Help - Jabiru VHF antenna

Australian designed and manufactured Jabiru Aircraft/

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rainier
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Help - Jabiru VHF antenna

Unread post by rainier » Mon Sep 03, 2018 12:03 pm

Guys - Rainier from MGL here. Need some help.
I have a customer in Oz - he has a Jabi with a dipol antenna of sorts fitted to the tail (in the rudder slot).
He fitted a V16 radio but is getting terrible feedback when pressing PTT. He is not very knowledgeable when it comes to these things but has sent me a few pictures.
From what I can see it appears the dipol antenna is connected using 75 ohm cable feeding to a point just before the radio where the cable goes into the radio (about 40 odd cm I guess from the BNC connector) where the 75 ohm cable is joined to 50 ohm cable. So this is a 1/4W matching stub - it seems. In addition however there is a clamp on ferrite at the BNC connector and an additional one at the joint between 50 and 75 ohm cable. The customer wrote to me that he was told these ferrites are essential. I cannot see these ferrites doing anything good here - it seems from posts I found online that this helps with radio noise (that would indicate some really bad issues - in other words the cable sheath is live and part of the antenna).

Something is not right with his installation (the RF feedback is really powerful). Can anybody confirm that this is indeed the way it is done on a Jabi and that it works - just want to make sure this is not just some poor attempt from the previous owner of this particular aircraft.

The rest of the installation - i.e. headset wiring etc looks OK to me from the pictures so I do not think the problem is located there. This is definitely RF related. But I am stumped on this one. I am somewhat suspicious of this antenna installation - hence the question.

Rainier
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Re: Help - Jabiru VHF antenna

Unread post by bluboyz » Mon Sep 03, 2018 1:21 pm

G'day Rainier,

Page(s) 91 & 92 of 344 in the Jabiru J430 construction manual covers the typical 1/2 wave dipole installed within the rudder and vertical fin gap.

https://jabiru.net.au/wp-content/upload ... Manual.pdf

Jabiru used to supply RG58 and 3 ferrite rings as per page 92 (and no other impedance matching)

Cheers Paul.
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Re: Help - Jabiru VHF antenna

Unread post by T4flyer » Tue Sep 04, 2018 9:29 am

Hi Rainier,

I also have lots of RF issues on my Jabiru, transmitting sends my IEFIS's Alt crazy, I have to disconnect my AP before transmitting! I have been looking at finding a solution, maybe fitting a new "conventional" antenna.

If you find a solution please post as I for one would be very interested.

Cheers, Don.
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Re: Help - Jabiru VHF antenna

Unread post by rainier » Tue Sep 04, 2018 4:16 pm

OK we know the dipol does not have the correct length as the rudder is not tall enough. That is not great.
However they metal rods are in close contact with the fibreglass resin and this has the effect of slowing down the speed the wave travels - that means the rods can be shorter. We don't know by how much and how repeatable this would be.

From recent comms with other owners of aircraft also struggling with dipols (not all are Jabis) it appears the use of these things still requires a bit of R&D.

The effect you see on your altimeter and likely other sensors is related. In simple terms your radio produces, depending on its power rating an A/C voltage of around 30 to 50 volts pp. The basic idea is to transfer this high frequency A/C voltage to the antenna and hopefully all of that energy gets radiated by the antenna - and that's all there is to that.
The trouble starts when things are not done right or incorrect components are used. That antenna cable can be viewed as water pipe with a given diameter. Your transmitter is pouring water down the pipe. Your antenna is taking it out at exactly the correct rate and getting rid og it. Now - your pipe can be leaky (Poor antenna cable - RG58 is quite good but does leak a bit, RG400 is much better). There could be a blockage somewhere in the pipe (Damaged cable or poorly joined parts). There could be a blockage or major leak where the pipe joins the antenna (You guessed it - that is where antenna matching comes into it). Finally the antenna itself might just not be tuned to the correct frequency range - and does nothing or little with the energy it is given.

So how does this matter ? RF energy wants to go somewhere once it has been created. If it does not get to the antenna and into the air it sticks around and causes trouble. It enters your aircraft's electrical wiring (usually through the grounds). And it might be say half of your power. This now gets into all of your electronics typically via the power supply. Internal filters, power supply regulators etc are easily overwhelmed and at least some of this makes it to the more sensitive parts of the electronics - like in your case the altimeter sensor.

The simple solution in all cases I have so far come across - get the RF power to go where it should. Anything else is pretty much like applying a plaster. Yes - it can sometimes be really tricky to find exactly what is going on. RF is invisible. Trail and error is the usual answer.

Back to the Jabi - I do think that antenna can work - but I am concerned about the antenna cable sheath, It is simply connected to the lower metal rod of the dipol and according to the builders manual three ferrite rings are placed over the cable right at the antenna. As we know ferrites (provided they actually have been formulated to be effective at 100Mhz and higher) will dampen the cable at that point. So these ferrites, while they will have some effect on the impedance of the antenna are mostly there to get rid of the signal on the sheath. They can't do a great job unfortunately -just dampen it a bit.
This means you have a live antenna cable going all the way from the rear to the cockpit. This is similar to having a really leaky coax cable. When you transmit - you are blasting RF all over the place inside your cockpit close to wiring and sensitive equipment.

Normally a dipol would be fed with a balanced cable - a good type would be a shielded one (so you have three conductors including the shield and the conversion to unbalanced 50 ohm would happen at the radio using a transformer or balun or any number of "tricks".

Alternatively - 50 ohm coax (RG400 is possible) right to the antenna and the conversion takes place there. But this has to be done right or else we end up with the live coax cable scenario again. That Jabi antenna can be made to work well - I am certain of that. But it is going to require a bit of effort. You cannot muck around with this.

Rainier
T4flyer wrote:
Tue Sep 04, 2018 9:29 am
Hi Rainier,

I also have lots of RF issues on my Jabiru, transmitting sends my IEFIS's Alt crazy, I have to disconnect my AP before transmitting! I have been looking at finding a solution, maybe fitting a new "conventional" antenna.

If you find a solution please post as I for one would be very interested.

Cheers, Don.
Who said the sky is the limit ? I think not.
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Re: Help - Jabiru VHF antenna

Unread post by T4flyer » Wed Sep 05, 2018 8:16 am

Thanks Rainier,

I am leaning towards installing a conventional antenna in the top skin above the baggage area. (no rear seats, Jabiru J170 two seater) I will also re-cable with RG400.

Interestingly, I have the exact same radio (Garmin SL40) that was fitted to my UL120, but the performance, especially when receiving a ATC transmission over distance has never been close to what I got when the radio was installed in my previous UL120. I recently checked the length of the antenna fitted to my buddies UL120 and was surprised to find it is quite a bit longer than the one fitted to my J170. It appears that due to the different rudder design the antenna on my J170 seems to have been cut shorter to make it fit in the available space.

Now I need to work out how to make and fit a suitable ground plan for a conventional antenna.

Cheers, Don.
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Re: Help - Jabiru VHF antenna

Unread post by rainier » Wed Sep 05, 2018 10:12 am

Regarding antennas - this is what I do if a plastic or wood aircraft is the platform.

I go to Midas and by a cheap car aerial - basic, straight whip. Nothing fancy, no twirly bits.
I then replace the cable (which is 75 ohm cable of really dubious quality) with RG58 (good quality variants) or RG400 if I can get my hands on it. But good quality RG58 (with nylon insulator, multistrand core) is just fine.
Then I cut the antenna to about 65cm length as measured from the base where it would normally meet the metal surface of the car. That tunes it to around 125Mhz.
Now the most important part - and the part most commonly ignored: The ground plane. This is really important. Without a ground plane that is correctly done your antenna is NOT GOING TO WORK. I wrote that in capitals not because my keyboard is stuck.

So what to do ? Easy: Take 4 lengths of ordinary electrical copper cable (just get some rip cord at the hardware store and split it). Each length about 70cm long. Take a few cm of insulation of one end on each cable. Twist the copper together and connect to the base of the antenna (inside of fuselage) Make sure you have good electrical contact to the base of the antenna - and that it will stay that way.
Now spread the 4 wires against the inside of the fuselage in such as way that when viewed from above it would look like a cross with 90 degree angles between the wires. I usually tape two to the one side and two to the other. Mc Guiver tape is the ticket here. Make the wire follow the inner contoure of the fuselage.

That's it - now if you have, take a SWR meter and check the antenna at various frequencies (say 118, 127 and 136Mhz). Take note of the readings - you should be able to take a guess where the lowest reading is. As here in SA most frequencies used are in the lower section of the band it makes sense to concentrate the tune there (I like about 125Mhz) - if you find the lowest SWR reading is at a higher frequency then take a good side cutter and clip about 5mm from the antenna top - check again. Once you have it close to 125Mhz - leave it there.
You should be able to get SWR readings around 1.2 at the minimum rising to no more than 1.5 at the band edges.

Now you have an antenna that is as good as anything you can buy for just pocket change.

If you have an all metal aircraft (RV or Sling for example) - no need for the 4 wires, the aircraft skin is a great ground plane. Just do not make the mistake of electrically insulating this from the base of the antenna by application of sealant etc.

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Re: Help - Jabiru VHF antenna

Unread post by rainier » Wed Sep 05, 2018 10:29 am

Another thing I should mention - if you do the above - try and get the antenna as far from where your headsets are going to be as possible. The more powerful your radio the further you need to get it away. Headsets tend to have little amplifier boards inside for the microphone - these can be very sensitive to receiving the transmitted signal be means of the various wires connected to it. The electronics there can be a little overwhelmed by this and even partly demodulate the signal to recover the transmitted audio. Now you have a situation that is very similar to standing on a stage with a microphone too close to a speaker. You get howling, distortion or a echoy bathroom like sound when you transmit.

This can also happen if your antenna or antenna cable has some sort of problem - this causes reflected signals that can enter the microphone inputs of your radio directly via coupling to the audio or even power supply wires.

This kind of thing is probably quite familiar to many pilots, it is perhaps one of the most common symptoms of a problem with the antenna installation in an aircraft.

Rainier
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Re: Help - Jabiru VHF antenna

Unread post by V5 - LEO » Wed Sep 05, 2018 11:13 am

...a nice read, maybe read the half dipole section and diagrams.

https://www.google.com.na/url?sa=t&rct= ... mCIS5s2Dkj
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Re: Help - Jabiru VHF antenna

Unread post by heisan » Wed Sep 05, 2018 12:01 pm

OK - waaayyyy too long since I have played with black magic (aka RF electronics), but it looks like this installation has no balun? If so, that will definitely make the shield live (with all that implies...). Putting some ferrites over the shield will suppress some of the side effects, but at the expense of antenna efficiency.
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Re: Help - Jabiru VHF antenna

Unread post by Firedog » Thu Sep 13, 2018 1:47 pm

Had similar issues with my Garmin SL40 in the Jabi. Michael at DART Aviation is quite a knowledgeable young man and he sorted it out for me on site at Rhino Park. Speak to Pieter at DART - it will be best if you can leave your aircraft there. There seems to be many Jabis out there with non standard wiring that could lead to RFI into your audio and other sensors. As mentioned before - fiberglass is not a very good protector against RF so make sure that all the screen wires on audio lines are connected properly.
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Re: Help - Jabiru VHF antenna

Unread post by rainier » Fri Sep 14, 2018 10:36 am

Sadly the Jabi in question does not have the range (or the owner the balls) the fly his craft to Rhino Park - he is in Australia.

However, the problem has been sorted. He bought the plane second hand and was very reluctant to fiddle with the wiring but eventually I managed to convince him that he had no choice. We simply ran out of options. Long and difficult story short - all that needed doing was to undo and redo the crews that connect the electrical lugs used on the antenna cable to the two aluminium strips serving as antenna.
He sent a photo - you can see clear evidence of galvanic corrosion on the screws - all of the plating is gone. Oxidization caused a bad electrical contact.
He will try and create a more durable electrical connection for the antenna.

The V16 radio has a built in VSWR meter so he used that to measure the Jabi antenna (after he fixed the contact). The results are pretty good. He is getting readings of 1.1 at the best frequency and 1.4 at the worst - this is very respectable. The antenna cable has been confirmed as RG400 - they put a red sleeve over it which confused me somewhat. Based on this I can safely state that the Jabi dipol at the back - using the three ferrites over the sleeve at the back (as per builders manual) is good and viable.

Just as an aside: The V16 has received Industry Canada certification today. This process used to take a week. It now takes 3/4 of a year ! So next time you think things work faster in civilized countries - perhaps not anymore.

Rainier
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