One antenna - Two Pro Sticks


#1

Now that I have my FlightAware 978/1090 antenna on it’s way, I am starting to wonder what is the best way to feed two pro sticks with one single antenna. My proposed setup would be (based on parts I already have):

Antenna -> 20’ LMR-400 run -> sma T and then each side of the t would feed into a filter attached to it’s respective pro stick. Both sticks would feed into the same RPi.

Is just a basic gold plated tee good enough instead of a combiner? Are there going to be issues having both sticks tied together like I’m planning? Or am I on the wrong runway all together?

Not opposed to another widget, just happen to have the tee sitting here in the spare parts bin.

Thoughts?


#2

The dongles tend to leak noise back towards the antenna. You will want a proper splitter/combiner with good isolation between ports, not just a simple tee.


#3

Thanks. I was wondering if they were noisy. You answered my question. I’ll start looking for a splitter/combiner.


#4

If you feel like experimenting and collecting data both ways, I’d be interested to see differences between the tee and the proper splitter. Maybe also 1 filter on the tee input vs 2 filters on the tee outputs.


#5

Sure thing. I was already thinking of the two vs one filter test. I’ve ordered the splitter. I’ll start taking notes! :slight_smile:

The site that I will be playing with is # 10130 - KPDX. Right now it’s the original FA antenna, 20’ LMR-400, FA Filter, FA Pro Stick. It had a bump in activity a week or so ago because I finally got the antenna about 19’ off the ground and outside (vs. inside attic).


#6

Tee is no good for RTL sticks as obj pointed out. The idea is to keep spurious emissions from the stick RF front ends from traveling between them directly through the tee.

Using a FA filter on each port, you are essentially creating a perfectly filtered path that blocks everything but the ADS-B frequencies. That means that any spurious emissions from one stick to the other that occur in the frequency band of interest, 1090, will freely pass between sticks, creating intereference.

This is a good deal:

ebay.com/itm/Mini-Circuits-Z … SwvUlWq5oG

This is a better deal if you are only interested in ADS-B 1090:

ebay.com/itm/MCL-ZAPD-2-8943 … SwAvJXBs5r

The MiniCircuits line is lab grade and normally costs about $75. They show up on eBay frequently for $15-$25. You’ll see many models, but stick with the
ZAPD-2 (best for ADS-B), or the ZAPD-21 (wider bandwidth, will allow UAT 978)

You will see no discernable difference between using one filter on the splitter input vs using one for each output. The splitter has plenty of isolation between ports, on the order of >26dB at ADS-B frequencies.


#7

I found this one online last night, and went ahead and picked it up. It’s close, cheap and the bandwidth is 700mhz - 1.4Ghz. I figured a tighter band wouldn’t be a bad thing. I saw the mini-circuits ones as well. I might pick one up just to have a spare to play with.

ebay.com/itm/261911413465?_t … EBIDX%3AIT


#8

I use Mini-Circuits splitters (mostly because I have them, and I use quality 50 Ohm feedline). For quite a while I ran a pair of SDRs from an antenna system that consisted of discone antenna -> high-pass filter -> Mini Circuits LNA -> mini circuits splitter -> SDRs.

I modified that setup to include a SAW filter after the LNA, which allowed me to bump the SDR gain up, getting better numbers.

Since switching over to the FlightAware filters and SDRs, I have one test setup where I use a Mini Circuits splitter fed by the antenna, with filter/sdrs connected to the splitter.

I connected a non-FLightAware SDR to the input of my spectrum analyzer a while ago, and yes, it’s a noisy little beast. Haven’t done the same with the Flight Aware SDR, but I’d suspect/hope that since it includes a LNA, that LNA would provide some isolation and reduce the noise.

bob k6rtm


#9

I went ahead and ordered a Mini-Circuits as well. I’ll post pics as soon as everything is installed. Thanks everyone for the help and information.


#10

Just an observation for those wanting to have one stick for ADS-B and one for UAT, the low rolloff for the FA Filter is just below 978, so it works great for both frequencies.


#11

Can you explain why you would want to do this? I’d be more interested in using two antennas with a single feed.


#12

Peter–

Please, NO on the SMA - T! Use a splitter. And you only need one filter, put it before the splitter. A nice little splitter is the Mini Circuits ZX10-2-20-S+ – look for them on eBay, or for any Mini Circuits splitter that covers to 2 GHz.

First, using a T produces a big impedance mismatch. Not as big a deal on receive, but it’s avoidable. Using a splitter matches the impedance, costing you a little in terms of signal loss, but not as much as using a T.

Second, the splitter provides isolation between the two SDRs. Not as big a problem with the FlightAware units, as they have an internal LNA which provides some isolation (or it should; I haven’t looked at one on my spectrum analyzer). Using two filters will give you more isolation on out of band signals, but I’d go with one filter ahead of the splitter.

73 de bob k6rtm


#13

What is the purpose of using the splitter? I assume to split the two frequencies but what benefit will that give compared to just one cable to the filter/pro stick?


#14

The whole reason for using the splitter is that you can then run multiple dongles from the same feed. If you look at my statistics, you can see that I have two feeds, they’re both coming from the same antenna, amplifier, filter and splitter. With this set up, I can change one system and leave the other consistent so that I can then compare apples to apples as to if the change was a net gain or loss.

Another reason is for running UAT and ADS-B dongles (different frequencies) so that you can pick up 1090 and 978 traffic simultaneously from the same feed assembly.

Also, as mentioned above, you want to have a good isolation between the receivers to give you the best chances of keeping them from interfering with each other.


#15

So my first splitter came in the mail this weekend. Was able to install it. Noticed a drop of about 15-20% of my messages/sec. Playing around with external pre-amp between antenna and splitter got my messages back to almost what they were before. I’m waiting for my mini-circuits splitter to arrive so I can compare to the existing one.

Just to ease everyone’s minds, the Tees have been secured back in the junk drawer they came from! :slight_smile:

New antenna and dongles show that they are out for delivery today. Hoping to get the old antenna swapped out with the new one when I get home from work tonight. Keeping my fingers crossed the weather plays nice.

And like was stated above, the reason I am doing this is so I can have one antenna feed multiple receivers.