The affect of raising an internal aerial by just three feet


#1

Pi3, Pro Stick Plus, home brew 2 element j-pole aerial.

At around 15:20 I took it out of the loft, mounted it on a three foot pole and slapped it back again about fifteen minutes later.

That’s more of a difference than I expected.


#2

to be honest - i do not really understand what you did and what the result is …

but if you are just talking about the 3 feet higher position it easily could depend to your roof construction or the improved mass of metal (the pole itself) at the bottom of antenna …


#3

I definitely see the step between before and after, but in the end, the lines seem to be connected at the same level again?

Three feet only makes a difference if it puts the antenna above surrounding objects. The horizon at a certain altitude is defined by d = sqrt(2 * radiusearth * altitude + altitude^2) = sqrt (altitude * 12742 * 10^3 + altitude^2). It makes the altitude of the antenna almost irrelevant. If your antenna was at 10 meters and now at 11 it would increase the distance to the horizon with only 550 meters. For low altitudes you could even neglect the altitude^2 and simplify to d (distance to horizon in meters) = 3570 * sqrt (h) with h = altitude in meters. I didn’t like it when I discovered this.


#4

It’s uncommon to have a clear line of sight to the horizon so raising the antenna above nearer obstructions is exactly the point.


#5

that’s what this meant - but in addition maybe it is just caused by the different behavior of the antenna when you add metal to the bottom …


#6

My home country is really as flat as a pancake with a minimum elevation of 7 meters below sea level and max 300 above. Especially in rural areas (or parts that we’ve reclaimed from the sea) having a clear line of sight to the horizon is quite common - at least for a high percentage of 360 degrees.

I’m with Tom that the improvement could have other causes that just raising it three feet.


#7

Sure! There are going to be other effects and it will vary from install to install. But you should think of the theoretical radio-horizon range as more of an upper bound on your performance, rather than a way to improve performance by increasing the height. If you have less range than predicted by the radio-horizon formula in some directions, then that may mean you have nearer-than-the-horizon obstructions and raising your antenna by a small amount may have a large impact if the obstruction is nearby.


#8

The pole it’s mounted on is specifically non metallic to deliberately rule out anything like increasing the ground plane.

Roof construction is constant all the way up.

I’ve re-run gain optimisation both this receiver and my externally mounted (and higher) receiver and although the external one still sees a lot more positions, they’re now very similar in the number of aircraft received although the patterns are noticeably different in certain areas.


#9

then - because you say ‘affect of raising an internal aerial’ the only left possible reason is a miracle :slight_smile:

p.s. here are my inside/outside reach patterns: Post your PiAware setup


#10

I’ve raised a receiver by three foot. I’m showing what the effect is. No more.


#11

Have a look at his configuration post of July 30th. @keithma seems to be quite educated in this field.

OT: I’ve lost you: you have an external FA antenna and an indoor J-pole? Do I understand correctly that you’ve moved the indoor J-pole to an outdoor mast? Which one is the indoor antenna you seem to use as a reference?

@obj: We’re on the same page :slight_smile: Holland (in this case) is just an example that sometimes the ideal theoretical situation can really exist.


#12

Apologies for the confusion.

My external system with the FlightAware vertical hasn’t changed, all I’ve done is literally just raised the internal homebrew J-Pole (and receiver and Pi) by three foot and then I ran the gain optimisation again. I hadn’t run the gain optimisation before I produced the images above.

I’m going to have to see if I can build another two element J-Pole and see if I can get it properly weatherproofed and mounted outside.