Surprised how much antenna height affects results

#1
I recently replaced my venerable Mk.3 spider antenna with a Flightaware pole, and was dismayed to see very little improvement in range. Just for kicks, I decided to try bringing a dongle up to my office with the original spider to see the difference.

The original install was an attic install of the spider, mounted on a pole at the peak of the attic:


I had originally mounted it to a telescoping pole to find the best position in the house where I'd get the best range and coverage. I found that a point along the southern wall peak surprisingly got the best northern coverage (or at least, the most complete radius.) Even with tweaks though, the maximum distance was still a disappointing 70-80 miles on average (with spikes near 80 miles to the northeast.)



For a while I was concerned that something in my antenna build was just messed up. Maybe the coax cable was bad? I briefly had the antenna directly connected to the dongle via only an adapter and was getting similar results. Maybe the BNC connector I used to build the spider was bad, or wasn't the right impedance:



I know 75 ohm BNC connectors exist, and have slightly difference mechanical specs, I looked and that wasn't it. I have dozens of these laying around, and adding a washer to mount the radials made this a quick build, so I built two more for testing to ensure something wasn't wrong. I used the second one to fine tune the radials and whip lengths, finding little difference until they were just too short and was getting very few hits past 20 miles.

I bought the Flightaware antenna on Amazon, and was excited until I saw that I had very little increase in range. My packets received increased greatly, as did the total aircraft detected in the range...but I had the same general limit as I'd had before.

For kicks and to experiment, I brought my Mk.3 spider up to my office with an older dongle, just to see what results I could get. I wasn't optimistic about it. I knew that the antenna would have a limited horizontal field of vision, my office has narrow windows, and the columns in between I believe are structural columns that probably contain steel beams, at the very least they'd contain dense concrete. As if that wasn't bad enough, there's a set of metal shutters along the windows that are permanently affixed to the building. The antenna would also have a fairly narrow vertical field to look through...like looking through a slit. The one good thing was my office is on the 7th floor, I'd estimate that with the mechanical level below me, I'm at least 75-85 feet above street level. Knowing that the couple of extra inches to mount along the top of my window wouldn't make that much of a difference, I built a small shelf for the spider to sit in that would sit at desk level. I work in a graphics shop where we produce posters, so I built out of a material I have plenty of: foam core board.



I suspect that the foam core is rf-transparent, at least at these frequencies, but I'm unwilling to try the microwave test with this stuff, so it's a complete guess. The antenna goes to about 4 feet of RG-6, then into the dongle adapter which is connected to my work laptop. Again, this was just to experiment.

I fired up the Windows build of dump1090 (not mutability, as I don't see a Win32 build for that branch.) I was almost immediately surprised by what I found. Not only was I getting a flood of packets, but I was seeing at least 175 miles of range, almost immediately. Throughout the the day I watched as that range filled in a bit more and I started seeing a much better footprint than I was at my house, with the same antenna.



Over four days of mapping, the best range I've seen was 275 miles, although I wasn't watching that flight at the time and I'm not logging on this machine like I do on my pi.

The strong directional reception I was anticipating...my office window faces directly north and I didn't expect much from northwest and northeast. What surprised me was the southern spikes. I believe I'm getting significant bounce from the building across the street. It's a tall brick building so it might be acting as a reflector, bouncing the signal 'over the shoulder' so to speak.

So far, my work antenna continues to receive more packets and more aircraft than my pi at home, albeit with a sharply focused receive pattern. I'm seriously considering moving my pi up to the office, or at the very least trying with the FA antenna to see if it receives even further!
 
#2
Hi and welcome,
I dream of 75-80 feet above street level!
As you say, height appears to be everything. I'm located in a river estuary, street level is about 12ft asl, Mounting the antenna anywhere below roof/chimney level cuts down the range with spikes corresponding to neighbouring obstacles. If I test an antenna out of the window of my den, hooked over the gutter, range is awful and spiky. Mounting it on the side of the house, about 12ft higher - approx 40ft above sea level, gives me a range almost equivalent to that which can be ideally expected on the website that member ab cd kindly posted some time ago http://www.heywhatsthat.com/
maxrange.jpg coverage0309.jpg
Oddly, I believe I have found an unconventional (and not very practical) way to get around 5-10 miles of increase. I have built 4 spiders, so far, using brass plumbing fittings as the base and either 1mm co-ax core, 1.6mm brazing rod or 2mm meter tail for the gp/whip (or in combinations). Double male F connectors being the electrical part of the deal.
So far it appears the mk2 version, with 1mm whip, 2mm (x8) gp is the best, but the whip, even when supported with insulation (WD40 spout) was susceptible to bird damage. The current version has a 1.6mm whip and the range is slightly cut back.
monopole.jpg

The longest ally tube I could get was 12' and allied to the highest I could fix it, due to ladder reach meant that the antenna would still be blocked by chimneys/tv aerials. To get the extra few feet I used lightweight 1" dia electrical conduit. This did the job, but was alarmingly "whippy". Eventually, due to back to back gales we've had here the last few months, plastic fatigue occurred and a rethink was required.
The current mount seems to have solved the whippyness. This uses 1.25" "push-fit" waste pipe - which fits snugly into 1.5" ally pipe. The tee piece and elbows are secured from twisting or coming apart by self tappers. Although it might not have been necessary, the long length of pipe was stiffened by self tapping a scrap length of 1" ally pipe to the inside, making it rigid.
first setup.jpg new setup.jpg

And the range dropped immediately by about 5-10 miles. The height was, within inches the same, the spider was the same one and the cabling was unchanged - Duh?

For the last couple of days we've had the remnants of that little bit of wind that hit the east coast of the USA passing through and it's been blustery, to say the least. I've been keeping an eye on the set-up and although it's been swaying a tad, it's not whipping like the last.

But the range has increased. My theory is that the whippyness continually alters the angles at which the antenna is mounted; this combined with the fact that the dongle is repeatedly receiving signals, increases the chances of picking up distant "pings" it would miss if the antenna was stationary.
Being an admitted electronic dunce, I'm prepared for this to be rubbished and won't take offence!

As the 1mm whip was superior, I'm going to try to "beef it up" by securing a plastic (hopefully rf transparent) covering to the f-connector over the bare copper. Likely candidates are a pen tube or something similar to the tubes that paint balls come in - lately, I'm sub-consciously scouring shelves as we shop!
 
#3
Oddly, I believe I have found an unconventional (and not very practical) way to get around 5-10 miles of increase. I have built 4 spiders, so far, using brass plumbing fittings as the base and either 1mm co-ax core, 1.6mm brazing rod or 2mm meter tail for the gp/whip (or in combinations). Double male F connectors being the electrical part of the deal.
So far it appears the mk2 version, with 1mm whip, 2mm (x8) gp is the best, but the whip, even when supported with insulation (WD40 spout) was susceptible to bird damage. The current version has a 1.6mm whip and the range is slightly cut back.
View attachment 2428
I'd never thought about building one with fittings like that before...it certainly looks a lot more durable than mine does (I've had to tighten the nuts on the radials several times so far...thinking about Loctite next.)

How are the radials connected to the base?
 
#4
Hi,
Drilled @45 deg, slightly oversize and soldered. I guess drilling horizontal and bending the radial would work, but it's easier to get the dimension accurate drilling at an angle.

I've tried various sizes of plumbing fittings from 15mm compression end caps through 22mm (best so far). I've got an immersion heater blanking plug (2.25" dia) to try when it gets a bit warmer in my shed.


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
 

ab cd

Senior Member
#5
In groundplane antenna, the dia of horizontal part is important. I tried old-timer "Coaxial Antenna", made of 3/4 Inch (20 mm) dia water pipe with end cap, but was not happy with results. I then tried a 54 mm narrow 250 ml drink can, and it showed improvement. I then tried a wider 68 mm dia drink can 330 ml 12 oz, and it gave excellent results. I then tried arger 70 mm dia & 100 mm dia cans, but there was no further improvement.

From above experiment, I concluded that if the horizontal disc of groundplane is 1/4 wavelength (69 mm), then the 1/4 wavelength 69 mm radials, the it gives best results.

Please note that in "Coaxial Antenna" & its wide-bodied varient "Cantenna", the vertical cylinderical part is actually a large number of Vertical Radials, touching each other.

I feel that this advantage of a 1/4 wavelength (69 mm) dia disc (circular bottom of the can) may also be applied to the Spider.. I have not tested it myself with Spider, but TomW did try a 2.5 inch disc + 69 mm radials, an it proved very good. Please see TomW's thread Side by side antenna testing. So try your luck with a disc of dia close to 69 mm i.e. a 2.5 inch (63.5 mm), or 2.75 inch (69.8 mm) or 3 inch (76.2 mm) dia disc, and hopefully you will get improved performance.


Conical Monopole.png



Cantenna - various dia cans.jpg



Cantenna and Coaxial Antenna.png
 
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#6
I feel that this advantage of a 1/4 wavelength (69 mm) dia disc (circular bottom of the can) may also be applied to the Spider.. I have not tested it myself with Spider, but TomW did try a 2.5 inch disc + 69 mm radials, an it proved very good. Please see TomW's thread Side by side antenna testing. So try your luck with a disc of dia close to 69 mm i.e. a 2.5 inch (63.5 mm), or 2.75 inch (69.8 mm) or 3 inch (76.2 mm) dia disc, and hopefully you will get improved performance.]
I did not have a chance to look this weekend, but I believe I have some scrap aluminum sheet that would be perfect to try this with. Would you attach the radials to the disc?
 

ab cd

Senior Member
#7
I did not have a chance to look this weekend, but I believe I have some scrap aluminum sheet that would be perfect to try this with. Would you attach the radials to the disc?
Good idea to use aluminum sheet to make a disc. Aluminum is easier to cut & drill hole than copper.

Yes, sure, I will attach radials to the disc. The disc alone will not give good performance. I will measure radial's length 69mm starting from outer edge of disc.

Here is how others have done it:

TomW's Spiders:
TomWs Spiders.jpg


blueskyspotter's Spider:
blueskyspotter spider.jpg . blueskyspotter spider upside down.jpg


F1Rocket's Spider:
F1Rockets Spider.jpg
 
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#8
As per my knowledge in groundplane antenna, the dia of horizontal part is important. You must try the "Coaxial Antenna", made of 3/4 Inch (20 mm) dia water pipe with end cap, but was not happy with results.
After you can try a 54 mm narrow 250 ml drink can, and it showed improvement.

pcb assembly cost
 
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