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Check your antenna

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Check your antenna

Postby Steevo » Tue Apr 14, 2015 12:01 pm

I thought I would do a write up on this as I have had my aircraft on the ground for weeks trying to find the solution to an interference problem.

The Problem

My aircraft has glass cockpit (MGL iEFIS) with 2 radios (Funkwerk ATR833 and MGL V6) freshly installed. Whenever the iEFIS was switched on, both radios became unusable. When I say unusable, I mean that with maximum squelch, across all frequencies was just electrical noise.

We tried every single possible solution to this including putting the EFIS components in a Faraday cage, re-wiring, shielded cabling, adding a DC-DC converter, isolating the iEFIS from the rest of the aircraft power. We even came to the conclusion that the iEFIS was faulty and the unit replaced. Weather, temperature and where you were sitting in the aircraft all changed the intensity of the problem. All coax cable was replaced with double shielded RG400 cable.

Virtually every solution had no affect on the problem and after 6 weeks of trying to fix it, I was almost at the stage where I was going to rip all the electronics out and go back to basic analogue instruments. I was on the verge of giving up.

Then we got on to the subject of antennas and I started to do research and learnt an awful lot about antenna design. First off, the VSWR reading was extremely high being between 2.8 and 3.4 across the frequency band. When I measured the tail mounted basic Jabiru antenna, I realised that the antenna was in-fact far too short for the airband frequency. By about 4 inches. I still wasn't convinced that this was the cause but was willing to try anything.

I eventually ordered a new antenna (a VHF-5i especially designed for composite aircraft). Straight away, you could see major differences between the standard Jab antenna and the new antenna. The new antenna was a lot longer and was balanced. I was still not convinced this was going to fix the problem so before installing it (which meant cutting the old Jab antenna out and bonding this one in) I ran a separate length of coax and just connected this new one externally.

To my amazement all the interference disappeared and even with everything on, both radios can now run on minimum squelch right across the frequency band. The new antenna was installed and the VSWR has come right down with it being under 1.5 in some places. All my problems had disappeared and it was a really simply fix after 6 weeks of hard graft and head scratching.

Doing a bit more research I could see why the old antenna was causing so many issues. When an antenna is too short, the radio waves still have to find a way to radiate and to do that, they need a specific length of antenna for that frequency. The only thing the radio waves had was the coax so they were using the coax as an antenna. The EFIS was putting out some Radio Frequency Interference (which all electrical devices do) and this interference would usually travel a few feet and then disappear in to ambient noise but because the coax was acting as an antenna, it was picking up the interference and essentially transmitting it throughout the whole aircraft.

Now I realise that not everyone has glass cockpit, and not everyone is getting interference. But that does not mean that you don't have a problem. If you antenna is producing high VSWR then you are potentially damaging your radio over a period of time. Anything over 3 is considered danger level that can cause long term component damage to any radio. Radios have a certain amount of protection against reflected radio energy, but they are not designed for excessive high VSWR. Even minor interference that you may be getting can be caused by your antenna being out of tune. The coax runs in the cabin so even though the antenna is in the tail, the coax can still be acting as an antenna so essentially, your antenna could be just a few inches from the electrically noisy engine. The perfect VSWR is 1:1, but that is paractially impossible to reach in an aircraft, especially a composite. Ideally, you should be looking for a VSWR of around 2 or less across the whole frequency band.

If you have a standard Jabiru, tail mounted antenna then the chances are that it is too short as the space in the tail is not tall enough on it's own so the Jabiru antenna has been designed to fit comfortably, not work at optimum performance. The antenna should be in 2 parts (dipole antenna), a top part, then a small gap and then a bottom part. Measure the 2 parts in inches and add them together. If the total is less than 43 inches then the antenna is too short and the chances are you have a high VSWR as well. If the antenna is like this, then you are almost certainly causing long term damage to the radio, have a risk of introducing interference that you struggle to get rid of, and you are getting a very reduced reception and transmission range to what is possible.

It would be good to check your VSWR as you may be surprised.
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Re: Check your antenna

Postby Garyo » Wed Apr 15, 2015 8:07 am

Will check the length of mine at the w/end, thanks for the info
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Re: Check your antenna

Postby NigelW » Fri Apr 17, 2015 4:41 pm

At the risk of appearing to be a total thicko! Where will I find it? I have looked for it before and can't seem to locate it. I have posted re my interference and am keen to try anything. :idea:
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Re: Check your antenna

Postby diablo » Fri Apr 17, 2015 5:23 pm

It is probably flocked on to your tail fin. Disconnect rudder cable at rudder and remove the three pins that hold it in place. The antenna should be tucked in the channel running vertically up the fin. The antenna is a bi-pole which should be tuned to 127 Mhz however on all of the jabs I have seen each pole that makes up the dipole if far too short.

The formula for calculating the length of the dipole is as follows:

Length of antenna in inches = 5905 x A / frequency in MHz (should be the middle of air band 127 MHz)

For a half wave dipole the length for a wave travelling in free space is calculated and this is multiplied by a factor "A". Typically it is between 0.96 and 0.98 and is mainly dependent upon the ratio of the length of the antenna to the thickness of the wire or tube used as the element. Its value can be approximated from the graph:

Afactor.gif
A factor table
Afactor.gif (3.68 KiB) Viewed 3639 times
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Re: Check your antenna

Postby Garyo » Sat Apr 18, 2015 7:06 am

You dont need to undoo the rudder ball joint, just pull the pins and fold the rudder to the side of the fin, then you will see 2 lengths of ali bar on the back of the fin
Former Owner of
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Jabiru 450 UL
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Re: Check your antenna

Postby NigelW » Sat Apr 18, 2015 8:29 pm

Thanks guys I'll give it a go.
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Re: Check your antenna

Postby NigelW » Sat Apr 25, 2015 8:24 pm

Here's the latest for those interested. Being a total novice with Avionics and the like I took up an offer from a nice young man who works on avionics for the RAF on Tornados and the like. Whilst informing that it was worlds apart from his day job he did have a crack. After doing lots of things and checking settings etc he did what seemed a real simple test. In effect engine running with lots of noise he swapped the radio aerial to the transponder aerial. Noise gone and all clear. Granted not a fantastically strong signal but clear as a bell.
His conclusion, either poor termination on the co-axial or naff aerial.
Next was have a look inside the fin to see said aerial. Nut off swivel to one side, the fin is enclosed with noting to see. So where is it? The fin was modified by MF to full height a few years back could this mean the aerial is elsewhere or am I totally useless and looking in the wrong place. Your views would be greatly appreciated. :D
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Re: Check your antenna

Postby Steevo » Sun Apr 26, 2015 4:20 pm

A transponder antenna and VHF antenna are completely different animals. The transponder runs at 1030MHZ and 1090MHZ whereas the VHF runs between 118MHZ and 137MHZ. But it has proved that by removing the VHF antenna has stopped your source of noise. A better test would be to borrow (or even make) a VHF antenna and connect that externally.

A VHF antenna can be made really easily and cheaply. All you have to do for a very basic antenna is buy a bar of aluminium about 1 inch wide and around 3mm thick (thickness really doesn't matter as such but 3mm will keep it a bit rigid). It should be 44inches long (not the exact size as it would need to be tuned in real life but 44inches will be a good starting point for a test). Then cut the bar exactly in half so you have 2 bits 12inches long.

Attach them to a length of wood vertical (a few pop rivets will do). Put then on the wood as if the bar wasn't cut with about 1/2 inch gap between the 2 halves.

Now take some RG58 coax cable and strip one end so you have the centre core and the shield. Attach the centre core to the top part of the aluminium bar and the shield to the bottom part of the bar. Attach them just by the gap between the 2 bars (in-fact, the gap is where the cables passes through).

All you need to do now is put a BNC connector on the other end and you have a VHF antenna. This will not be perfect but it will be a good enough test to see if your antenna is the problem. Just attach to the back of your radio and stand the antenna on the elevator so it is vertical.

The coax and aluminium is dirt cheap. Should cost you no more than about £20 for everything.

Regarding the position of your antenna, I will describe it step by step as it is on my one.

At the tail, remove the 3 screws, 3 pins and take off the rudder (you might have to remove the nut and bolt to the control cable or just get someone to hold it there while you check). If you then look in the groove where the rudder was, it should be bonded in that groove. It may be hard to see depending on how the aircraft was built as it is bonded in with fibreglass. If someone has thrown loads of fibreglass over it, It may just look like a groove. At the very least, you should be able to see the cable poking through about half way down.

It is also possible that someone could have removed it in the past and put one in the tailcone. They are available for the tailcone but that is not as supplied by the factory or as in the build manual. It may be easier to trace your coax cable from the radio and see where it goes.
Jabiru J160 & Jabiru SPL-450
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Funke ATR833 OLED/Dittel KRT2
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MGL Xtreme EMS
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Re: Check your antenna

Postby NigelW » Sun Apr 26, 2015 5:09 pm

Thanks Steevo, I did have reservations but now I can try and prove it one way or another. On the hunt tomorrow for the bits and bobs you describe. I just want to get this sorted and enjoy flying.
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Re: Check your antenna

Postby diablo » Sun Apr 26, 2015 5:51 pm

I think Steve means to cut the 44" aly strip down the middle so you have two 22" strips (not two 12" strips). Just correcting Typo ...

Fully agree, using your transponder antenna will damage your radio if you transmit. It is fine to test reception, but on transmit, you will get most of the signal reflected back due to a very high SWR. If the radio has no protection (older radios do not), you will burn out the power circuitry. Could end up being a very expensive test ...

Check out antenna section for further details on making your own high performance antenna. Using this method I made one with an SWR of 1 at 126.5 MHz! Cant get better than that!

http://jabiruownersgroup.org/viewtopic.php?f=49&t=519
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