
Op. Temp.
-30C through +85C
Price (US$ retail) $79
Tri-M
Engineering's ACTIVE POWERED GPS ANTENNA SPECIFICATIONS
Manufacturer
Tri-M Engineering / Tri-M Systems Inc.
Antenna
Big-Brother
Dimensions (mm)
48 x 58 x 15
Mount
Magnetic(2 x magnets), and/or taped screw holes and/or permanent mounting
plate incl.
Connector
Tri-M universal FME to BNC, MCX, SMA, SMB, TNC
Power
5Vdc @ 28 mA max
Gain (db)
30 cable loss included
Cable length (m)
5
Cable type
RG-174
Cable loss
2 - 3 db
Noise figure(db)
1.5dB typical 2.5dB max
Gain Pattern
+5db@90 deg.
+3db@45 deg.
-1db@10 deg.
Axial Ratio
3db max
Rejection
20dB min @ Fo +/- 50Mhz
Op. Temp.
-30C through +85C
1.00 Application. These Specifications apply to the electrical and mechanical performance.
2.00 System
This antenna system consists of two function blocks listed below.
2.1 Receiving antenna
2.2 LNA
3.0 General
3.1 Environmental Conditions
3.1.1. Operating Temperature
-30 deg C to + 80 deg C
3.1.2. Storage Temperature
-40 deg C to + 100 deg C
3.1.3. Relative Humidity
20% to 95%
3.2 Electrical Specifications
3.2.1. Input voltage
5.0v DC +/- 0.25 V
3.2.2 Power Consumption
32mA MAX (Room Temperature=30mA Max)
3.2.3 Output connector
BNC
3.2.4 Cable
1.5DS-QEHV (RG 174 equiv) 5 metres Black.
3.3 Mechanical Specifications
3.3.1 Mounting
Magnet mount
3.3.2 Pulling force of magnet
1.5 kgf min
3.3.3 Water proof
Tested to JISD 0203 S2
3.3.4 Shock
10G 10 mSec Half Sine
3.3.5 Vibration
10-200 Hz Log sweep 3.0 G
Sweep time 15 minutes 3 axis
3.3.6 Magnet tenacity
To stand vehicle speed 180 Km/h
3.3.7 Cable Pulling off force.
5kgf Min (Room Temperature)
3.3.8. Bending test
1000 cycles 90 deg left and right
3.3.9. Dimensions
50.5 mm x 42 mm x 13.8 mm
3.3.10 Weight
50 g max without cable & connector
4.0 Antenna
4.1 Frequency Range
1575.42 +/- 1.023 MHz
4.2 Gain
90 deg 3.0 dbi Min
20 deg -5.O dBi Min (Mounted on 65mm Square Ground Plane)
4.3 Polarization
RHCP
4.4 Axial Ratio
90 deg 4.0dB Max
10 deg 6.OdB Max (Mounted on 65mm Square Ground Plane)
5.0 LNA
5.1 Frequency Range
1.575.42 +/- 1.023 MHz
5.2 Gain
28 dB Min
5.3 Noise Figure
1.5 dB Max (+25 deg C +/- 5 deg C)
2.2 dB Max (+80 deg C)
5.4 Out of Band Rejection
fo=1575.42 MHz
fo+/-20MHz 7dB Min
fo +/- 30MHz 12dB Min
fo +/- 50 MHz 20dB Min
fo +/- 100 MHz 30 dB Min
5.5 Output Impedance
50 ohms
5.6 Output VSWR
2.0 Max
6.0 Total Specifications (Through Antenna, LNA, Cable and Connector)
6.1 Frequency Range
1.575.42 +/- 1.023 MHz
6.2 Gain
23 dBi Min (Mounted on the 65mm X 65mm
square Ground Plane -30 deg C to + 80 deg C)
6.3 Noise Figure
1. 6dB Max (+23 deg C)
2.5 dB Max (+ 80 deg C)
6.4 Output Impedance
50 ohms
6.5 VSWR
2.0 Max
6.6 MTBF
1,012,146 hours calculated
18ma 10.5ma 26.5ma 17.8ma
These figures indicate that the Mighty Mouse antenna will give the longest battery life when the external antenna is operated from the handheld GPS receiver's internal AA batteries. For instance, with a G-II+ (draws about 90ma), battery life would be about 10% better (about 2 hours longer) using the Mighty Mouse as compared with either the Lowe or GA-26 antenna.
Tangential Sensitivity:
The following is an average of 8 measurements for each antenna
over a period of time so as to tend to average out any minor pattern variations
and also to average out systematic measurement errors.
Lowe ---- Mighty Mouse ---- Big Brother ---- GA-26
+9
+5
+10
+7
These results show that the Big Brother offers slightly better performance
than any of the other antennas. The Lowe's and Garmin antennas are
close behind and the Mighty Mouse slightly below the Garmin unit.
NOTE that even the Mighty Mouse is 5db better than the built in G-II+ antenna.
In actual practice, we were frankly unable to tell ANY difference between the four antennas in "walk around" tests in the woods. All four antennas provided great signals and the variations in the woods from the signal attenutation and multipath in the woods were much greater than the differences between the four antennas.
We noticed that the Big Brother slightly overloaded the front end of the G-II+ when the antenna was "out in the clear". We asked Garmin how much "RF headroom" was available in Garmin GPS products in case one or another external amplified antenna might output large signal levels. Garmin advised us that their recommendation is that the maximum signal level out of the coax cable not exceed 20db gain relative to an unamplified antenna (the built in one). We noted that the Lowe's, Mighty Mouse, and GA-26 all fall generally within this range, but the Big Brother has about 10db of extra gain. We asked Tri-M System about this and we were told that the Big Brother was normally recommended only for those situations where the extra gain would be used to overcome the cable losses of longer cable runs. We still feel comfortable in recommending the Big Brother for situations where low signals are expected such as under heavy tree cover. It is possible however that situations may occur where multipath coupled with the extra strong signal levels output may result in poorer overall performance than would result with an antenna having lower electronic gain.
Approximate Street Prices of Antennas Tested
Lowes ---- Mighty Mouse ---- Big Brother
---- GA-26
US$63
US$64
US$74
US$70
Where you can buy these antennas:
Garmin
Lowe's (US
Dealer = CommSystems Intermountain)
Tri-M Systems
Please report any comments or errors noted to
Joe Mehaffey