FlightAware Discussions

Antenna Efficiency

For those two points, a low-noise amplifier located at the antenna, can make them irrelevant.
So what truly remains an issue is the height above obstacles. And of course that includes the walls when located indoors.

I basically doubled my message and planes count just by raising the antenna somewhat higher (and I am yet to clear all the obstacles in the way of horizon line-of-sight)

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Looking at my “nearby”, I see one with outstanding performance, and several with very similar performance. The polar view is even the same, indicating that traffic patterns have some impact on coverage numbers.

I would be inclined to build a spider. I have built a few over the years, for different radios.
There aren’t any parts stores here, like there were down in the Silicon Valley ;-(

My best with the little antenna is no ground plane, sitting on a metal arm touching the skylight.
The 170mm pan seemed to hurt.

Today, I think I’ll try a spider, using the existing base, and some 67 mm pieces of 12AWG copper house wire.

FlightAware conveniently sent me a welcome letter with a shopping list
5 meter cable
FlightAware Antenna

I might just put the FlightAware antenna on my Christmas list, and mount it on the outside of the skylight housing, if Santa brings it to me.

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You could order 2 or 5 meters of CFD200 or LMR400 which is already terminated with SMA. (on amazon for example)
That should be 10 to 20 $.

Then you chop of one end and build the spider into the cable by shoving some copper rods into the insulation.

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I thought maybe my little antenna had bad wiring in the base, which I’ve seen mentioned in discussions. I only saw one aircraft on SkyView.
I wondered how many I should see, so I went to a FlightAware Live view.
Hey! Only one plane within 50 miles of me.
That makes it hard to measure performance.

My range seems to be about 60 miles at high altitude. 50 miles at 20,000 feet.

SFO airport is 55 nautical air miles, but the altitude of most flights would be low.
My range in that direction is about the Golden Gate Bridge, 40 NM at 10,000 feet.

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I can get 60 miles with the default RTL-SDR antenna sitting on my basement window bars(3ft above ground).
In my attic, at about 60 ft AMSL I can get over 100 miles.
Chimney antenna at 70ft AMSL I can get out to 200NM.

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Try this one. It needs only a 1m piece of satellite coax like RG6 or WF100, and a F-connector.

Quick Spider

You will also need RG316 F-female to MCX-male Pigtail. It has added advantage over a solid connector that it will not transfer mechanical stress of heavy RG6/RG58 cable to dvb-t’s delicate mcx socket.


I unscrewed the antenna, leaving the base on a 67mm aluminum disc.
I saw a flight at 20,000 feet, 42 NM out.
With a 67mm piece of steel wire threaded onto the base, 30,000 feet, 56 NM.
A five legged spider (six, but one is along the mounting arm), with the legs 45° down at the edge of the 67mm disc, so 67+67+67 per leg, underneath the disc, with the mag base on top.
That doesn’t seem as good, for range. Should the legs be at the edge of a 67 mm disc, or closer in? The one built on the “Easy” SO-239 looks like the angle of the legs meets the base of the antenna at 45°, but the Coke can with the splayed sides looks like a 67mm disc with legs at the edge.

NO disc with Spider, only vertical and radials.

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I took the disc off, and made the top bars between the down legs 30mm, which is the base of the original antenna.
100.7nm at 29,000 feet. Another at 88NM.
I’m using 17ga galvanized wire, because I happened to have a roll.
Should I use some 12 or 14 ga copper instead?

Lots more flights visible, but that’s just evening traffic, in general.
Comparison to flightaware.com/live on another monitor is helpful

This is ok.

Not necessary.


Can you please take a close-up photo of spider and post it?

I’m currently visiting a friend in California but even in the UK am not particularly interested in aircraft long distances away. In UK I am close to Heathrow so see much interesting traffic at around 8000 feet.

May experiment with different antenna length upon return to UK.


08 15

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I really think that the suggestion with the wire rolled around the base pin is not great. It creates an inductance at that point - remember the high frequency signals are traveling only on the outside portion (skin) of the wire.
I had the same antenna and I have cut the wire with a pair of pliers at 65mm (there is a portion of un-shielded wire in the base too). And then used the lid of a mushroom can as ground.

For the situation of the OP, those walls can be shielding the signal, and the only ones that he can “see” are the ones that are bouncing trough the skylight, on the side walls. So antenna doesn’t have as much importance as luck (plane to be in the right place geometrically).

65-67mm on this antenna is right in the middle of the coil. (picture of the original with a ruler, above).

I could solder a copper wire directly to the threaded stub (or to a nut), since this seems to be the best antenna.

Looking at the HeyWhat’sThat map, my current pattern looks like the one that I can expect.
Today is 21,230 <50, 2,889 50-100, 288 100-150 NM. The 50-100 is 13%. It was 17% earlier today. With the original antenna, it was around 7%, but that number varies with air traffic, which is quite variable. My sky is not full of airplanes. The area around SFO is in the shadow of some mountains for me. I have seen 20 at a time. I have seen none, or two. Comparing to Flightaware/Live shows that I’m not missing much in the coverage pattern that is becoming familiar. I’ve started watching the Age column in SkyView, to know what the maximum range is of a flight before it disappears.

This antenna is supposed to be optimized for ADS-B. It’s 170cm with a coil at 50cm for 10 loops. Is it a 3/4 wave antenna? What if I mimicked that length and coil for my spider legs? What if i used a straight 3/4 length 201mm for my legs? 201 mm legs and aerial, straight, soldered to the stub?

Length of whip is not critical. I have tried whips in lengths 52mm, 67mm, and 72mm. There is some noticeable difference, but less than 10%.


Not necessary. Few turns of wire over the threaded stud are ok. The turns are for mechanical reason to hold the wire in place. Electrically, the turns do not form a coil, as almost entire RF current directly passes through the first point of contact between wire and stud, and does not follow the coil route.

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Clarence, when you refer to polar plots is that on the Flight Aware stats or FlightRadar24 page?

I have the same R-Pi feeding both FA and FR24 and whilst direction is similar range seems to vary if I view each individual stats page.


I am referring to the piAware ADS-B stats page.

This spider is working well.
The 50-100 message count is 20% of the <50 number. That’s three times what my lowest antenna setup had.

My spider legs are not electrically connected to the ground of the antenna/cable.
I presume that is okay, since the magnetic ground plane antenna would normally be sitting, I suppose, on a car roof with no electrical connection.

The braid of coax is solidly connected to the 1 inch dia magnetic base. Through the capacitance formed between antenna base and the ground plane, the groundplane gets connected to braid of coax, even though there is no direct connection.



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