GPS antenna question

[fixit]

I have an ICOM GPS that has a bad external antenna, Do I have to get a ICOM antenna or will a Garmin or whatever work also??

[David-C]

Good question!

Have you tried a word search on the internet?

Good luck!

David

[fixit]

Dave, Most all units are 12 channel units, It works and will see what happens. I have an Icom 360 GPS and am using a Garmin antenna.

[reelclass]

I to have a ICOM GP 270ML and need an antenna. After talking to the dealer rep in Washington state he said that more than likely the problem was a blown antenna due to electrical storms. I agreed as we had some wicked stomrs for a week and I left the antenna mounted on my grady white offshoe 24 all winter. It worked fine thru the eary spring and after the storms the next time I tried to power it up it would not pick up the satelites needed to give data for locations. But here is the info you want I have just ordered an antenna from the internet should be here by thurs fri next week. If it works good I can leave you a b-mail and give you the sellers info good luck and tight lines

[Grady23]

Why not test the antenna w/ a digital OHM meter. There sholud be a lable that shows the impedance. Just set your meter to OHMs and test from the outside metal ground/inside center pin.

[fixit]

Well, Here is the scoop... The Garmin antenna works better than the Icom did EVER !!!! Full satellites from shore to the canyon with no problems.......

[fredhead]

Grady23 originally wrote:
Why not test the antenna w/ a digital OHM meter. There sholud be a lable that shows the impedance. Just set your meter to OHMs and test from the outside metal ground/inside center pin.


The impedance listing is (for lack of a better description) the "resistance" to the radio frequency (RF) AC signal. Impedance is a combination of DC resistance, and capacitance and inductance, which only affect AC signals, and change with frequency of the AC signal. For instance, your 50-ohm VHF antenna will not read 50 ohms between the center conductor and the shield of the cable using the ohmmeter.

The digital ohm-meter uses a DC voltage (usually a 9-volt battery) to test resistance, and cannot test impedance. It can, however, be used to check for an open or shorted cable. I have seen some antennas (in my 28 year career as a Navy and civilian electronics tech) that read a DC short, so there's another reason not to use it to test an antenna.

Since the GPS antenna is a receive only, there is no simple test for it. For a transmit/receive antenna (i.e. VHF) an SWR (standing wave ratio) test that compares your forward and reflected transmit power is a simple test of your antenna.

Also, your GPS antenna probably has some circuitry inside, such as a preamplifier to boost the received RF signal before it is sent to the receiver. This compensates for losses in the RF transmission line. The antenna may be good and just the preamp blown. A lot of times it is powered by a DC voltage carried on the RF line. You can test for this DC voltage by using a DC voltmeter on the back of the receiver (antenna connection) between the center conductor and shield. If you have voltage there, there is probably some circuitry inside the antenna assembly, and a diferent manufacturer's antenna may not work.