How much better is an external antenna?

FlightAware ADS-B Flight Tracking
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I’ve had my antenna mounted outside on the chimney for the past week or so now and have enough data to make a comparison with the same system mounted in the loft.

The setup is as follows:-

Antenna: COL1090/5-H Vinnant from eBay.
LNA: RTL_SDR.com triple filtered ADS-B LNA.
Receiver: Airspy Mini running at 20MHz.
Raspberry Pi 4

The LNA is mounted on the mast below the antenna. It’s not waterproof itself, so I found a small waterproof junction box on eBay that it fits quite neatly inside:

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The PG9 size cable glands are large enough for an SMA connector to pass through but nothing larger, so I removed one of them and used some self-amalgamating tape to protect the connection on the output. It also means that should any water somehow get inside it will drain out the bottom rather than accumulate. The picture doesn’t show the silicone rubber gasket that seals the lid.

The antenna and mast were mounted on the chimney using a standard TV antenna mast and mount:
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The total height above the ground is about 12m.

So here’s some data - settings and hardware between the two are identical and only the position is different. Gain is set to 18 on the airspy.

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The median max. range has increased by 12.3% from 201.2nm to 226 nm.
The median message rate has increased by 11.8% from 1945/s to 2175/s.
The number median number of aircraft received has increased by 28.7% from 226 to 291.

It’s clear from the graph that the number of messages received per aircraft has increased across the board, and the peak number of aircraft seen has increased from 323 to 417, or 29.1%.

A pretty worthwhile improvement for the cost of the mounting hardware and some lunch for my brother who helped me put it up.

It’s useful to see where the increase has come from:

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Range has increased in every direction, but especially to the North and South, which was expected because that’s the direction the gable ends of the roof are, with brick walls that were obstructing the signal in those directions. There is also a good increase to the West.
The distributions show that proportionally more aircraft are seen at longer ranges, and the signal histogram shows that there are far fewer weaker signals.

The high-altitude heatmap shows the difference in coverage best:

Inside antenna:

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Outside antenna:

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The areas of reduced shading are quite obvious. There are still some shaded areas that are due to a few tall buildings nearby. The elevation plot clearly shows the improvement as well:

Indoor antenna:

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Outdoor antenna:

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And just for completeness, here is a comparison of the rtl-sdr v3 dongle with antenna in the loft, compared with the airspy with antenna outside:

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There’s no contest really.

8 Likes
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:+1: :+1: :+1:

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A comparison with the same dongle would be quite nice. Using two different dongles seem to be a bit unfair to the loft antenna :slight_smile:

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I might hook up the rtl dongle to see how it performs on the external antenna at some point. Most of the post compares the same dongle though.

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I thought you were asking a question, not making a statement about the improvement of inside vs outside antenna placement. :smiley:

Of course once I read the post I see you were making a statement!

Yeah, there is no comparison between the two.

The next comparison is elevation of the antenna. The gains are pretty big up until you reach the theatrical maximum reception range. I’m sure there is a curve of diminishing returns of elevation = increased range.

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Raising the antenna doesn’t affect the theoretical curvature-of-the-Earth radio horizon directly very much (e.g. the difference between 10m AGL and 20m AGL is only around 5km range); but it has a large effect if it can get you above nearby obstacles that are blocking the view of the horizon.

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Yeah, going up 10’ got me about 25nm in range. I’m sure some of that was obstacle avoidance. We’ve got some big pine trees around here.

For testing I just tossed up a second antenna about 10 shorter then the other.

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I considered using a 3m pole, but decided against it because it would have required a more substantial mount, and disturbing the rendering on the chimney which is in dubious condition. Re-rendering it is not a job I want to create at the moment. I don’t think it would give much advantage anyway, because the buildings causing obstruction are all taller than that anyway.

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I might hook up the rtl dongle to see how it performs on the external antenna at some point. Most of the post compares the same dongle though.

Please do, I’m in the process of building a new and upgrading an existing site. I’m curious how the V3 performs against the Airspy Mini under similar conditions.

Cavity filter + FA Prostick Plus over here works better than without filter.
Another dongle with the Triple Filter/LNA performs slightly better than Cavity+LNA. However, cavity filter does give some extra ESD protection.

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If you just want a comparison of the two dongles under the same conditions then I have this:

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The rtl-sdr.com dongle is red and airspy blue - both using the same antenna and rtl-sdr.com LNA, but with the antenna inside.

1 Like
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Unless you mean the RTL-SDR was tested on the antenna inside and the Airspy on the antenna outside, thanks! I guess I misread you earlier posts, because you said:

here is a comparison of the rtl-sdr v3 dongle with antenna in the loft, compared with the airspy with antenna outside

Antenna inside or outside would be different conditions.

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The original post was comparing airspy with inside and outside antenna. The one I responded to you with was inside, just the dongle changed.

I just put that to show the overall difference from when I started making changes.

1 Like
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Awesome, thanks for clearing that out.

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Very good shout. I seriously considered using my chimney as well but that would have meant a lot of work beforehand as I have no idea of its condition. The last thing I’d have wanted was for it to come off the top of the house during a storm.

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It’s intact, but quite crumbly - it’s ok for now, but physically disturbing it by strapping a mount to it would probably make it fail fairly quickly. The smaller mount fitted on the already excluded brickwork which is solid.