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Ep1607 ADSB Reception


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I'm enjoying this series quite a lot. I got interested in the SDR by watching your TPM episode. The SDR's are quite contagious.

A couple of thoughts on the latest show. I'm really disappointed in the number of planes you received with your antenna, given the location. I would have expected you to have received 3x the numbers you did.

I have a couple of thoughts based on my own experiences. The first thing that jumps to mind the the amount of RF-noise you are getting off the active usb extension cord. I had one that completely overloaded my rtl-dongle with rf. Just by adding a short passive usb extension on the end, the noise floor drops dramatically. I've also added ferrite chokes to help as well.

Your right about signal loss on the coax. The closer you can get the RTL to the antenna the better. However, that's not always practical. I'm using an in-line amplifier designed for satellite TV. Its approximately $9, and adds 20db of gain from 950-2150 mhz.. With this added at the base of the antenna, I can easily run 20 feet of coax to my SDR. I'm using an antenna similar to the one you built, but only 11 segments. It was 12, but there was an accident installing it. :)

I moved away from the USB extension to a raspberry pi. Hence the user name "PiDAR". I've attached a gps to the pi with a PPS signal giving me a stratum 1 time server for my network, as well as allowing precise time stamps for calculating positions of aircraft not broadcasting their location (MLAT - Multilateration).

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I'm in the Little Rock Arkansas area and am receiving aircraft well over 200 miles out, 80-90 aircraft at peak times.

A friend of mine, in the same area, built an 16 element antenna and used the amplifier I suggested .Yesterday he recorded 104 aircraft at once, with a max of 250 miles out.

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When I read about collinear antennas it was said that it was important for your segments to be flat/level as opposed to twisted. Looking at the way the cable was put inside the pipe, it's almost certain to be twisted and supposedly that's detrimental to your signal.

Now, I have ZERO actual experience with this so YMMV but if you're not getting very good reception it's a quick and easy thing to verify and/or adjust.

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My antenna is very similar. The 1/2 inch PC pipe doesn't have a lot of extra room in it. With the top of the coax anchored at the top of the pipe, the weight of the coax should pull it pretty straight. I don't think that would be hurting them that bad. I've never given it much thought, but it is an interesting theory.

Edited by piDAR
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It surprises me that a 16 ele collinear would outperform the 8 ele , after all the planes are at quite a height and should be perfectly obtainable with a quarter wave with radials provided it's out in the clear.. I believe the 8 ele is basically just a 4 ele with 4 elements for phasing meaning a collinear made from stacked J poles with either coiled or sidewards elements should be smaller with the same amount of gain. A slim jim normally gives a reasonably low angle of radiation with very little building know-how or parts/time etc. Australian CBers make coaxial collinears for UHF. these are normally mounted on a wooden dowel with cable ties - this holds it straight.

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I would have used something like a J pole antenna so that way you get a lower take off angle.

There are plans online that mirror the Arrow Antennas J pole. It would IMO be eaiser to build and perform better than the colliner. The PVC had to effect the resonence of the antenna.

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First off.... the idea of having 16 elements is for gain... The gain is produced by a couple of things. First is the amount of area of the entire antenna. If you look at it simply as 16 elements have twice the area as 8 elements.... second, the 16 element antenna has it's gain at a lower angle... lower to the horizon. If you want downward beam tilt... if your using a collinear on a high tower to talk to mobiles on the ground, you want the angle of the horizontal beam to be lower than the horizon.... With aircraft, they don't do so well at low altitude :rolleyes: ... so our ADS-B antennas do just fine with up-tilt... To get that tilt angle in the beam, to lower it, make the elements 2% longer that the formula... for higher beam tilt, make them 2% shorter.... One more thing... A quarter wave has somewhere around 20-40 ohms impedance (depending on the ground side of the antenna... ie radials, car roof...etc.) so you want the antenna to match the impedance of the cable/antenna port of the radio..... so.... If the impedance along a feed line repeats itself every 1/2 wave.... AND you use 1/2 wave elements.... Then you need a 1/4 wave whip at the top of the collinear.... Remember the input connector to most RTL dongles are 50 ohms.... try not to change this in your feed line and antenna.... (Hint...RG6* is 75 ohms.... why do you use it?)... answer, its cheap, easy to assemble with the solid conductor... and "F" connectors are good to 4GHZ.... So many things to think about, so little time!!!

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I would have used something like a J pole antenna so that way you get a lower take off angle.

There are plans online that mirror the Arrow Antennas J pole. It would IMO be eaiser to build and perform better than the colliner. The PVC had to effect the resonence of the antenna.

The "J POLE" is a neat antenna... easy to make and it is grounded to help with lightining... but, it's no match for the gain you can get from a properly built collinear... The PVC you want to stay away from is... the gray or black types.... to many metals used in making them... White PVC pipe will only affect your antenna in the slightest.... Fiberglass is your best friend.... nearly zero affect...

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The "J POLE" is a neat antenna... easy to make and it is grounded to help with lightining... but, it's no match for the gain you can get from a properly built collinear... The PVC you want to stay away from is... the gray or black types.... to many metals used in making them... White PVC pipe will only affect your antenna in the slightest.... Fiberglass is your best friend.... nearly zero affect...

http://www.tgc-ares.org/DBJ-2%20Roll%20up.pdf

Mr Fong goes into some good detail on the effects of PVC. The DBJ 1 and 2 are both great antennas. If you look at the take off angle from the eznec models it has a take off angle of about 45 degrees. That would give you the farthest reach.

I like colliners but at VHF they are getting pretty big with lots of points to have a failure.

No matter what antenna you go with it beats the pants off the stock one! P;us at the size you are messing with for ADSB you can make a whole lot of antennas with not too many supplies. I would like to see Darren to stick his antenna on a VNA and get some specs.

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2% at 1090Mhz is a shaving or a slip with th knife! I doubt the RTL is 50 Ohm it's more likely to be a 75Ohm device as it's a TV receiver. J-Pole's won't have that low of an angle of radiation as although it's 3/4 wave in length it's a half wave with a J matching stub. Slim Jim's are a natural progression to a J-Pole and will lower the angle. W7PP is right about the downward tilt, this is why I suggested that a grounded quarter wave or a dipole should not work so badly..

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Don't get me wrong jobdone or W7PP (nice station) I don't think a jpole is the end all be all. Just throwing something out there.

I think it would be cool to see them team up with Ben Heck to make a phased vertical ground plane that rotates and then if you select a flight number it would track that flight. Pretty much like sats but no need for an elevation rotor.

I did this project a few years ago and had good results

http://files.bluecrow.net/Amateur%20Radio/Antennas/2m-phased-array-ant.pdf

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"""I doubt the RTL is 50 Ohm it's more likely to be a 75Ohm device as it's a TV receiver""" Yes it receives TV signals, but the hardware that attaches it to the antenna...ie. the MCX connector, is 50 ohms.... Along with that, I have seen many extra, made for ASD-B, preamps, antennas etc, that are almost always produced using 50 ohm connectors.... Some of these include... "N" connectors, SMA, RG316 coax... If your only clue to 75 ohm is that they are used to receive "TV" signals, then the overwhelming use of 50 ohm connectors should indicate that they are indeed 50 ohm devices....Every 75 ohm device I've seen on the net is prodced by hobbist... Just saying... :rolleyes: ! Oh, and by the way, 2% of a half wave using 82% VF coax is about 2 1/4mm. Remember when you change the elements for down tilt, you change the SWR.... It's a balancing act.

Note on "N" connectors... I list them as 50 ohm because the only place on earth you will find the 75ohm "N" is in the cable TV business. They need to maintain low loss without impedance bumps along their lines and connections. B)

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