External antenna

[HMR]

I mounted an external amplified antenna but to my surprice the performance was not as expected.

After some time I figured out that I had problems with antenna signal closed loop. The antenna is mounted a little too cloose to the GPS and the 276C leaked signals that was picked up by the antenna creating a signal shortcut. Same phenomena you see when you hold a microphone to close to the loadspeaker.

I had to mount a metal shield between the 276C and the antenna before it worked as it should.

 

[Deleted account 220427002]

why did you not just mount it on the beak?

 

[Deleted account 220427002]

or

http://www.ukgser.com/forums/showthread.php?s=&threadid=25807&perpage=16

might help explain the bad signal too

 

[HMR]

Tuned In,
the reason for mounting the antenna where I did was that I wanted it so see as much sky & horizon as possible. I considered mounting it where you did but I didn't like the idea of having 30% of the sky cut away by the bike.

I guess there is no measurable difference in the performance....

 

[PanEuropean]

I mounted an external amplified antenna but... I had problems with antenna signal closed loop. The antenna is mounted a little too close to the GPS...

Interesting that you should encounter that on a motorcycle installation. There is a general rule in aircraft GPSR antenna installations that no antenna should be closer than one meter (one yard) from another GPS antenna. I didn't think that would be an issue with automotive installs, but, I guess it is something that has to be considered.

On my bike, the antenna is about 6 inches away from the GPSR, mounted on top of the right hand brake fluid reservoir (just inboard of the throttle grip). It has worked OK in that position with a great variety of GPSRs (296, 25xx, 26xx, 27xx, 7xxx, etc.) but maybe this has to do with some other factor - perhaps I have a different antenna than you do, or the cabling is different, etc. It has quite a good view of the horizon from that position - my body blocks about 15% of the sky view, but that doesn't seem to make much of a difference to performance.

One thing you may want to experiment with (to eliminate the need for the metal shield behind your GPSR) is shortening the cable between the external antenna and the connector so that the cable is no longer than it reasonably needs to be. Normally, external antennas are supplied with about a 3 meter (3 yard) long antenna cable, which is far more than we need for motorcycle use. The easiest way to get the cable shortened is to take it to an avionics (aircraft electronics) shop at an airport near you. These shops have the tools and test equipment needed to do a 'military spec' job of fitting a new connector onto the end of the cable, and then testing it to make sure that the connection is solid and performs as it should.

Michael

External Antenna fitment - my moto
<a href="http://www.hostdub.com/PanEuropean:Remote_on_Moto_2720:full"><img src="http://www.hostdub.com/albums/PanEuropean/Remote_on_Moto_2720.jpg" alt="hostdub.com"></a>

 

[BurnieM]

I too am a little suprised that you had problems.
What makes you think that you were getting residual interference from the GPS ?
Certainly the internal frequencies used are; very far away, not a harmonic and low power.

One thing that I have seen is that if you mount the remote antenna on a non-metal surface then the signal received is often poor, worse than the standard antenna. I guess most remote antenna require a ground plane, certainly the ones from San Jose Navigation do.

I do question whether a remote antenna is required on a modern receiver.
It certainly made a difference on my GPSmap 176 but my GPS V and GPSmap 276C work fine and acquire quickly without one.

 

[PanEuropean]

I do question whether a remote antenna is required on a modern receiver.

My experience has been that any Garmin GPSR since the StreetPilot III works just fine without an external antenna, as long as the GPSR is no more than 15° off horizontal or vertical. Once the GPSR is tilted more than 15°, the performance of the built-in antenna starts to suffer. In such cases, using an external antenna that can be mounted flat helps restore performance to what it would be if the GPSR itself (with its built-in antenna) was horizontal - as it likely would be on a car dashboard, but often is not when mounted on a moto.

I think the point Burnie is trying to make - which is that an external antenna will not, a priori, improve satellite reception - is worth noting. In other words, if you are not encountering degraded satellite reception as a result of tilting your GPSR, then there is no benefit at all to be gained from purchasing an external antenna, unless (for example) you plan to mount it on a tall mast at the back of your bike so that it has a clear view of the sky in every direction.

Michael

 

[HMR]

In regards to the comments from Pan & Burnie:

- The reason for using an external antenna was that the original antenna could not point upwards without being in the way for the instruments.

- The antenna cable is cut down to the shortest length possible. About 1 m.

- I don't know much about antenna technology but I mounted the antenna on a metal console that is electrically connected with the frame with the purpose to create an earth plane. Maybe the surface is not big enough?

- One reason I had for not mounting the antenna on the plastic brake fluid container (or on the beak) was that I wanted to mount it on an metallic earth plane. A can lid, as someone else used, will not work without wiring it to the frame.

- How did I find out what was the problem? The GPS worked OK but the satellite indicator bars where much lower than with the original antenna. If I put my hand between the antenna and the GPS then 7-8 of the bars directly went up to 100%.

I was quite surpriced that it didn't work without the shield. I have seen several installations with the antenna close to the GPS that works without problems. Maybe my antenna has higher gain than usual or maybe my GPS leaks more than other devices?

 

[PanEuropean]

I don't know for sure if a GPS antenna needs a metal surface below it or not. I kind of think it is not required, because my Volkswagen has two GPS antennas embedded in the rear window glass (no metal surface there), and a third antenna mounted under the front fender (wing), sitting on top of a large foam molding that lies underneath the fender. The front fenders of my VW are plastic.

Michael

 

[BurnieM]

Depends if the antenna is designed to be ground independent or not.
Most (all?) antenna with a magnetic base are designed to use the metal they are sitting on as a ground plane.


This is my setup, unit is about 40 degrees from horizontal;

The only time I have used a remote antenna (San Jose MK-76/BNC on a tin (can) lid velcroed to my brake master cylinder) is when we were collecting tracklogs for a mapping project and I had to do 20km thru a dense, wet forest

The antenna was originally purchased for my GPSmap 176 and it noticably improved signal strength and reduced acquisition time on this earlier generation receiver.

 

[shedracer]

I have a Garmin GA 26 antenna BNC, which I use and it came with a aluminium mount with 3 little suckers to mount to a windscreen. Now obviously the magnet will not stick to aluminium and the rubber base that incorporates the 4 little magnets has to be removed so you can screw the antenna to the mount. So Garmin do not insist that their branded antena must be mounted to a steel surface to ground the device. Saying that though, I have noticed an improved signal using the antenna with the magnet base on the outside of a vehicle, but I would assume this is just because of a clearer sky view.