VHF aerial

[BrendAn]

My old Jabiru has a double VHF aerial. I don't know the correct name for it. It has the normal aerial on top of the fuselage and has the same underneath in the cabin. Is that a good setup or should I fit a new one.

[RFguy]

Modern Jabirus usually have : the antenna in the tail is a centre fed dipole. 

the antenna in  the midships is often a quarter wave whip with sometimes a limited ground plane wire pair. 

 

Your setup is I guess typical of early jabs. these antennas are just end fed , but most of them are designed to be mounted against a metallic aircraft skin. 

 

The setup will be fine if it is currently working, it is hard to improve on it if the cable and connectors and radio are good. 

 

[Rastus]

It is called a dipole antenna.  They are used in places where there is no electrical groundplane.  Picture a metal Tecnam with an antenna on top.  The aeroplane becomes the groundplane for the antenna.  A dipole is generally more efficient than a simple antenna.  I would leave it alone.  Over to you RFguy 😋

 

Robert

 

See RFguy has just beat me. 🤪

[BrendAn]

Thanks for that. It's all there so I will use it.  

[skippydiesel]

If you want one, its easy to make a groundplane for a non metal aircraft  - I used flat aluminium rod (from memory 1.6 x 25 mm) for the longitudinal axis (cant remember the length but probably as long as the aircraft would accommodate on the inside floor of the fuselage)  now and builders (annealed) soft aluminium flashing for the horizontal axis. The flashing can be molded to the contours of the fuselage & glued in place with silastic. Where they cross ( as near to centre as practical) you need an aluminium plate with a hole in it for the cable/connector and 3-4 smaller holes to mount the antenna - worked a treat.

[BrendAn]

Had a good look today . It all appears to be in good condition. 

[skippydiesel]

No offence, but to my untutored mind,the groundplane looks to be far too small

[BrendAn]

38 minutes ago, skippydiesel said:

No offence, but to my untutored mind,the groundplane looks to be far too small

it was a certified trainer so i assume the radio must have worked ok.

[jackc]

Go find an antenna analyser, NOT a SWR meter and sweep the Aviation Band and see where it is best resonant, hopefully on you rmost used frequencies.  One of my first jobs when I get the Super Bin Chicken home, along with a NEW Becker radio, the MicroAir 760 will be replaced ASAP. 

[RFguy]

1) antenna analyser - Talk to the local ham radio club, someone or 5 will have one you can borrow or use 

2) yes, that ground plane is too small. although better than nothing. 

You need a minimum of two radials, preferably going in opposite directions (electrical balance) each 59cm long if they are thin  wire, or 55cm if they are something like 1/2 or inch wide flat strip (which is better than thin wire) .  These could be attached to the existing ground plate, such that the total length from the antenna base to the end of the strip or wire was one of the above numbers......

 

 

[BrendAn]

56 minutes ago, RFguy said:

1) antenna analyser - Talk to the local ham radio club, someone or 5 will have one you can borrow or use 

2) yes, that ground plane is too small. although better than nothing. 

You need a minimum of two radials, preferably going in opposite directions (electrical balance) each 59cm long if they are thin  wire, or 55cm if they are something like 1/2 or inch wide flat strip (which is better than thin wire) .  These could be attached to the existing ground plate, such that the total length from the antenna base to the end of the strip or wire was one of the above numbers......

 

 

but surely it would have worked ok to be in a training school for years.   i guess most training calls are in the circuit so it may have been good enough for a couple miles. 

[skippydiesel]

Working okay is a bit like saying "sh'ell be right". This is a long way from even good trans/reception let alone excellent.

 

Most of us have experienced poor quality radio communications and hoped our own efforts would be a significant level better.

 

I would ask does the pilot (A) of this aircraft clearly hear transmissions from the pilot (B )10+ Nm away and conversely does pilot B clearly hear the transmission from pilot A - if not then something needs to improve.

 

A decent groundplane  is usually so easy and cheap to make/install - you may be surprised at how your transceivers performance improves..

[aro]

5 hours ago, skippydiesel said:

No offence, but to my untutored mind,the groundplane looks to be far too small

I think it's a dipole so a ground plane is not required. That is what the black bit hanging down is.

[Bernie]

Was easy to check radio in times back...ask ATC for QNH.

Bernie.

[RFguy]

talking to other aircraft in VCY or in circuit a wet string will work. 

 

Try talking  to CENTRE on one of their channels- you can look up where they are on the VTC / VNC. That will give you a good long known path, and the equipment at the CENTRE base station end probably works to good specification.  (instead of trying to talk to another unknown aircraft performance system) .  Ensure you are at least 500' airborne to remove ground reflection effects that would vary the test on ground.

 

An  average RAA plane talking to a aviation CENTRE base should be able to communicate over 300nm line of sight if there is earth curvature clearance. In space, the radio should be able to clearly communicate around 600nm.

So this test, ensure you are line of sight to the base, that probably requires a fair dial of altitude .

Choose a base that is 100nm away. Bases are located on tops of mountains - that gives the altitude-distance line of sight calc  a head start.  Get up to 6500' and you should be able to hit the CENTRE base 100nm away if there is no huge mountain range in the middle.

 

The other way. Use a aircraft radio and a 40dB attenuator on the antenna at the aerodrome.  The 40dB attenuator divides all transmitted and received signals by a factor of 10,000 and so  will simulate 100x the distance, so if the aircraft you are talking to is 1nm away, electrically it is 100nm away.

 

I would expect comms to at least 4nm with this setup, simulating 400nm. The 40dB attenuator is about $30 to buy... you can put one on top of a portable radio also, the rubber duck of the portable is only about 10% efficient (-10dB) so a 30dB attenuator on the radio would do it.

 

-glen - yes I do this for a living 🙂

 

 

 

 

 

 

 

 

 

[skippydiesel]

9 hours ago, aro said:

I think it's a dipole so a ground plane is not required. That is what the black bit hanging down is.

Again from the uninitiated - I had a dipole set in my glass aircraft - it was okay but nothing compared with the conventional antenna with decent groundplane that I installed later (chalk & cheese).

 

True the dipole may have been poorly executed (I don't know) and I did splash out & purchase a new radio at the same time but the difference was genuinely astonishing.

[aro]

17 minutes ago, skippydiesel said:

the dipole may have been poorly executed

That's always possible.

 

I'm no expert, but my understanding is that a dipole is the ideal configuration. With a large enough ground plane ("large enough" depends on the frequency) we can make something that acts like a dipole at radio frequencies, without the bulk of the second element. There are complications, e.g. the shape of the ground plane can affect the radiation pattern from the antenna.

 

Again, my understanding is that the SWR check at aviation frequencies is the easiest way to check the performance of the antenna. I see some people are recommending something else - I would be interested in more information about why.

 

20 minutes ago, skippydiesel said:

I did splash out & purchase a new radio at the same time

That is at least as likely to have made the difference as the antenna.

 

I don't think its as simple as making a bigger ground plane. You would probably need a new antenna designed to work with a ground plane, which I think has a different connector from your current antenna. That means you probably need to install a BNC connector or run a new antenna cable.

 

I wouldn't do anything unless you know you have a problem. If you do have problems and the setup is the same as other Jabirus, I would look for problems in the current setup before replacing it. I don't think Jabirus have a problem in general with radio performance.

[jackc]

The Microair 760 was never a great radio and as far as I know there have be no design changes either, the business has  changed hands but products are still the same.  I am pedantic about radios and have simply decided to replace mine, then I know I am good to go, reliably. 

[RFguy]

The Microair is OK- from a specification point of view, it is OK . I have found factory alignment a bit wanting... I can always get much better RX and TX specs with correct alignment.... 

 

My company has a new aviation radio in the works, ... 4 channel simultaneous receive, built in UHF CB , engine static elimination, etc. fully digital... no adjustments internally...$2k retail. 

Edited October 17, 2022 by RFguy

[jackc]

Glen, when will it be available?

[RFguy]

January I think . maybe December for the first versions of the enclosures.... the enclosure is the tricky bit to get right, that needs a bit of attention right now. 

Currently parts shortages mean the hard to get parts get used on (more) profitable products....  contact me directly...

[Yenn]

I made my ground plane from brass shim about 15 thou, just a square souldered up at the size that would fit inside the fuse. Very light weight and works well.

The antenna is 1/8" SS cut to suit the middle of the airband for length.

[skippydiesel]

2 hours ago, Yenn said:

I made my ground plane from brass shim about 15 thou, just a square souldered up at the size that would fit inside the fuse. Very light weight and works well.

The antenna is 1/8" SS cut to suit the middle of the airband for length.

As I understand it any conductive material (mainly metals) will do.

For ease of acquisition (local supply/low cost), low weight and corrosion resistance, I went with aluminium.

The builders aluminium flashing is great stuff - can be molded to conform to almost any shape and trimmed with industrial scissors (could probably use any scissors but may not do the tool much good)  or tin snips . For stability & durable attachment, does need a sandwich of aluminium plate (s) where your antenna base comes through.

[Yenn]

There is one thing to consider with using aluminium. It corrodes easily and can result in poor electrical connections.

[RFguy]

IMO, Aly is fine if you use gal (heavy zinc)  fasteners, (avoid stainless) and if you really want to go to town, use Aluminium electrical jointing compound , which I think is some zinc based compound. 

If you must joint copper and ally etc or ally and something, use a nice fat gal washer between the metals. 

 

Edited October 19, 2022 by RFguy