Avionics guys- I need to pick your brains again!

[danny_galaga]

 

So my radio isn't brilliant. And I'm not very methodical when it comes to trouble shooting. But what I have noticed is the signal seems a lot clearer when I'm turning to base. I didn't know what to make of that until I realised the signal doesn't seem as good on climb.  It finally occurred to me what might be happening the other day so I took some measurements today.

 

I have a dipole antenna. When I installed it I felt quite chuffed because it seemed kinda vertical and was a tidy install. As you know, dipole is simple but due to the length is a bit tricky to find somewhere near vertical, else you have to bend it.  But how close to vertical is my installation really? I imagined it was something like 60-70°. But that was an optical illusion due to me being distracted by the door frame. See attached pic. In fact, when taxiing, it's about 45° 😲. So this is my rough estimate of angles:

 

Taxi: 45°

Straight and level: 50°

Descending on final: 55°

Climb: 40° !

 

The question I guess is this: I think I  have more than one problem, but given the variation in quality of signal seems to be related to aircraft attitude, and looking at the general angle mine is at, will I at least reduce or eliminate one problem by making my antenna vertical? My solution would be to move the base of it (bottom right in that first pic) about 400-500mm back so it's not running against the aluminium upright and poke the aerial through the top of the fuselage. This crossed my mind when I was first installing it but I opted for the simplest solution knowing I could try something else  later.

 

 

[skippydiesel]

Just a comment on the dipole antenna

 

I only have experince of one such installation - in my composite ATEC Zephyr. It worked, just!

 

I replaced it with a conventional antenna /ground plain and the performance of the transceiver improved 100%.

 

A proper electronics person may be able to explain/speculate why. 😈

[sfGnome]

I’m not an RF engineer, but I don’t think that the angle is an issue. Most external aerials appear sloped back. Possibly the problem is that the aerial (if I read your description correctly) is running along the aluminium upright which puts a big earth plane in totally the wrong place. I think that dipole antennae work well in composite aircraft where there is little metal to get in the way, but you have lots of metal tubes running everywhere. 

[danny_galaga]

No, its not running along the upright.

[danny_galaga]

10 hours ago, sfGnome said:

I’m not an RF engineer, but I don’t think that the angle is an issue. Most external aerials appear sloped back. Possibly the problem is that the aerial (if I read your description correctly) is running along the aluminium upright which puts a big earth plane in totally the wrong place. I think that dipole antennae work well in composite aircraft where there is little metal to get in the way, but you have lots of metal tubes running everywhere. 

I should add that the installation instructions specifically said that vertical is the ideal, but that it's not always possible so that you should aim for as close to vertical as possible or vertical and bent. It radiates in two toroidal (I think?) patterns so vertical allows for the best propagation. This is what made me think that poking through the top would be ideal.

 

Clearly ~45° is not very vertical 😄. Funny I didn't realise before. I guess hope, and laziness springs eternal