Author Topic: Reduction of (number of) antennas  (Read 3736 times)

Kevin W

Reduction of (number of) antennas
« on: May 08, 2020, 02:26:30 pm »
So currently I have 3 antennas dedicated to pilot aware, an 868 colinear for Pilotaware, an 868 colinear for Flarm and a multiband colinear being used for ADSB. 

I need to free up the Multiband colinear for other purposes, so am going to try splitting the Flarm aerial between the adsb and Flarm dongles.

The 3db loss from the split I can make up with an LNA.  The worse 1090 reception due to a shorter and 868 tuned antenna i will have to live with - but adsb is the least of my worries given the higher power transmissions.

Any thoughts?  I will have to wait till we get some gliders back in the air to really tell the difference  of course!

Cheers
Kev
« Last Edit: May 09, 2020, 07:20:23 am by Kevin W »

Kevin W

Re: Reduction of (number of) antennas
« Reply #1 on: May 09, 2020, 07:27:53 am »
P.s.  Whilst the splitter I am using is designed for 75 ohms, apparently they are not horrendous at 50 ohms, and the lna should more than make up for any losses:

https://ka7oei.blogspot.com/2019/12/using-tv-type-75-ohm-splitters-and-taps.html

exfirepro

Re: Reduction of (number of) antennas
« Reply #2 on: May 09, 2020, 09:11:51 am »
Hi Kev,

I haven’t tried this myself (yet), but I know one of our group, Paul Ruskin, did a considerable amount of work on using shared antennas via splitters in the early days of OGN-R. I know he set up at least some of his stations using antenna splitters. I had several e-mail exchanges with him about it at the time as I was considering doing something similar with my own stations.

I haven’t seen Paul on here for a while, but you could try ‘PM’ing’ him through the message board. If you can’t get him that way, PM me and I will give you his e-mail.

As always, please keep us informed on progress.

Best Regards

Peter
« Last Edit: May 09, 2020, 09:44:12 am by exfirepro »

PaulRuskin

Re: Reduction of (number of) antennas
« Reply #3 on: May 10, 2020, 09:47:40 am »
So currently I have 3 antennas dedicated to pilot aware, an 868 colinear for Pilotaware, an 868 colinear for Flarm and a multiband colinear being used for ADSB. 

I need to free up the Multiband colinear for other purposes, so am going to try splitting the Flarm aerial between the adsb and Flarm dongles.

The 3db loss from the split I can make up with an LNA.  The worse 1090 reception due to a shorter and 868 tuned antenna i will have to live with - but adsb is the least of my worries given the higher power transmissions. 

Any thoughts?  I will have to wait till we get some gliders back in the air to really tell the difference  of course!

Cheers
Kev

Hi Kev

I used 50 ohm splitters to share the Flarm and PAW antennas.  It works well.

You lost 3 dB on each of the antennas - but the range is fine for the Flarm, and in the PAW case it's easily made up by the 9dBi antennas I was using. But you will I think lose that much by using a 75 ohm splitter anyway.

In your case you could do that, or you could try sharing the Flarm and ADSB antennas.  Generally there's enough power in the ADSB signal for the mismatch not to be a problem (I use an 868 co-linear as my ADSB antenna because I have a spare, and it works fine).

I had a batch of ~12 splitters donated to me which I used, but I've used a few of these  https://www.instockwireless.com/power_divider_pd1020.htm   I think that's what Orwell has (plus a GSM filter).

I've also just tried this https://www.m2.wifi-antennas.co.uk/2-way-wifi-gsm-splitter-unit which you can source from the UK.

I'm not sure it's worth using an LNA for the ADSB to make up for the splitter gain.  You've probably already got plenty of signal, and the issue seems often to be signal / external noise rather than signal / receiver noise.  In that case, your LNA merely amplifies both the signal and the external noise.  The exception is when you can put a narrowband filter in, which can help (which is why I've got a GSM filter at Orwell - a cell tower was installed 100m away).  But no harm trying. 

You're not really trying for extreme range - what you want is good signals from more local aircraft.  It may amount to the same thing though.

Paul
« Last Edit: May 10, 2020, 09:49:50 am by PaulRuskin »

Kevin W

Re: Reduction of (number of) antennas
« Reply #4 on: May 11, 2020, 08:55:25 pm »
Thanks Paul,

Interesting that you share the PAW and FLARM - I steered away from that due to the transmit on the PAW.  It has to deaden the Flarm Rx whenever it transmits anyway I suppose, but being directly connected feels like it would be worse?  Although I suppose the 2 antennas next to each other with their respective gains, vs the port isolation in a splitter - directly connected might be better!

Flarm and ADSB Rx wise I use flightaware pro dongles with the front end filter built in - the Flarm one I replaced the 1090 filter with an 868 one and that seems to work well.  Agreed I don't need the LNA for the ADSB, but didn't want to loose the 3db to 4db on the Flarm side.

Just tried it, but I get an issue that the SDR's show as a DC short - so they kill the LNA power supply, doh!  Setup without the amp for now then till I find a dc blocking splitter.

Cheers
Kev




Kevin W

Re: Reduction of (number of) antennas
« Reply #5 on: May 12, 2020, 12:16:51 pm »
Some more info as my research improves.

Resistive splitters - the £5 satellite aerial types have a 6db + insertion loss on each output, you need a Microstrip type to get down to the 3db + insertion loss on each output.

This site was very informative:  https://www.microwaves101.com/encyclopedias/resistive-power-splitters

Paul_Sengupta

Re: Reduction of (number of) antennas
« Reply #6 on: May 26, 2020, 03:12:55 pm »
What's the reason you particularly want to get rid of one antenna? The ADS-B antenna can be fairly unobtrusive, just a quarter wave ground plane or a half wave dipole (maybe end fed like on the PAW) would work ok given that it's out in the clear. A colinear ADS-B antenna might extend the receive range from 150 miles to 200 miles, but I don't think the object is to get that sort of range!

Kevin W

Re: Reduction of (number of) antennas
« Reply #7 on: May 26, 2020, 03:56:50 pm »
Hi Paul, a valid point, but as well as the feed up to 360 radar I use the feed for FR24 and other purposes, hence wanting to keep it a reasonably good feed.

Paul_Sengupta

Re: Reduction of (number of) antennas
« Reply #8 on: May 27, 2020, 01:28:06 pm »
If it's out in the clear, a quarter wave groundplane or half wave dipole will get you a pretty good signal. Or you could make a coaxial colinear in a bit of plastic pipe. I made one, works well!

PaulSS

Re: Reduction of (number of) antennas
« Reply #9 on: March 07, 2021, 01:48:27 pm »
A bit of thread resurrection (again) but does anyone have any suggestions for a simple two way splitter for an ADSB antenna?

What I'd like to do is feed both my ATOM station and FR24 dongle (on its own Pi) from the same ADSB antenna. I would prefer not to have to plug more things into electricity if I can help it  :)

Kevin W

Re: Reduction of (number of) antennas
« Reply #10 on: March 07, 2021, 04:34:56 pm »
Hi Paul,

You can split at the avr-tcp level, saving yourself duplicate sdr's and decoding cpu?

Just run fr24feed on a different host and connect it to port 30002 of the ground station.

Your fr24feed.ini should contain:

reciever="avr-tcp"
host="ip:30002"

Etc

Cheers
Kev

PaulSS

Re: Reduction of (number of) antennas
« Reply #11 on: March 07, 2021, 05:09:50 pm »
Thank you for your reply, Kevin. I understood "Hi Paul"  :)

At the moment I have an ATOM station that Keith built and supplied me. I connected the antennas and switched it on and it worked. I didn't have to do any Sudo stuff or anything.

I also have a separate Pi which feeds FR24. I did actually set that up myself with a LOT of help from the FR24 Forum and I even managed to get some graphs and a radar thing with some Dump 1090 info on it (whatever that is). At the moment the ADSB antenna for FR24 is just a small thing sat on the window sill. It still gets traffic at about 110 miles but I was getting 250 odd when I had a proper antenna attached to it (now being used for my ATOM station).

We'll be moving house (hopefully) in Apr and I would like to set up my ATOM station's antennas externally but also have to be sympathetic to my wife's 'instructions' regarding the fact that it is a Victorian house and doesn't need to look like GCHQ. To that end I would just like to have the one ADSB antenna and feed both the ATOM and FR24 in the simplest way possible.