External Ae Dilemma

[+tirediron]

This thread is a continuation of the discussion started in the "Getting Started" forum, which was becoming rather more technical than was appropriate!

 

Okay... Just tried a little experiment with my 76CS and Glisson external AE. After five minutes outside with decent sky exposure (my partially tree'd front yard) I had a solid lock on four birds with all signals at the half-way point or higher on the signal strength indicator.

 

At this point, I plugged in the external AE, and picked up two more birds, and all signal-strength indications went up to nearly full. I left it this way for approximately a further five minutes, at which time I covered the external AE with a large sheet of 18AWG aluminum.

 

After five minutes in this configuration, the unit displayed the three of the original four satellites at a slightly reduced signal strength. Does this mean that the internal AE is still in use? It would appear so to me.

 

Note that combining the signals from the two antennas is not necessarily easy. The best solution might require two signal processing units and then compare the output from one with the output from the other.

 

My understanding (Granted, I haven't researched it too deeply - I plan to soon) is that both the quad-helix and the patch are both variations of a basic long-wire AE, in which case (I assume) they simply operate as one, larger AE when the external is plugged in.

 

Having babbled on ad nauseum about a topic that I know a great deal less than I should, I will take the coward's way out and e-mail the tech weenies at Garmin and see what they say.

[+KYHilltopper]

Good luck getting the "tech weenies" at Garmin to answer any email. But on the other hand, I will replicate your experiment with my GPS and cover both antennas (one at a time) to see the difference. I am convinced that the antennas are in parallel (sp?) and the only down side of doing that is that the impedence of the circuit will change and some of the combined signal will be lost. You will, however end up with more signal than if you only had one antenna. Receiver systems have used "co-phased" antenna systems for years with the result as above.

 

Clint

[peter]

Okay... Just tried a little experiment with my 76CS and Glisson external AE. After five minutes outside with decent sky exposure (my partially tree'd front yard) I had a solid lock on four birds with all signals at the half-way point or higher on the signal strength indicator.

 

At this point, I plugged in the external AE, and picked up two more birds, and all signal-strength indications went up to nearly full. I left it this way for approximately a further five minutes, at which time I covered the external AE with a large sheet of 18AWG aluminum.

 

After five minutes in this configuration, the unit displayed the three of the original four satellites at a slightly reduced signal strength. Does this mean that the internal AE is still in use? It would appear so to me.

Hard to say without seeing how you positioned the "large sheet" and whether signals could still get in under it and be reflected to the antenna. I'd be more convinced if you wrapped the antenna in aluminum foil or even just closed your hand completely around it. The latter is sufficient on mine to make the signal strength (really SNR) bars all go to zero if I cover the Gilsson even if the internal antenna still has a clear sky view. Conversely if I cover the internal antenna while the Gilsson is conected it does not affect the signal bars at all.

 

If your 76cs is working properly only one antenna at a time should be active. Once the unit detects current being drawn by the external Gilsson antenna it should disconnect the internal antenna. Getting signals from more than one antenna can lead to problems with interference. The wavelength of the GPS signals is about 8", so if one signal were delayed by only 4" relative to the other then it would be out of phase and the two would subtract from each other. So with a varying delay between the two antennas you'd be having the signals sometimes adding and sometimes subtracting resulting in fluctuating levels and poorer reception. Of course if the signal from one of the antennas is substantially stronger then the interference problem may not be significant. Still better to avoid it altogether though.

[+tirediron]

The large sheet (24" x 24") was placed directly on top of the external AE which was sitting on the ground. IMHO, zero room for signals to "slide in underneath" but a valid question! I will try again with the tinfoil after I have gone shopping..

[+tirediron]

If your 76cs is working properly only one antenna at a time should be active. Once the unit detects current being drawn by the external <snip>

Okay.... but do you have a reference?????

[peter]

If your 76cs is working properly only one antenna at a time should be active.  Once the unit detects current being drawn by the external <snip>

Okay.... but do you have a reference?????

Unfortunately not from Garmin, but Dale's unofficial Garmin Manual

http://www.authorhouse.com/BookStore/ItemD...z%252boniI%253d

says: "Garmin supports two kinds of remote antennas. One uses a very small mcx connector and when plugged into the unit it removes power to the internal patch antenna. To do this Garmin has a circuit inside that senses the power drawn by the external unit and switches the antenna source." This is also consistent with how things work with my eMap and from reports of others who tried connecting unpowered antennas and found they needed to draw power from the MCX connector before the antenna would be 'seen.'

 

However, I have not done any tests on a 76c/cs model and I believe Dale's manual also predates this model. I'll be interested in how your test comes out with the antenna fully shielded. I'm surprised there was still a signal with the sheet of aluminum on top of the Gilsson.

[+=BB=]

Just a technical note:

 

When you replicate the experiment - as noted, you should cover the entire GPSr and likewise for the antenna. In addition to this, you should consider grounding the metal cover to provide a Faraday shield.