by: Dale DePriest - all rights reserved.
While re-radiating antennas have been around for a few years now this new unit from PC-mobile.net breaks some new ground. All previous systems were designed specifically for in car use and required a 12 volts source to power them while this new unit will run off of 3 AA batteries. In addition the unit does not require that you purchase a special antenna only for this purpose.
The Re-Radiating Antenna set consists of 2 parts, an active antenna (GAS76X or GAS76B) and an appropriate power/transmitting Unit (GARTX/GARTB). You can also use the active antenna for another GPS device which has the appropriate external connector. So it is best of both worlds! In addition the GARTX or GARTB can be purchased separately if you already have an external antenna that you want to use. The older 5V antennas are not suitable for this use.
The power/transmitting unit consists of a battery pack that holds 3 AA cells and a re-radiating antenna attached with a 1 meter cable. A mating connector mcx or bnc on the other end of the battery pack completes the design. The batteries can be either alkaline or rechargeables and provide a very long service life. PC-Mobile expects a set of alkalines to last 200 hours and a set of NiMH to last 100 hours or more between charges. This, of course, is dependent on the battery drain of the active antenna you choose. The battery pack also has an on/off switch and a blue LED indicator to show when power is on. Since the batteries are likely the same as you use in your GPS they could also serve as emergency backup batteries if the ones in your unit run down. This makes the unit ideal for backpacking use where an external antenna can provide increased performance under heavy tree cover.
The power unit has a capacitor inside to block the DC voltage from reaching the radiating antenna which consists of a stiff wire in a loop shape. It does not seem to be tuned to GPS frequencies but does transmit sufficient signal if held in close proximity to the GPS internal antenna. The recommended attachment is to just use a rubber band around the GPS antenna and then insert the radiating antenna under the rubber band. I found that some signal coupling occurred with the antenna separations of up to about 4 inches but the closer connection is much better.
I compared a Garmin emap using a direct connection with the external antenna versus a re-radiating antenna connection using the same amplified antenna and a connection with a rubber band holding the loop antenna in direct contact with the unit. I found that the performance was similar under both static and dynamic conditions. One problem with any re-radiating antenna is that the internal antenna is still capable of receiving signals directly. This make a direct comparison under real world conditions difficult since the direct reception can sometimes aid and some times hinder the reception of a given satellite. Overall the results were very positive.
The attachment of the loop antenna to a gps is fairly obvious. Just lay the antenna over the top of the patch antenna and secure with a rubberband or tape. For helical antennas there might be a temptation to loop the antenna over the end of the stick. This is not the correct way to install the loop antenna. It should be laid beside the antenna lengthwise and then a rubber band or tape should be used to secure the two together. You might need to slide the connection up or down the antenna rod to find the most sensitive spot but it is generally near the center of the rod. For more tips on using this antenna consult the pc-mobile web site.
While this antenna can aid in reception under difficult reception
conditions it an be essential when the car has a windshield that blocks
reception or other situations where using the internal antenna is just
not satisfactory. I tested this combination on several Garmin units and
units from Magellan, the Navman iPAQ sleeve, and several Compact Flash
receivers hooked to a PDA.
The antenna shipped with the unit is a high gain external antenna that is sold separately as well. It will work on many gps receivers as an independent external antenna providing excellent performance. This means that the combination of this antenna and the power box provides all the antenna needs for now and any future GPS you might buy. The specifications indicate a full 26 dB gain which means it is very sensitive while only requiring about 12 mA of current. It will work on 2.5 to 5 Volt systems which makes it ideal for use with the GARTX or GARTB power unit or any of the GPS units that accept external antennas. I found that it outperformed the standard mouse style antennas often sold as external antennas for Compact Flash GPS units. The antenna weighs about 2 oz and another ounce for the cable which is 10 feet (3 meters) long.
The antenna in the picture has a right angle MCX connector which is a favorite among many GPS systems but there is also a model available with a BNC connector if needed. The cable is long enough for most uses but PC-mobile also has extension cables available. There is plenty of amplification in the unit to use an extension cable.
The antenna has a powerful magnet permanently attached to the back which permits easy installation on top of your car. The cable can then be threaded into the car between the door and door jam or via an opening in the window. Power to the unit and the signal share the same cable. Of course the antenna could also be used on the dash if you didn't want to locate it outside, but there is better performance from the roof since there is a better sky view and the roof itself provides a ground plane for the antenna. Generally I was able to pickup 1 to 2 more satellites using this antenna and had improve performance with WAAS enabled receivers as well. For hiking use the antenna can be placed on top your your hat with a washer or other piece of metal inside the hat to hold it on using the magnet. Other locations are certainly possible as well. Using an external antenna in this fashion permits the GPS itself to be carried in a pocket or case clipped to your belt.
A good sky view is often underestimated by many users. Some believe that if they can get reception from the seat of their car or while the GPS is in their shirt pocket then all is well. Unfortunately these conditions can often lead to inaccurate data since the vehicle or the human body can shield the signals leave the GPS with poor satellite geometry to use for the position solution. In addition an automobile can contribute to multipath reception errors and will cause the position solution to drift even when the vehicle is stationary.