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Brian Searle

I think? There for I am, I think?

Date Joined Jan 2007
Total Posts : 213

Posted 15/Oct/2009 9:00 PM (GMT 0)

As most of have a transceiver fitted in our Microlights, all be it a portable one like a Icom A3 how well dose yours work. Mine is fine 90% of the time but that odd 10% I can be just a few miles away from an airfield and all I get is. “Station calling you are unreadable” I have been reading about tropospheric ducting and other stuff that is interesting but does not fix my problem. Best thing would be to get feedback from other users. Post how well your system works and say what you have done to make it better and what problems you are having with your radio.

Martin Watson

BMAA 5370

Date Joined Jul 2007
Total Posts : 687

Posted 16/Oct/2009 7:57 AM (GMT 0)

Brian that's interesting - I have exactly the same experience. Icom A2. Ive had everuything from comments complimenting me on the quality of the transmitted audio to "station calling you are unreadable" when only 3 miles from the ground station (Sywell for example) which is a pain if you need acess to airspace. No variation in radio or other kit or how it is set up, and not even consistent in any one flight, so I'm sure its not down to things not working at my end.

All very odd

Martin

Joan Walsh

Microlight flying's for fun

Date Joined Oct 2005
Total Posts : 1048

Posted 16/Oct/2009 10:13 AM (GMT 0)

Oh, message lost in the ether.... Try again (quite ironic really).

I've noticed that it's possible to get an uneven signal strangth around the aircraft with a less-than-ideal aerial installation. Next time you get the problem, try changing heading by, say, 30 degrees and retransmit your message. You may find they get much better reception.

I've found my signal seems to be weakest straight ahead, ie when heading straight towards the ground station.

Joan Walsh BMAA 2916
Saxon Microlights

Surely there's more to life than this

Andy Fell

Wants to be up there!

Date Joined Dec 2006
Total Posts : 1899

Posted 16/Oct/2009 10:22 AM (GMT 0)

Yes it can often be down to antenna radiation pattern (both on the a/c and on the ground station).

http://en.wikipedia.org/wiki/Radiation_pattern

The aircraft (particularly for a pod mounted antenna) is usually worse from behind, since the structure is shadowed by the pod/pilot and engine. Hence why a kingpost antenna is theoretically better - and usually is.

For ground stations it can somtimes be down to what the landscape is doing in that particular area. signals can (and always do) travel by a direct route (i.e. straight from one antenna to the other) and also by multiple indirect routes (bounces of the side of hills/valleys/buildings etc).. when the delays from the indirect propagation add up to be in such a phase which is equal and opposite to the direct signal, you get cancellation - thus a crap signal. When this happens you generally get the signal fading in and out, quite quickly as the aircraft is moving and changing the relative phase of the incoming signals (same as the high speed fading you hear with an FM radio station in your car when the signal is getting weak).

Other effects can be a result of tropospheric ducting (http://en.wikipedia.org/wiki/Tropospheric_propagation) at VHF which means that distant stations have longer 'range' than normal.. these signals can sometimes break through in to the receiver circuit and effect the gain control of the receiver.

There are lots of these sort of effects but it's mostly atmospheric or down to bad frequency planning (not so common).

I have found with the kingpost antenna that the system is fairly omnidirectional... I don't get much variation in antenna pattern so it's pretty good from any angle. If your radio system is setup and working as best as it can, there's nothing else you can do about it.

Well.. you did ask.

Andy
Quik G-GTFC

Post Edited (Andy Fell) : 16/10/2009 12:41:58 GMT

Phil Rigby

Never afraid to speak his mind

Date Joined Jun 2008
Total Posts : 339

Posted 16/Oct/2009 11:21 AM (GMT 0)

Thanks to Joan and Andy for the discussion on aerial installations.

My aerial is installed IAW standard SKR build guidelines above & just behind the pilot, with NO ground plane, apart from the nearby tubular structure. (See photo). Is this good, bad, or indifferent in terms of location? I've had Joan's 'straight ahead' symptoms with both Norwich (several times) and Sywell once. Should I think about repositioning the aerial?

Are there good versus 'less good' aerials? If so, what's a good one? Is it possible to have an intermittent fault with an aerial?

I'm grateful for any advice,

Phil

Riggers

[NAFF ONE]

Norfolk Air Force

G-SIRE

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Martin Watson

BMAA 5370

Date Joined Jul 2007
Total Posts : 687

Posted 16/Oct/2009 11:59 AM (GMT 0)

The "straight ahead syndrome" would explain mine - me, the fuel tank and the engine between the aerial and the ground station, on the occasions its happened.

I'm talking about being only a few miles away - in visual contact with the airfield which is below the horizon(of course) so this may further worsen the signal strength going in the right direction from an aerial mounted on top of the rear fuselage.

Certainly not down to anomalous propagation effects in my case ( I know from my ham radio days what havoc, or fun depending on your point of view, can be caused by tropo ducting)

Martin

Ginge Sunley

Ginge Sunley BMAA 1265

Date Joined Oct 2006
Total Posts : 1387

Posted 16/Oct/2009 12:17 PM (GMT 0)

Repositioning could well help Phil, the aerial as fitted to our T600N is mounted on the top of the fuse tube about a third of the way back to the tail, and as is Joan describes. The installation on our TST points downwards and is threequarters of the way back to the tail and very rarely gives any problem and then only on the ground.

Ginge

Steve Taft
BMAA 3138

Date Joined Dec 2006
Total Posts : 191

Posted 16/Oct/2009 2:02 PM (GMT 0)

Phil,

If you wish to experiment you could try locating an antenna under the fuselage. I do not know what the belly pan is like on a SKR but I use one in two different installations with great results (trike & 3 axis), a ground plane is normally easier to fit down below and I have had very good results just using the rubber icom antenna on the trike. If i were to use an A6 in a trike I am sure I would not have had any problems with range. The only drawback with a belly antenna is that it can sometime limit comms on the ground.

Steve

Andy Fell

Wants to be up there!

Date Joined Dec 2006
Total Posts : 1899

Posted 16/Oct/2009 2:24 PM (GMT 0)

Hey Phil,

I would be tempted to do as Steve says. You can put a reasonable groundplane inside the covering, underneath the fuz (make it super lightweight of course).

Although you do have some groundplane (aircraft frame) what is happening is that you in the cockpit, and the aircraft frame, are causing a shadow to the antenna which essentially needs to transmitt/receive downwards... you are blocking the antenna's view towards the ground stations.

Rick Goddin changed his setup and attached the antenna to a jury strut connection (the top bolt close to the underside of the wing).. I believe he said that helped a lot. In a way, the frame of the wing will act as a ground plane (signal wavelength is around 2.5Metres, so you can have good sized 'gaps' in the groundplane), not the best groundplane.... but the point is that the antenna has a good view towards the ground.

If you don't need to worry about the performance while on the ground, underside of the fuz would be the best position.

Jeremy Harris wrote a great little article on antenna installations.. think it's on the BMAA site somewhere...

Andy
Quik G-GTFC

Post Edited (Andy Fell) : 16/10/2009 14:28:32 GMT

Andy Fell

Wants to be up there!

Date Joined Dec 2006
Total Posts : 1899

Posted 16/Oct/2009 4:19 PM (GMT 0)

Forgot to say though Phil is that if you are intending on fitting a transponder you would be better putting the transponder antenna on the underside of the fuz too... so perhaps the jury strut bolt may be a better option for the VHF antenna.

Andy
Quik G-GTFC

Rick Goddin

WFAeC - the Club to aspire to!

Date Joined Apr 2008
Total Posts : 1607

Posted 16/Oct/2009 4:28 PM (GMT 0)

Hi Phil

On G-HMcB the VHF antenna is on the starboard wing at the front upper strut position, hanging down, with the co-ax running through the strut and into the cabin at the lower strut bolt position.

On the attached photo you can just see the antenna - it has a white blob of PVC on the end (not for flight!) to prevent my walking into it.

I have the Filser transponder antenna and ground plane in the rear fuselage floor just forward of the front ventral fin. The stub is just visible in line with the "B" of the call-sign.

This set up works well for both transponder and VHF (ICOM A3E)

Cheers

Rick

"Fly higher - with the WFAeC"

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Alan Batchelor
BMAA Member 1257

Date Joined Mar 2009
Total Posts : 115

Posted 16/Oct/2009 4:52 PM (GMT 0)

It's not unusual to get good reception to and from Sywell at range yet to be told you're unreadable in the circuit.

It's happened to me a couple of times, and after landing the guy in the fuel kiosk said he heard me perfectly on his hand-held. So my guess it's a multipath oddity which effects the Sywell set-up particularly badly. Could be reflections from the considerable acreage of hangar roofs!

I fly a Skyranger with the antenna on top the fuselage just behind the cockpit in the standard set-up. My recommendation for antennas though is never use a rubber duck or a tuned antenna, just the simple straight or cranked monopole whip - which will be about 21 to 22 inches long - and good quality low loss coax (double screened if possible) to keep out unwanted emmissions from both transponder (yep, they do radiate in the VHF range!) and the GPS which also radiates.

Alan

Andy Fell

Wants to be up there!

Date Joined Dec 2006
Total Posts : 1899

Posted 16/Oct/2009 8:24 PM (GMT 0)

Alan, a 'rubber ruck' (aka donkey dick

) is a helical antenna. It is no more or less of a tuned antenna than a simple dipole or monopole whip. Since a helical is more compact, it is not as efficient as a dipole, although it is tuned to the correct frequency.

I'm not sure that a transponder radiates at VHF (don't think my TT21 does) because it would not pass all of those aviation EMC specs if it did... Maybe the older ones do... It's the transmitter power amp keying up that can give a bit of trouble.. This is not radiating at VHF, it's the sharp transients doing it. Similar effect is when a mobile phone goes off near the car radio..

Agree on the GPS, the internal microprocessor runs at VHF and goes straight into the receiver at airband! That's why I opted for 96C, as it seems to be much quieter than the 196/296... they must have a different processor running a lot more efficiently in this model, since it also runs a lot longer on half the amount of batteries!

Andy
Quik G-GTFC

Post Edited (Andy Fell) : 16/10/2009 20:32:14 GMT

Alan Batchelor
BMAA Member 1257

Date Joined Mar 2009
Total Posts : 115

Posted 16/Oct/2009 8:44 PM (GMT 0)

Andy

Yes I'd agree with almost all of that. But whilst the transponder radiates it's wanted transmission on 1030 or 1090MHz, (can't remember which is which now - one is transmit and one is receive) it will also radiate unwanted sidebands on VHF, whether these are generated by the internal rf circuits or whether they're generated by virtue of switching transients. I have experience of two systems, both Garmins with MG serial altitude encoders, which do sporadically interfere with the VHF radio - interference which goes away if the VHF antenna is removed!

The rubber duck as you say is a helical antenna and quite inefficient, so should be avoided for best communications.

I also believe that Glass Cockpits can/do radiate significantly, as I know of one Skyranger where the VHF antenna had to be mounted on the tail fin to get as far away from the cockpit electrical environment as possible.

I think best quality wiring practice is always the key to getting an acceptable VHF/Intercom environment, whether it's on a flex or on a three axis machine. My previous mount was a Pegasus Q, and with an A20 Radio and a king-post mounted antenna I once had a VHF radio chat with a guy somewhere in Yorkshire, some 150 miles away from me at that time.

I was recently asked to look at a radio system in a Chipmunk because the performance was so poor - what a nightmare with old wiring and junction boxes marked with the old Air Ministry markings! Nothing beats good wiring and best quality coaxs.

regards

Alan

Andy Fell

Wants to be up there!

Date Joined Dec 2006
Total Posts : 1899

Posted 16/Oct/2009 8:50 PM (GMT 0)

Interesting, I'm surprised they get away with that (the garmin transponders that is!)

Andy
Quik G-GTFC

Alan Batchelor
BMAA Member 1257

Date Joined Mar 2009
Total Posts : 115

Posted 16/Oct/2009 9:08 PM (GMT 0)

Problem is Andy, that you don't know what's actually radiating. Odds are that it's the interface wiring or the power wiring, as the SSR antenna is unlikely to radiate much anyway at VHF due to it's very short physical length. In an all metal aeroplane, with the wiring tucked away behind a steel or aluminium panel, you'd expect significant 'internal retention' of these unwanted VHF interferences, but with our 'cloth aeroplanes' we get what we get, unless we adopt excessive shielding measures. Strange thing is with these Garmin systems that the interference is not tied to the SSR response - there's a small 'R' displayed when the unit responds to an SSR interrogation, and if the interference was tied to transients you'd expect it to appear with the 'R' and disappear again shortly afterwards. Could be harmonics of the serial altitude data which is transferred to the transponder every 1 second. (ie 1 second altitude update rate). Not too serious that it can't be got around with judiscious use of the radio squelch control, so as it's working to an acceptable standard I'll leave it alone.

regards

alan

Andy Fell

Wants to be up there!

Date Joined Dec 2006
Total Posts : 1899

Posted 16/Oct/2009 9:15 PM (GMT 0)

Indeed. Many an installation needs a bit of filtering on the wiring!! I did put a ferrit on the 12V supply and it helped a little.. I've found that with the 'R' display (actually on the TT21 its a little icon) I can just about correlate it to the small amount of noise on the headset. It's a bit tricky though because where I live (Cambridge aera) the thing is firing off all the time.. It must be the TCAS from the Commercials going into Stansted and Luton setting it off... even when I'm on the ground!

But like I said earlier, the noise is so low level I just ignore it .. can't hear it in flight anyway.

Andy
Quik G-GTFC

Alan Batchelor
BMAA Member 1257

Date Joined Mar 2009
Total Posts : 115

Posted 16/Oct/2009 9:36 PM (GMT 0)

How nice to have had a thread of seventeen interesting posts without it getting hijacked for personal or political reasons!

Phil Perry
Never afraid to speak his mind

Date Joined Jan 2007
Total Posts : 235

Posted 16/Oct/2009 11:42 PM (GMT 0)

Compared with other radio systems, Commercial airband radios are pretty well as deaf as a post, to preclude the reception of co-channelled signals from so far away from your station as to be fairly useless. and it follows that a small discrepancy in feedlines / tuning and or situation of the antenna can make a huge difference to the overall perfomance of our "Flea Powered" airborne radio systems.

Joan pointed out a popular problem with directionality of signal transceive, where there is a null created by the mass of the aeroplane due to poor siting of the antenna. Bear in mind that with only five watts or less of RF energy, assuming that the system is working at optimum, the inverse square law bit of the laws of physics states that a signal will degrade as the inverse square of the distance travelled. . . . so if Sywell ( Or Heathrow, etc etc... ) have their squelch (mute ) turned up a bit hard, or they have a slight null in a particular direction and you're there...... well they may not hear you. This does not mean that your system is in any way faulty, it's just that, that's life, when using the low powered systems that we do.

By all means carry out all of the checks that the preceding posters have suggested, and then you've tried your best.

No aircraft mounted antenna will radiate perfectly in all directions, this is a fallacy, but you can site the aerial with a bit of forethought, feed it correctly, shield the system if you have to, and then if that didn't work, come back on here and ask again ! ! ! Maybe we can try more than one aerial on your plane, like the police cars in aAmerica and Australia, who use phase and frequency diversity systems with multiple radios and a signal tracking computer to mix and sort the best signal AND frequency band for the distance, velocity of vehicle and location required. . . . .

There is always someone on heere who will help

Phil P.

BMAA 2395

Paul Lintott

A bit on the mouthy side

Date Joined Jan 2008
Total Posts : 97

Posted 17/Oct/2009 12:59 AM (GMT 0)

Brian, First prize is the antenna installation, I have spent many years with rf and installations with trikes, vehicles, boats and military applications. Thats normally the fault.

The Garmin 296 is a reliable source of rf emission, this coupled to a carbon fibre prop.

We have in South Africa a very good propellor called NC props, they perform as intended, brilliantly, very light but make a very good radio shield and reflector. Now until we tested them properly couldnt believe the radio could not tx through them. Ie behind the aerie with a antenna on the front. Second is the ground plane, a trike does by design default does not have sufficient surface area for a 1/4 wave monopole. Hence a King Post dipole will perform well. (Look at this first)

Also a wild card (look at this also) that we have seen is the fact that the ICA3/22 series are old in the tooth, the BNC female sockets are now getting worn and we have seen the physical connection inside the radio solder joints cracked. It is a good idea to use the antenna BNC male plug connector to be a right hand device which reduces mechanical load on the radio. This is a very common fault with the ICA 5/23 series incidentally. Also creates a problem that we had when initially using the VXA series as they have a SMA male - BNC female adaptor and need a rubber supporting collar which prevents mechanical damage. If installed in a haphazard fashion we have had one socket self extract from the radio body, then we learnt the way they are installed !!
(The VXA s outperform those old radios and the ICA 6 in any event)

Transponder antenna, I used a Indian sourced ally curry pot base (about 350mm dia ) the side of the pot cut away to make stand off legs, the base radius makes a neat finish for my old Mode C transponder mounted under the seat. In those days a very simple King post dipole as far away as possible and never had a problem with noise ingress at all. I have a TT21 on order so will be able to report back very soon. Our trikes dont have pods so we have less "real estate" than you chaps.

For info, with our local laws flying under the TMA of a large international airport we have to have Mode S, strobes, two radios (and two antennas), landing lights. Ballito, KZN. No visitors and call the tower before we take off (by phone)...Its hard work to fly a trike now. Days of hop in and fly seem to be over. We will be needing a small power station to run this lot next...

Good Luck Paul

The older I get the faster I was...

Phil Rigby

Never afraid to speak his mind

Date Joined Jun 2008
Total Posts : 339

Posted 17/Oct/2009 6:07 AM (GMT 0)

Wow! What a thread!

Thanks to one & all for the learned radio discussion - there's so much here that my engine fitter's head is still spinning, but with a little time, there seems a fair bit I can try before consigning my mate's A6 to the bin.

Thanks again,

Phil

BTW: Ref Composite props shielding transmissions: thinking about it, every time I've experienced radio problems, (basically, I can hear everyone else, other airborne traffic can hear me, but the ground station can't), I have been approaching the ground station, such that the propeller would have been 'in the way'. Maybe a coincidence, but it's happened so far with Norwich, (regularly), Farnborough, Beccles, Sywell...

Hmmn: the engine fitter's solution could be to gear the radio to the camshaft, so that the radio only transmits between the rotating prop blades: well, it worked for machine guns....

Riggers

[NAFF ONE]

Norfolk Air Force

G-SIRE

Post Edited (Phil Rigby) : 17/10/2009 06:31:32 GMT

Martin Watson

BMAA 5370

Date Joined Jul 2007
Total Posts : 687

Posted 17/Oct/2009 9:02 AM (GMT 0)

Ah! I'm begining to see daylight!

My probs were not down to poor installation because during the same flight everything could be OK with another ground station. But the poor audio quality reports (note - they can hear a transmission, but sound quality was poor so they couldn't make out what I was saying) always happen when the composite prop is between the antenna and the groundstation. If the rapidly rotating blades interupt the signal as they pass through, this could cause a rapidly vaying signal strength at the ATC receiver (varying at the rpm of the prop) which would create a buzz or rasping audio quality. Maybe...? What do folks think?

Martin

Roger Mole

Appreciative of hand start in cold weather

Date Joined Jun 2008
Total Posts : 753

Posted 17/Oct/2009 9:47 AM (GMT 0)

Phil Rigby said...

Hmmn: the engine fitter's solution could be to gear the radio to the camshaft, so that the radio only transmits between the rotating prop blades: well, it worked for machine guns....

Who was that comedian Phil who used to do the routine with a microphone that kept 'breaking down' so while he was speaking there were constant gaps in what he was saying - was it Parrotface (remember him?)

Anyway whoever it was, that's who you'd sound like

I'm sure you'd have ATC in stitches

NOT!

Still Having Fun Getting G-MYRO Flying!

Dave Smith
Someone shut this guy up!

Date Joined Dec 2005
Total Posts : 1039

Posted 17/Oct/2009 10:42 AM (GMT 0)

Poor transmitted audio is usually the critical problem from microlights - especially open cockpit ones. Of course, the controller can report 'Strength 2' on a really powerful received transmission - he still can't understand most of what's being said. In more enlightened amateur world, both carrier strength and understandability are rated separately. More useful information.

I always like to hear the transmission on the ground if I'm 'debugging' someone's radio system.

You need a good microphone with a foam protector, held close to the lips, a good radio interface, and a radio that's up to spec. I was amazed at how poorly one major manufacturer's 'noise cancelling' microphones were at cancelling extraneous noise.

Dave

Phil Rigby

Never afraid to speak his mind

Date Joined Jun 2008
Total Posts : 339

Posted 17/Oct/2009 10:59 AM (GMT 0)

Roger Mole said...

I'm sure you'd have ATC in stitches

NOT!

Roger,

With my standard of radio work, ANYTHING would be an improvement...

Phil

Riggers

[NAFF ONE]

Norfolk Air Force

G-SIRE

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