GPS Fishfinders - external vs internal antennas

[gregk9]

Any preference here? I'm assuming the external would get better reception, no?

 

[russ010]

I've got internal on my 788ci - and I have had no problems.. even when it was under the tree the other night

I would go with internal if you have an open boat - it's one less thing to worry about running wires for or worry about getting damaged.

 

[longjohn119]

Any preference here? I'm assuming the external would get better reception, no?

It depends on the receiver. Most external antennas I see need 3.3 to 5V from the antenna socket because they have to have an amplifier to overcome the high losses in the cheap coax they use and/or too small of an antenna element. If there isn't any voltage from the antenna jack then none of the amplified antennas will work at all. Most internal antennas, nearly always undersized, use an amplifier between the antenna and receiver input. Make sure your antenna input jack has voltage output before you buy an external antenna, the manual will tell you if it does.

My current GPS setup is a GPS receiver built into a Gobi 1000 cellular data modem which wasn't enabled by computer manufacturer (Acer) and thus had no antenna 'cut' for GPS frequencies (The GPS antenna input also doubles as a diversity antenna input for 3G which is how Acer had it set up) There is also no voltage output in the antenna jack so I'm using passive antennas with high quality hi-frequency coax. The antenna I'm currently using is a 'patch' antenna taken from an Eagle 240S/GPS with a bad LCD and mounted on a larger base plate (Unetched PCB board) to get a little 'gain' from the antenna. It actually works amazingly well considering I whipped it up in under 15 minutes, I haven't experienced any dropouts and average 8 satellites and an HDOP of 1.2 or less even with the binimi top up. (I have more coax on order and will eventually lengthen the antenna feed so I can velcro the antenna on top of the bimini which should increase reception) I still plan on building a couple of quad helix antennas for it eventually because they are superior in every way (except space needed) to the much more common (and usually undersized) patch antennas in most GPSes.

 

[redbug]

internal will work fine my hds7 lowrance works fine. i also have a lcx27c that has an external antenna that works fine but I had to fish the cable through the decking to the dash

 

[longjohn119]

There is absolutely nothing wrong with the antenna/front end of the Lowrance/Eagle GPS receivers, even the build quality on the entry level 240S/GPS is outstanding, that's exactly why I took the antenna from it and will eventually hack out the postage stamp sized antenna preamp/quartz filter circuit ahead of the GPS receiver chip. It's a much better design than the ones built into the aftermarket external patch antennas and since it's only trying to overcome a slightly undersized antenna instead of cheap lossy coax it's low gain, and thus low noise. Me I prefer full sized passive antennas and quality coax because of their higher sensitivity and zero noise figure but I'm actually making maps not just using them so I'm shooting for HDOP of less than 1. I mention HDOP because it's a measure of the quality of the signal which is more important than the signal level because the signal level also includes all the noise that degrades accuracy.

If a person is using a decent quality handheld GPS he'd probably get better reception and accuracy by buying and using a mount and getting his hands and body away from the internal antenna than by getting an external antenna in the less than 50 buck range. All of the under 50 and many of the over 50 dollar antennas I see use coax that really shouldn't be used at 500 Mhz much less 1.5 Ghz. What good is a 10 dBi gain antenna when you hook it up to 10 feet of coax with a loss of 7-8 dB at 1.5 Ghz that will only get worse over time as the cable and connections degrade? (This is a problem common to aftermarket WiFi and Cell phone antennas too) Their main benefit is getting your hands and body away from the antenna and you can do the same and better just by buying a mount and using the internal antenna/amplifier that's matched to the receiver and not just a one size fits all compromise.

 

[wasilvers]

I've got internal on my 788ci - and I have had no problems.. even when it was under the tree the other night

I would go with internal if you have an open boat - it's one less thing to worry about running wires for or worry about getting damaged.

I have the 798ci (internal) and can get a reading in the garage and next to a window in my living room. Internal is one less thing to hookup. Only issue I have is if you mark a location going north, then go back south, you could be a good 15 feet from it - assuming it marks 'just the right spot".

 

[gregk9]

Only issue I have is if you mark a location going north, then go back south, you could be a good 15 feet from it - assuming it marks 'just the right spot".

Do you think that's an antenna issue or the unit itself?

 

[wasilvers]

Only issue I have is if you mark a location going north, then go back south, you could be a good 15 feet from it - assuming it marks 'just the right spot".

Do you think that's an antenna issue or the unit itself?

It's just the difference of the transducer being 8 feet behind my internal gps unit. If you have an external antenna, you just mount it above the transducer and you are as close as you can get.

 

[longjohn119]

Only issue I have is if you mark a location going north, then go back south, you could be a good 15 feet from it - assuming it marks 'just the right spot".

Do you think that's an antenna issue or the unit itself?

It's just the difference of the transducer being 8 feet behind my internal gps unit. If you have an external antenna, you just mount it above the transducer and you are as close as you can get.

The transducer has nothing to do with the accuracy of the GPS, your GPS antenna is in the exact same place in the boat both times. Only if you are making a depth map does the relationship between the GPS and transducer needs to be taken into account. Programs like Dr Depth allow you to calibrate but it doesn't change your GPS location, only your depth readings at a given location.

What you are experiencing is most likely a degraded HDOP,(horizontal dilution of precision) in fact I can estimate your HDOP which is between 2 and 3 (meters). A quality GPS with a good clean signal from 6 or more satellites should have an HDOP and hence an error of 1 meter or about 3 feet although many manufacturers settle for a HDOP of 2 or less. Most GPS units will allow you to take multiple readings and then average them out to give you better accuracy. A lot of units also suffer from poor screen resolution, for any decent kind of accuracy you need a pixel for every meter or less and some units the screen (Or built in map) resolution is 2 of 3 meters per pixel so it's easy to be 8 ft off if you are just one pixel off your mark.

 

[wasilvers]

Let me start by saying, I understand GPS units can be off just because of the design and limitations. My theory uses a perfect world situation, where I mark a spot with the gps and I can return to the exact spot again.

That being said, if you mark a spot on the GPS, where the antenna is in a different location than the transducer, it will mark the spot of the antenna. If you approach the waypoint from a different direction, you will (in a perfect world) be off by the max of the double the distance your gps antenna is from your transducer when you first marked the location. I scratched these out to illustrate...

Say you find some structure with a transom mount transducer and mark it with your internal antenna gps unit - like this....

Then you do a 180 and return to the waypoint with your unit. The unit will return to the same place it originally saved, but the transducer will be farther away from the structure originally marked. Like this...

It isn't a real big deal and really hasn't affected my gps use yet. I use it to find areas and fish. It might be more to think about if I fished cribs and such. But I haven't found any yet :roll:

Will

 

[longjohn119]

It doesn't matter, your transducer has absolutely nothing to do with GPS position, your position is determined by the placement of the antenna and nothing else. The only error from the transducer would be the depth not position. 90+% of GPSes don't even have a transducer nor have any need for one.

 

[longjohn119]

You also have to remember that if you see structure (Or a fish) on your sonar screen, you are already past it and have to back up to get back on it. With my style of fishing I've learned to use that to my advantage. Since I mainly anchor in current which means my nose is pointed upstream and I fish off the back of the boat, I don't want to be right on the spot but a few meters upstream. So I motor upstream over my GPS waypoint and when I see the structure I'm looking for on my sonar screen (At this point I'm already past it upstream) I just coast a couple more meters and drop my anchor and slide back a bit and I'm right in front of my target and I can work pretty much all sides of the structure.

If I need more accuracy I depth map the area, create a 3D map and use it to put me within a couple of feet. For example this old barge piling/rock pile which is only a couple of meters across at the top and I don't want to have to float over it to see it on my sonar but just nose up to it so I don't disturb the fish. I use this map and although it doesn't show it here it places my boat on the water's surface and draws a line down to show me exactly where I'm at in relation to the bottom structure. (These maps also rotate on both the horizontal and vertical axis) In this case I could just as well turn my sonar off since It's not going to show anything since I not going over my target and besides, I already have my depth data