As I switched to a 22-cm antenna + 1-m cable (from the 66cm FlightAware antenna, no cable), I expected three things: a) fewer airplanes, b) fewer positions, and c) shorter range. Well, fewer planes and fewer positions turn out true, but the shorter antenna got much better range.
Together with antenna, I also replaced the Pro Stick Plus (blue) receiver with new unit. Is the increased range a result of variation in receivers (purchased one month from each other) or is there some other explanation?
Some controls I applied/missed:
Fixed receiver gain immediately before and after the switch.
Same elevation immediately before and after the switch. Caveat: The 66cm antenna was placed horizontally immediately before the switch. However, post-switch range is better than most days when the 66cm antenna was vertical and higher than immediately after the switch.
When I placed the top of the 22cm antenna at the same height as the 66cm antenna had been, the range was better than the best day observed with the 66cm antenna. Caveat: On the best day observed with 66cm, the receiver was on AGC, not at the current gain level.
“Range” is evaluated by the absolute number of positions farther than 160km. (Because the 66cm antenna received more total positions, the ratio of this number in total is also larger with 22cm antenna.) On the first day after the switch, the absolute number was 10x that before, and the maximum range also increased.
The switch is a result of physical damage to the USB connector on the old unit, so a side-by-side comparison between the two sets is not immediately feasible. (I will test the 66cm antenna with the new receiver unit after I gather more data.)
The plot thickens. I actually had a 4th prediction: lower message rate. At the beginning, this seemed to be true (with quite some caveats). But at least since I raised top of the 22cm antenna to the same height as the 66cm one late yesterday, message rate has skyrocketed, surpassing the best rate I had observed with the larger antenna + old receiver at the same gain. (Message rate is generally lower when using AGC.) This breaks the 4th prediction. At this rate, the smaller antenna is going to surpass both planes spotted and positions, further breaking the 1st and 2nd predictions. So, in the end, the tiny wire antenna beats the large rod antenna in all measures unless the old receiver was severely damaged or defective.
Surely, the old receiver had received two or three impacts estimated at between 2g and 5g. (The last blow dislodged the USB connector.) But electronically, I cannot imagine these impacts having an effect on any of surface mounted components or the PCB. (I did measure DC voltage after USB connector. There was no difference.)
Let’s see if I can keep the numbers straight here…
In a perfect world (and antenna manufacturing),
1090 mhz vertical antenna lengths:
Full wavelength: 10 5/16ths Inches or 0.262 meters
5/8ths wavelength (very common): 6 15/32nds inches or 0.164 meters
1/2 wavelength: 5 5/32nds inches or 0.131 meters
1/4 wavelength: 2 19/32nds inches or 0.065 meters
A dipole antenna is typically 1/2 wave length each side (each element being 1/4 wavelength)
By the math (and some great websites that do it for you), your 66 cm antenna is MOST LIKELY either a 2 meter or 440 mhz type antenna. Depending on how I break it down (1/4, 1/2 wave) I come up with 170mhz through 849 mhz
22 Cm is ~ 8.66 inches
at a 1/2 wave dipole antenna (4.33 each leg), it’s tuned for 1296 mhz, which would explain why you’re getting such better results.
Antenna’s receive and transmit off the sides, not the top straight up or out the bottom. Without getting into a long antenna theory discussion, yes you can make an antenna transmit/receive out the end, but not typically for a ‘wire’ antenna, and there is the ‘polarization’ of the ‘antenna receiver to transmitter’ to factor in. Aircraft have vertical polarization, which is why our antennas go up and down. If you lay it on the side, it’s horizontal polarization. WILL one receive the other, MOST the time but it’s VERY specific and usually not very good. (If I’m remembering all my past reading correctly that is, someone feel free to correct if I’m wrong).
I set up my PiAware a week ago. I took a Pi out of my cabinet, downloaded the image to an SD card, hooked up my RTL-SDR receiver I had in the same cabinet, Looked up what the length of a vertical antenna should be for 5/8ths wave, went to my shed, found an old piece of CATV hard line (the kind that runs on the poles down the street), sliced the aluminum shielding into 3rds, cut it to the correct length, it already had an N connector on it, so I put an adapter on it to go from N to SO239 connector on it, and screwed a piece of coax I had laying around to that and put a SMA adapter on the end to connect to the RTL-SDR and VIOLA! I’m picking up aircraft like crazy.
Do a search for ANTENNA LENGTH CALCULATOR, figure out how long you want your antenna to be (Full, 5/8, 1/2 or 1/4 wavelength, FULL or 5/8 is what I recommend), then get a piece of coax, measure the length on one end, carefully remove the covering, spread the shielding wire (outer wire) apart down at the bottom of your measurement and pull the center conductor through that, twist the shield together , and BOOM, you now have a dipole antenna. Hook it up (TV coax is an F connector, CB/Amateur Radio TENDS to use PL259 connectors for VHF/HF, some use N connectors) with an adapter to the SMA connection and see how well it works… You’ll probably be pleasantly surprised.
There are designs out there I’ve seen that make a vertical antenna for this using a soft drink can for the ground plane (part that connects to the shield), or things like that, and some simply had a piece of wire stuck in an adapter for the center element (CANTENNA’s for ADS-B would be your search).
If you know any amateur radio operators who might have an antenna analyzer, they might be able to check your antenna’s and tell you exactly what freq they are designed for… Antenna lengths without a tuner are very critical when transmitting, but can have a large impact on receiving as well.
As to the “horizontal” orientation I mentioned, that was just a temporary condition immediately before replacement due to “injury” to the old receiver; in most of its history, the rod antenna was installed vertically. Comparisons are drawn to “standup” days, not to those “lying down” days.
[66cm/26in, every calculator i could find lists the max full wave length @ 1090mhz is 10.xxx"; why is this sooooo looooong?]
From the image it looks like the antenna may be a sleeved dipole + whip + impedance matching design, or something similar. This design is somewhat of a collinear design thanks to the additional whip which also gives a flatter radiation pattern with more gain direction out towards the horizon. These antennas are omnidirectional (they receive equally from all directions) and have a higher gain compared to most other omnidirectional antennas, but their radiation pattern is flattened and directed more towards the horizon. This is a good thing for receiving planes that are far away as they will be at lower elevations, but aircraft at higher elevations relative to your antenna may be received poorer. Although, it is likely that any aircraft at high elevations to your position will be closer to you anyway, and thus have a stronger signal making the reduced gain at higher elevations less important.
By M. Austin on January 2, 2019
Would be interesting to tear it apart and see exactly how it ticks…
An antenna WILL receive (although poorly) on the 3rd octet of a freq ie; 440 mhz is the 3rd octet of 144~ mhz … it’s how we fine tune to a degree when doing amateur radio foxhunts.
Again I’d be curious how just a bare bones dipole or vertical antenna compares in your situation…
Yes, it does but the filter is located after a preamplifier. The problem is you can overload the preamplifier with out of band signals which then generate intermodulation products which reduce the performance. Adding a filter in front of the preamplifier can vastly improve things if this is your problem.
That’s a cool dissection! I thought I knew about quarter-wave resonator, half-wave resonator, and all. But I must admit that I have never studied antenna design. What does the phasing coil do exactly? Several DIY designs don’t include one.
Good read. Thanks again. Now that we are into antenna design, I have another question. I had always thought that top feed and bottom feed are equivalent. But when I turn the FlightAware rod upside down, message rate drops considerably. Why?
No, I did not open mine as I do not want to spoil it and lose $45.
This photo was posted by Flightaware staff couple of years ago. There were no comments/write up on the photo, I have added those.
Without removing plastic pipe, cannot take any measurements except at the N connector, where it shows open circuit between shield & core. May be someone else who has opened it can answer your questions.
Antenna’s come in various flavors…
Beams reach out in a strong line with a null to their rear as they have a driven element, with directors to the front and a reflector piece behind the driven element.
Verticals have the driven element that goes up, and the ground plane is …the ground… be that a shielding type device (the silver part on the aforementioned picture) that provides the counterpoise to the driven element, a car body that provides the counterpoise with it’s metal surface, or as I mentioned, even a soda can (http://www.radioforeveryone.com/p/coketenna-easy-cheap-effec.html).
Dipole antenna’s work by having the driven (or as it were, the receiving) element on one half, and the counterpoise on the other side.
Essentially, when you turn it upside down, you’re blind on the coax feedline side (the null) of the antenna.
Radio waves are electrical energy flying through the air. They bounce off metal things, they HATE pine trees (sigs reflect like crazy off them), bounce off the earth, and even the atmo electrical regions. Ever heard anyone talking about CB radios and talking ‘skip’? It’s where the signal bounces through the various layers and reaches farther than it normally would. Right about the daybreak line (about a half hour before to about a half hour after sunrise) I can talk on lower HF freqs from North Carolina to Australia and New Zealand when the atmo is favorable to cross equator comms like they’re next door. About any other time, I can’t.
Basically if I put up an HF vertical in my yard, I need something for counterpoise, so I would put wires connected to the outer shield of the coax along the ground (called RADIALS) and it would enhance the transmission and reception of the antenna. The more I put down, the better it works. Same with putting on on a car. It’s location on the car depends on the radiation pattern believe it or not. Center of the car, the pattern is almost circular. If I put it on the back left corner of the car, the radiation pattern is off the right front (over the most metal). Back center means I have an ellipse off the nose of the vehicle more so than any other direction.
Hope that all makes sense… I guess I COULD have said think of it like the beam of light from a flash light. The signal comes into the light from the sides… and if it’s dark, it’s not seeing any signals…
My 15 MHz bandpass filter finally arrived, and I have had installed viedehopf’s graphs1090 just in time for a comparison. (“Receiver 2” is currently fitted with the FlightAware brown rod.) Daytime noise dropped 6 dB, and nighttime noise dropped 10 dB! This improves usable dynamic range by 3 dB to 5 dB.
Reduced noise on the high-gain antenna, combined with mounting the rod vertical on empty hamster cage (from horizontal) means that this experimental feeder now surpasses my PiAware feeder with short antenna in both aircrafts and positions reported, even though the rod is on the “wrong” side of my building (facing away from most traffic with two more dry walls) and 1.5m lower.
Of course, low antenna gain isn’t the only factor against Receiver 1. As I barely diagnosed a day ago, that setup is suffering from COVID interference. Not that carbon-based SARS-CoV-2 somehow infects silicon-based receiver, but a bad HDMI adapter from workplace that invaded my home due to COVID shelter-in-place spills extreme RF leakage to saturate the preamp just as it did - and more severely so, when the brown rod was attached to it.
As I have experimentally determined in the other thread, sharp weekday elevation in noise level is attributable to this bad video adapter. (Again, a shoutout to @wiedehopf for the wonderful graphs1090!) I still need to further diagnose the smaller noise elevation during weekend days.