Ground plane sizing for radio, transponder and PilotAware

Transponder, radio and PilotAware antenna fitting in a non-metallic skinned aircraft

There is often talk around the hangers about poor radio signals and how some aircraft can be heard from great distances while other fail to make contact with a ground station even nearby. One of the issues often overlooked is a missing or wrongly sizes ground plane. There are many myths about the purpose of a ground plane and I hope in this short article to give you guidance on the correct size and purpose of a ground plane.

Before we look at the ground plane lets set expectation on maximum distance you can be heard.

The civil air band radio is in the VHF (Very High Frequency) band and under normal weather / tropospheric conditions, it requires radio line of sight between the aircraft and the ground station; thus, the maximum range is the distances to the radio horizon from the aircraft to the ground station, for simplification we are assuming the ground station is at sea level.

The radio horizon distance is given by the formula: D= K√h

Where: D = distance in nautical miles (NM), h = height of aircraft’s antenna above ground level and K = 1.23 when h is expressed in feet

So, when we are flying at 2,000ft the maximum range to a sea level ground station is 55.00NM or at 3,000ft 67.34NM

The same is true for transponders, CAP1391 devices e.g. SkyEcho and PilotAware transmissions, however the limited power and antenna arrangement of CAP1391 and PilotAware means the range is less.

What is a ground plane is and why we need one?

Keeping this simple, it provides a ¼ wave monopole antenna (also know as a Marconi) with its required counterpart allowing the flow of the electrical current which generates the electrical and magnetic waves that make up the RF signal. The earth acts as a type of electrical “mirror,” effectively providing the other quarter wavelength making it equivalent to a vertical dipole. This in turn helps present the correct impedance to the transmitter, thus allowing the maximum power transfer into the antenna, which results in the most efficient transmission of the signal. Most antennas sold for Microlights and light aircraft are ¼ wave.

Ground plane

Does size matter? 

I’m afraid the answer is yes it does! Bigger is generally better, but again there are exceptions and a minimum size for the efficient working. The minimum size is a radius of ¼ the wave length of the signal being transmitted.  In basic RF theory we were taught that bigger is better, although the benefits of size are less important above 3 times the wave length. In aviation while we are flying along we want our signal to be picked up both above and below the aircraft, the larger the ground plane the bigger the blind spot we will have on the underside of the ground plane.

Is the shape important? 

No, a ground planes can be square, circular or many different shapes. Say you had a metal aeroplane, the whole fuselage is the ground plane for your radio antenna, and your transponder antenna. So long as you have at least ¼ wave length in all directions, then the shape doesn’t matter too much. However as we want the lightest ground plane we can get, a circle is the optimum shape and supports equal propagation of the signal in all directions.

What should the thickness of the ground plane be? 

Any, you can use foil tape or discs of different thicknesses so long as it is mechanically robust enough and conductive.

Can it be a mesh or does it have to be solid? 

It can be a mesh (the tighter the mesh the better) or even multiple radial wires, typically the more the better 8 or 16, but for our purposes a solid circle is normally the easiest and best option.

What should it be made of? 

Copper, however it can be any material which is a good conductor. Aluminium is a good conductor and can be used, however you need to be mindful of the galvanic effect between a stainless-steel antenna and the ground plane. This may lead to connection issues in the future, thus I would recommend a thin copper disc.

Why do some avionics companies state to use a square ground plane? 

Well I assume this is just convenience, so long as the diameter of the circle below can fit within the square it will work fine. Just make sure the antenna is in the middle!

What does all of this mean in practice? 

Minimum diameter of ground planes in aviation should be:

N.B. Always round the ground plane size up not down.

Transponder: frequency 1090MHz, Wave length 275mm, ¼ wave length 68.75mm

Minimum ground plane diameter 137.5mm

Radio: lowest frequency 108MHz, Wave length 2775.85mm, ¼ wave length 693.96mm

Minimum ground plane diameter 1,387.9mm

PilotAware: external antenna fitting, frequency 869.5MHz, Wave length 344.79mm, ¼ wave length 86.19mm

Minimum ground plane diameter 172.39mm

 

Antenna Theory Is Complicated

Antenna theory is complicated, but if you have a ground plane with the correct minimum size and the antenna mounted in the middle this will be one less problem.  But let’s quickly prove this is fact and not just some text book exercise. As an extreme lets consider an antenna without a ground plane vs the same antenna with a correctly sized ground plane.

An external PilotAware antenna without a ground plane is put under test using a low cost VNA (Vector Network Analyser) 

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As can be seen opposite the resonant frequency of this antenna is not 869.5MHz as it should be! It is showing a resonant of over 1GHz. The impedance and VSWR for the antenna was also way out, in fact the reflections from the imbalance could damage the transmitter or at best the radiated signal will not be optimal.

Let’s add a make shift ground plane out of kitchen foil of the recommended size above.

opposite you can see the effect of adding the ground plane. The resonant frequency has shifted down to be close to the 869.5MHZ that we are looking for, the VSWR was down to acceptable limits and the impedance was close to 50 ohms.

Hopefully this graphically demonstrates the importance of having a correctly sized ground plane!

 

One last Fact! The monopole antenna was invented in 1895 by radio pioneer Guglielmo Marconi

PilotAware and Skydemon the perfect VFR pairing?

Twin Otter

I had an exciting and varied day Sunday, first I taught lesson 7 from the new syllabus (climbing and descending ) ending up with a bit of lesson 6 (straight and level) revision in poor conditions, where the horizon was barely visible. The student did very well in difficult conditions and its always nice when they say they really enjoyed it.

Twin Otter

Today was for me about PilotAware and Skydemon, regular readers will know I think that currently, this is the best possible combination; during the lesson in the haze it was difficult to see other aircraft but, with PilotAware and Skydemon I could see the other aircraft, which we knew was out there, as it was from our club. The indicated position on Skydemon allowed us to quickly locate it and ensure we remained a safe separation. +1 for the combo.

PilotAware

Next up was my opportunity to occupy the co-pilot seat in a Twin Otter. The Twin Otter has Mode S transponder but, no ADS-B Out and no ADS-B In, thus they wanted to see PilotAware in operation. The Twin Otter in question was Skydive one the parachute aircraft, the test was very successful, in part because most aircraft at our home airfield have ADS-B Out or PilotAware. On the first flight we got a traffic service from Lakenheath as normal and shortly after we were showing a contact on Skydemon, the pilot said it will be interesting if ATC reports the same or not, just as he finished speaking or the radio came traffic information relating to the aircraft. This was interesting and helped show how pilot aware could help. On the next lift an aircraft was showing ahead and, on the left, Skydiveone would have normally turned left but, an alternative routing was taken to maintain separation.  Now the worth of PilotAware and skydemon was proving interesting! Then a little later in the flight we asked for clear drop and ATC came back with another aircraft in the zone, we had heard on the other frequency that an aircraft was leaving the circuit and with the traffic information we were able to positively ID the aircraft, thus there was no need to abort the drop. Fantastic news for both the safety and the cost of a go around!

Collision alert on Skydemon
Collision alert on Skydemon
Proximity Alert on Skydemon
Proximity Alert on Skydemon

 

 

 

 

 

 

 

 

There was few other contacts that helped too but, above was enough to convince them the effectiveness!

 

Then after a few lifts in the Twin Otter, I got to be the passenger in our C42A while my friend and co-owner did a few circuits.

What is EC or should I say Electronic Conspicuity or even ADS-B In and Out?

PilotAware

What is EC or should I say Electronic Conspicuity or even ADS-B In and Out?

And why do we care anyway!

In today’s world of aviation, technology is becoming the norm and if that helps to increase safety then I for one am in favour of it and I hope you are too.

It was not long after I got my NPPL that I was using and EC in the form of the then beta product of an upcoming technology called Pilotaware. Fast forward a number of years and Pilotaware, SkyEcho 2, Stratux, Power FLARM and more are all affordable in cockpit EC devices. Add to these the traditional transponders which have various modes A, C and S as well as ADS-B Out and some systems having ADS-B In and then there is weather reception, TCAS and secondary surveillance radar (SSR) too!

So as a member of both the UK Microlight and GA community what is best for me and what do all the TLAs (Three Letter Acronym) mean?

ADS-B stands for Automatic dependent surveillance-broadcast, for your starter (yes I know that is a four-letter acronym).

But first to set the scene, I would not consider myself an expert, but I do have some knowledge having researched the subject to enable me to write my transponder decode application which is available for free download from this site.

The short version of this blog is that I remain happy with my current combination of devices, which consist of ADS-B Out, and Pilotaware for ADS-B In and Pilotaware device reception including OGN (Open Glider Network) when near a ground station that is up-linking and all displayed on SkyDemon.

 

The Transponder

The transponder is a powerful device and comes with various modes depending on its age and capabilities. Older aircraft have Mode A & C with newer Aircraft/transponders having Mode S also

Mode A: In response to Mode A interrogation the transponder transmits an identity code for the aircraft in the octal range 0000-7777 the Squawk code.

Mode C: Transmits the aircraft’s pressure altitude automatically and augments mode A hence sometimes called mode A/C

Trig Transponder

Mode S: Mode S has over 17 million unique 24-bit aircraft addresses known as the ICAO number or mode S address allowing the unique identification of every aircraft, altitude reports in 25 feet increments and the call sign (or tail number) is transmitted along with other information also.

Unlike traditional Secondary Surveillance Radar (SSR) stations which elicit multiple replies containing the same information from all aircraft within their range, Mode S makes selective (Mode S is abbreviated from Mode Select) interrogations of each specific aircraft. ‘All call’ interrogations are also made to identify new aircraft to be interrogated. Mode S transponders are backward compatible with Mode A/C radars.

Civilian Mode S supports a number of different messages Each message has a particular purpose. The formats DF0, DF4, DF5, DF11, DF16, DF20, DF21 and DF24 are used in civil aviation at present.

Number Message
DF0 Short Air to Air ACAS
DF4 Surveillance (roll call) Altitude
DF5 Surveillance (roll call) IDENT Reply
DF11 Mode S Only All-Call Reply (Acq. Squitter if II=0)
DF16 Long Air to Air ACAS
DF20
DF21
Comm. B Altitude, IDENT Reply
DF24 Comm. D Extended Length Message (ELM)

 

Location Reporting

Now some people think as they have a mode S transponder it transmits their position and that their exact location is displayed to other aircraft, but this is not the case. Location information is transmitted in the ES (Extended Squitter) messages DF17 and DF18.

 

DF17 1090 Extended Squitter
DF18 1090 Extended Squitter, supplementary

 

For a mode S transponder to transmit ES DF17 (we will come to DF18 later) the transponder must be connected to a position source i.e. a GPS, and these come in different levels of certification from uncertified up. This configuration is known as ADS-B Out.

It is DF17 (and DF18) that devices like Pilotaware and SkyEcho use to show the exact location of other Aircraft. For non-ADS-B Out aircraft, they use the signal strength only to show proximity but, not location and they show altitude received too.

If only ADS-B Out gives the position how does Flight Radar 24 and 360Radar work their magic and show position of the other aircraft without ADS-B Out? Well, this is done by a system called MLAT which is out of scope for this blog as it requires multiple ground stations.

So DF18, this is essentially the same as DF17, however, it is the message used by a new breed of EC devices which are low power and low cost. They are regulated under CAP 1391. CAP 1391 (First published 2016) specifies Electronic Conspicuity devices that have the ability to signal their presence to other airspace users, thereby turning the “see-and-avoid” concept into “see-BE SEEN-and-avoid”.

It must be noted that CAP1391 states that they must not transmit if carried in an aircraft equipped with a Mode S transponder as this will result in mode S messages and DF 18 being broadcasted.

A system receiving DF18 implicitly knows it cannot interrogate that device, hence no aircraft can transmit DF18 if it has a mode S transponder as it will block or confuse SSRs (CAP1391 6.35). You cannot switch your mode S transponder off due to SERA.13001.

SERA.13001 requires the pilot of an aircraft equipped with a serviceable SSR (Mode S) transponder to operate the transponder at all times during flight.

Additionally, these new devices are intended for UK Annex II aircraft; non-complex EASA aircraft of <5700kg MTOM and for gliders and balloons. (CAP1391 Executive summary 7).

Lastly, they do not allow access to TMZs.

 

GPS

Mode S sends more information then just the location received from the location source, it sends information about how reliable that data is and if it were to fail, the size of containment that is required and more.  The data that most advertisers quote is the SIL value but, this is just one of the parameters.

Much is made of some of some of the new devices having a SIL1 GPS  in their ads, however, most have an SDA of 0 which means they are not trusted!

SIL (Source Integrity Level) field is used to define the probability of the reported horizontal position exceeding the radius of containment defined by the NIC value. 0 is unknown integrity or untrusted!

SDA (System Design Assurance) field defines the failure condition that the ADS-B system is designed to support.

NIC (Navigational Integrity Category) is reported in conjunction with the SIL, NIC of 0 is unknown integrity or untrusted!

 

Can I be seen on TCAS?

Yes, if you have a mode C transponder because most TCAS system utilises an aerial array to determine the position of other aircraft using mode C transmissions.

TCAS II has to be linked to a mode S transponder if fitted so its presentence is encoded in the mode S messages. But TCAS systems below version 7.1 don’t use the ADS-B out messages from other aircraft.

TCAS II Hybrid Surveillance does use Mode S messages but currently has limited adoption.

Hybrid surveillance is a method that decreases the number of Mode S surveillance interrogations made by an aircraft’s TCAS II unit. This feature, new to TCAS II version 7.1, may be included as optional functionality in TCAS II units. TCAS II units equipped with hybrid surveillance use passive surveillance instead of active surveillance to track intruders that meet validation criteria and are not projected to be near-term collision threats. With active surveillance, TCAS II transmits interrogations to the intruder’s transponder and the transponder replies provide range, bearing, and altitude for the intruder. With passive surveillance, position data provided by an on-board navigation source is broadcast from the intruder’s Mode S (ADS-B Out) transponder. The position data is typically based on GNSS and received on own aircraft by the use of Mode S extended squitter, i.e. 1090 MHz ADS-B, also known as 1090ES. Standards for Hybrid Surveillance have been published in RTCA DO-300. The intent of hybrid surveillance is to reduce the TCAS II interrogation rate through the judicious use of validated ADS-B data provided via the Mode S extended squitter without any degradation of the safety and effectiveness of TCAS II.

 

What to buy?  

Should you buy a CAP 1391 device such as a SkyEcho 2? Well that’s up to you but, if you have a Mode S transponder you will need to disable the only feature it has over Pilotaware, which costs less and has mode compatibility with other systems.

If you don’t have a Mode S transponder then Yes, it is an option and will increase your visibility, but not to most TCAS equipped aircraft including military jets and helicopters as they are looking for mode C transmissions.

 

Conclusion

Finally, for me a proper transponder with ADS-B Out (certified GPS or uncertified) is also essential in the increasingly busy skies for your visibility and Pilotaware as your ADS-B In device gives you the best low-cost solution currently.

Today I was told a story of an instructor doing a PFL and they got a Mode C alert on Pilotaware but, at first could not see the other aircraft. Then at a low level saw a chinook below them. The instructor’s aircraft also had a transponder with Mode A/C and S also with ADS-B out, so the TCAS in the Chinook would have been going off too. “see-BE SEEN-and-avoid” in action.

 

 

Little Snoring and the 60th anniversary of the McAully Flying Group

Today we thought we would be brave and visit it a predominantly GA fly-in our C42 Microlight!

The Fly-in was at little Snoring an ex RAF base for the Mosquitos (and other aircraft) opened in 1943, sadly only one of the runways remain operational. The fly-in was hosted by the  McAully Flying Group who are celebrating their 60th anniversary. Two C42 aircraft from Chatteris flew over and that was 66% of all the microlights there, the other was an EV97 Eurostar. I guess due to the weather there was not as many as was hoped for but there was sufficient planes to make it worthwhile and as with my last two flights and blogs there was more changes in PilotAware and SkyDemon to test, I think I’m get obsessed with this combination now!

So there are a number of changes too both pieces of software, but the main items of interest for me today was the PilotAware’s native connection in SkyDemon and a PilotAware patch to fix the issue with not being able to access the internet while connected to PilotAware.

The native PilotAware connection in SkyDemon connects a lot quicker the old method of Flam, which took a few seconds and would show “seek satellites”  while it connected, the new method seems to connect instantly!

And the patch also worked fine allowing us to use live date while navigating.

The whole setup for the third consecutive flight remained rock solid.

Below you can see the combination showing an aircraft near us (we both took off from the same airfield) it is showing in the main screen and on the traffic radar.

The actual flight to Little snoring was uneventful, however the return was to be a fun trip the first C42 return back to Little Snoring so we decided to wait before leaving, eventually a break in the weather came and we confirmed on a couple of weather radar sites that we could fly out and down between two fronts.  In a couple of places we were down to 650ft but as the radar had shown we were soon through the worst of it and flying back to Chatteris behind the front that moving through.

We were not operating at minima, but the training we had for that was certainly a help as was SkyDemon confirming the presence of obstacles on route well in advance so we could make certain we had good visual contact and this helped lessen the workload in the cockpit.   As it turned out if we have wait 3 more hours we would have had a perfectly fine flight back, however in many ways the practice of navigating in low visibility and at low altitude was quite satisfying and Simon who was flying that leg did a great job ably assisted by myself!

A quick blog about a short flight testing PilotAware with SkyDemon

So a great day to fly on Saturday, however our microlight was booked out for the whole day, well actually it wasn’t, it was just that a booking failed to be cancel that a member of our group said they had cancelled, man vs machine! Not to worry Sunday was looking promising, but as it turned out it was not so good, rain and low cloud was the order of the day. I did get a 10 min flight in and my friend Simon and I went up for another 20 mins later too.

Hardly worth blogging about really, but I had built my PilotAware into a single case and wanted to ensure it worked so 10mins and a further 20mins was indeed worth it, well for me any time spent aloft is worth it!

So this is what PilotAware looked like before I rebuilt it:

 

And now it’s more robust in a single case:

 

 

I have also removed the casing from Both the GPS and DVB-T TV tuner dongle RTL2832U that is used for the ADS-B receiver. The ADS-B receiver runs quite hot so I have ordered some SMD heatsinks for it, but they have yet to arrive.

So on the first flight I had it wide open to all traffic and wow with the change of aerial to the “Rubber Duck” meant I could see traffic everywhere, all way out or way high! It could have also been that I had enabled the feature “Mode-CS (Beta)”.

The first thing to note was I was seeing myself as a target on my Skydemon, When back on the ground I added the Hex of our aircraft and rebooted the PilotAware, this resolved the issue of seeing myself. Next I downed the range from long to medium and the height in Skydemon from 50,000ft to 10,000.

So on our next flight and with Simon at the controls I had time to play with it and we were picking up traffic, but with the new settings the screen was a lot less crowded, however we did not see a Mode S equipped aircraft below us and landing and I’m unsure why that was, it was showing in the Traffic screens so I will be asking that question to see if it is my configuration.

Overall I would say this is a great supplement to your own lookout, which remains the primary way to observe in VFR.

Next up I have ordered a plug to connect my headset to the audio out so I get any warnings without needing to watch the screen. I will let you know how I get on with that soon.