GDL 88 Installation Question

[PR474]

I am about to have a GDL 88 and Flightstream 210 installed in my Garmin 530W equipped M20J. The avionics shop wants an extra $1000.00 to install the "diversity" antenna system (top and bottom antennas). Does anyone have just the one antenna installed and is it worth the extra money to have both.

[Marauder]

Just now, PR474 said:

I am about to have a GDL 88 and Flightstream 210 installed in my Garmin 530W equipped M20J. The avionics shop wants an extra $1000.00 to install the "diversity" antenna system (top and bottom antennas). Does anyone have just the one antenna installed and is it worth the extra money to have both.

I'm still in the prep mode for my upgrade. I think most owners have opted not for the diversity option. As I'm sure you have figured out, it is designed to ensure full TIS-B coverage. Not sure how much value it has on our size plane. 

I'm curious to hear what others think.

[jonhop]

I'm planning to do the same install this summer and would be interested in your final cost of installation.

[Bennett]

I just have the single bottom antenna installed, and TIS-B works just fine.

[Bob_Belville]

38 minutes ago, Bennett said:

I just have the single bottom antenna installed, and TIS-B works just fine.

Ditto. I've had the GDL88 for 3 years and have only the one antenna. As far as I know I've not missed anything.

[dmedders]

Our shop recommended against diversity.

We have two belly mounted antennas.

Cheers,

David

 

[carusoam]

Welcome aboard, David.

Best regards,

-a-

[flyboy0681]

I'm into the first week of using the GDL-88 with a bottom antenna only and I can't imagine the traffic picture being any better than it is. 

[Jerry 5TJ]

So how do you know what you're missing? I have the GDL-88D on the Ovation with both top and bottom antennas.  Do I see more traffic than with one antenna and no diversity?  Do more aircraft see me?  Probably yes but I have no data. 

Maybe the question should be: How many non-cooperating targets are out there that you don't see on the traffic display no matter how many antennas you have?  On one short flight I went past two gliders and one tow plane and none of them showed on the screen.  They had no transponders and no ADS-B out. Legal in that airspace.

At work, decades ago now, I had access to engineering data on diversity transponder vs single antenna transponder reply probability. (Happened to be Mode 4 SIF data but close enough for us) On a pulse-by-pulse basis diversity made a big difference but on multi-pulse basis the advantage was much reduced.  I can visualize a similar situation with ADS-B UAT or Mode S ES: You may miss one message but a few seconds later you get the next one and the traffic is displayed.  

Glad I have diversity and glad the previous owner paid for it.

 

[larryb]

The ADSB transmitter is 100 watts. At the distances we really care about, 5 miles or less, I think we will receive the transmission from that target with a single antenna. The transmissions are once per second. Like Jerry says, if you miss one you'll get the next 1 second later. I got the single antenna for mine.

[PR474]

Thanks for the responses. Sounds like the second antenna may not be necessary, so I think I will go with the single antenna and save the 1k, or maybe upgrade my transponder while they are working under the panel.

Thanks again,

Paul

[TWinter]

I guess I'm in the camp of do the "Diversity" install. I have the two antennas. Not sure what I would be missing without, but I guess my thought was "While they are there go ahead and do it", I'm sure it would cost later if they had to re-wire and re-run wires. This philosophy might get me suspended from the CB Club for a week or so, but if you can swing it why not now than later...assuming you might want it later.

 

 

-Tom