Tuning the DVB Antenna for 1090MHz Specific reception

[Chris7777]

Hold on - 69mm is a little too long!

There needs to be a correction for the diameter of the antenna element.
This is called 'end factor correction' K.

65 mm will resonate at 1090MHz.

If you want the explanation read on.

Using V = f x λ we get:
wavelength = Velocity / frequency
wavelength = 300,000,000 / 1090,000,000 = 0.275 metres or 275 mm.
¼ wavelength whip antenna will be 275mm / 4 = 68.8 mm, all correct so far. So what am I beefing about.

As I said 68.8 mm will not be resonant at 1090MHz, it will be 'overtuned', it needs an end factor correction to bring it into resonance.

End factor correction is dependent on the ratio of the half wavelength to the thickness of the antenna element.

In our case 137.6 mm to 1.5 mm or 92:1

By looking this ratio up in any good antenna theory book the antenna length should be 0.96 times its calculated size.
68.8 x 0.96 = 65.4 mm.

So there you have it 

[exfirepro]

Chris,

You have a valid point, but to be accurate, the speed of light in air is nearer 299,700,000 m/sec, which when applied to your calculations makes the antenna length 65.989mm.

But remember, this is a "receive only' antenna, working with very strong signals compared to the P3i side, (or the likes of FLARM). For most users, the reason for trimming the antenna is to ease fitting on top of an aircraft coaming against a sloping windscreen, rather than for accurate frequency response.

In practice, the 'actual' length isn't critical - anywhere between about 65 - 75mm measured from the base will work fine. It's more critical in fact not to cut it too short as this could have a detrimental effect on the measured strength of the signal passed to the software, which is then used to trigger the bearingless Mode C or S alerts.

Regards
Peter

[Paul_Sengupta]

A quarter wave whip should also have a ground plane.

[exfirepro]

True Paul,

But again, much less of an issue for receive only.

P

[Paul_Sengupta]

Also note that the antenna and screw doesn't go right to the bottom of the base...

But there's a bit of metal and a magnet which might affect things.

And transponders are high powered.

I use a "pepper" GSM antenna. Receives out to 40-60 miles which is good enough for traffic purposes.

[Ian Melville]

I think a bit of thought needs to go into the ADSB antenna choice.

It's all very well saying that you get aircraft out to 40 - 50 miles, however if they are high level CAT, then you could be hiding a problem it's lower GA aircraft. CATs positioning and high power assures a strong signal is received, but GA which may be below and the transponder on the underside will be at a big disadvantage.
My preference would be to fit a tuned, omnidirectional antenna to get the strongest signal possible then restrict volume of hits in the PAW software by having a bubble around your aircraft of say 40km

Can the radio experts tell me if a mag mounted antenna is using the metal sheet it is connected to as a ground plain, or as a director/reflector. I don't get the fact that it is not electrical bonded to the base of the antenna, yet it seem to be vital the screen is bonded to the magnet. Can electricity jump through paint⚡️😳

[Paul_Sengupta]

Yes, there will be capacitance between the internal plate and the metal it's stuck to.

I may only get 15 miles on GA transponders...but I think that's ok.

The problem will come when things like the SkyEcho are to be received, then I guess one will need a good ADS-B antenna.

[exfirepro]

Can the radio experts tell me if a mag mounted antenna is using the metal sheet it is connected to as a ground plain, or as a director/reflector. I don't get the fact that it is not electrical bonded to the base of the antenna, yet it seem to be vital the screen is bonded to the magnet. Can electricity jump through paint⚡️😳

Ian,

I wouldn't call myself an 'expert', but the simple answer is 'YES', the ground plane of a magnetically mounted antenna, fitted on a steel car roof for example, is capacitively coupled to the 'earth' braid of the cable feeding the antenna whip or 'monopole' and forms an integral part of the system. For this reason, magnetic antennas (or any other monopole whips) should be mounted as near as possible to the centre of the ground plane. Failure to do so causes the transmission pattern  to be 'skewed' with maximum transmission effectively across the longest length of ground plane away from the antenna itself.

Without a ground plane, a high proportion of the energy from a monopole transmitter will be radiated at a very high angle and horizontal propagation will be limited to a relatively short distance. The antenna is also unlikely to prove a good electrical match to the coaxial feed line - setting up standing waves which can wastefully (and in some cases destructively) reflect power back into the transmitter. Unless capacitively connected, it is therefore essential that the antenna ground plane of a transmitting antenna is electrically connected to the outer braid of the coax at the feed point.

A good ground plane both reduces the angle of maximum transmission (effectively providing 'gain' to the antenna) and improves 'impedance matching', lowering standing wave ratio (SWR) and ensuring that maximum power is transferred from the transmitter feed line into the antenna. With a 'perfect' ground plane extending infinitely in all directions around the monopole however, antenna impedance will be about 35 Ohms, which is not a very good match to coaxial cable. For transmit antennas, the impedance of a monopole antenna is often increased to match 'standard' 50 ohm coax by replacing the horizontal ground plane with a set of 'radials' angled downwards at approximately 60 degrees (see attachment).

With receive only antennas, the same principles apply, but matching is generally less critical, though the closer to the ideal we can achieve, the stronger the signal passed to the receiver itself is likely to be.

Lots of reading out there if you want to get technical.

Regards

Peter

[Chris7777]

Hi Peter

Thanks for making me look at that calculation again.
I had made a mistake. I must check my working more carefully. 

My calculation should have been 300,000,000 / 1090,000,000 = 275 mm and 275mm / 4 = 68.8 mm, then 68.8 x 0.96 is 66.055mm

I didn't think that using 299,700,000 would make that much difference, only being 1 part in a 1000, and it doesn't, you got 65.989mm

But as you say this is less critical on receive.

Thanks for the wake up. 

[exfirepro]

Hi Chris,

With PAW's 1090 MHz side being 'receive only', things aren't so critical, especially for ADSB targets, which are generally strong signals from high power transmitters and present us with accurate GPS derived position data. The main concern with 1090 MHz, is that 'bearingless' Mode C and S alerts are triggered by the strength level of the received signal - and dictated by the detection 'Range' you have selected in the PAW 'Configure' Screen. To be effective the alert triggers rely on a well sited antenna to ensure we receive the maximum signal strength possible from the other aircraft. Tuning the antenna obviously helps, but we have found that an antenna which is too long is generally far less critical than one which is too short, which is why I always warn users against over shortening the antenna, but am not too worried about leaving it slightly long.

Of more importance is antenna positioning. Although the antenna in theory receives signals equally from all round the aircraft, it is very difficult to position it in such a way as to completely avoid screening by aircraft metalwork, engines or occupants. In this respect, we generally advise users to position the antenna where it will get the best clear view to the front of the aircraft as this is the direction from where we normally get the least warning of approaching aircraft due to the combined effect of closing speeds.

For metal bodied aircraft, an alternative would be to fit 'twin' 1/4 wave monopoles externally mounted in the clear above and below the fuselage, and use the body of the aircraft as their common ground plane, but this is technically fairly difficult to achieve a good impedance match to the receiver. It might be worth a bit of experimentation however, when I can find the time.

Best Regards

Peter

[rogellis]

.
Interesting topic, that needs to be highlighted once again.

One question:   If you tune the aerial to 70mm, will this make the aerial/dongle combination more efficient?   ie, draw less electrical power, because it is only looking at specific frequencies?

We are running the PAW from battery, and any conservation would be appreciated.  Judging by the heat it puts out, the ADSB dongle is the biggest consumer, and lessening that would be helpful.  It seems odd that the receive-only ADSB aerial would draw more current than the PAW transmit aerial....

Thanks,
R

[exfirepro]

Hi Rog,

Tuning the antenna shouldn't make much difference in practice to the electrical power draw as the desired receive frequency is set in software, so the SDR will simply ignore 'off frequency' signals. Using an antenna tuned to the correct frequency however improves the signal strength (and therefore reliability) of the received signals on the chosen frequency. As I have said several times, however, (despite the precise calculation discussions above), exact frequency tuning is far less critical for the relatively high power 1090 MHz ADSB, Mode C and Mode S signals which the SDR Dongle is looking at, compared to the much lower power P3i signals which the other (Bridge) part of the PAW is looking for.

Best Regards

Peter

[rogellis]

As I have said several times, however, (despite the precise calculation discussions above), exact frequency tuning is far less critical for the relatively high power 1090 MHz ADSB, Mode C and Mode S signals which the SDR Dongle is looking at.

Indeed.  We broke our aerial, and still got 100k range on the ADSB.

But theoretically, if the ADSB aerial was inclined against the canopy by 25º, should we lengthen the optimum to 84mm?  Or do radio waves not behave like that?

Cheers,
Ralph

[Ian Melville]

No they don't behave like that. They are tuning forks for radios. The length gives the tone, no matter what angle you hold it.

Given the power of the transponders 25 degrees probrobly make little or no difference to the range. More important not to have blind spots rather than extreme range. Anything more than 50km is sufficient.

[rogellis]

I have shortened my antenna to 69 mm (19mm base plus 50mm stick), in line with the comments here. Making a 1/4 wave antenna.

However while doing so I noticed that the original antenna stick was 115mm long, plus rhe 19mm base, which makes a 134mm antenna.   Is that not close enough to the 138mm 1/2 wavelength, to be an even more effectove antenna?   

If the antenna was 138mm, would it be even more sensitive?

Thanks,
R