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 

[danny_galaga]

Update. I figured it couldn't hurt to move it. Only thing of course is because of the length, unless it's in the tailplane, something's gonna poke out. So 3D printer to the rescue 😀 

 

There wasn't a lot of traffic this morning. I did a circuit without no one else on the radio. From that brief flight it seems that making the aerial vertical has made it sound (to me) more consistent. That is, whether I was on the ground, or downwind or turning to base it sounded much the same .

It was getting a bit breezy for this low hours pilot so I took the opportunity to work on tidying it up. I was hopeful that someone might be in the radio down the other end and I wanted to test the difference from port and starboard 'broadside'. You'll notice because the aerial is close to starboard, it's closer to some tubes.

 

My general understanding is the dipole radiation pattern is two torus. Basically two doughnuts with a very smaller hole. I have one tube virtually in the centre where the two 'doughnuts' are 'stacked', so I figure it won't have much effect.

 

The top tube will have more of an effect. As you can imagine, the number of degrees of 'shadow' it would cause depends on its diameter and the distance from the aerial. This is pretty close. So I would say maybe 40mm of the aerial just there is 'shaded' by about 35°. It gets too complicated to imagine how much effect that is though, because the rest of the aerial around it must radiate some energy into that space. As well, I believe a metal object close and in the way will re-radiate some of the energy, thus 'filling in some of blanks' so to speak. The signal won't be as good in that general arc but more than zero.

 

ANYWAY, with all that in mind, I taxied the plane to have the starboard side perpendicular to the runway so that if there was someone down the other end, my signal should be somewhat shielded by that tube. Someone was down the other end. They read me 4. I then turned around 180°. They read me 5. This all adds up. 

 

So, in the end it would seem it was worth fiddling with the aerial. On the ground, slightly weaker signal on the starboard side, but still readable. In the air, for most circumstances it should be fine above, below and behind. I feel for any installation the engine in front will cause some shadowing. And starboard, from a distance should be fine as the radiation 'normalises'. Close, the total of the signal will be stronger so I imagine still readable.

 

Probably would be better to have it in the centre, but I would probably have to run a new cable to reach. This was much easier 😄

 

[jetboy]

Could you mount a normal 1/4 wave whip on a small plate on the central tube structure of the cabin? 

 

Mounting a dipole partially inside a metal cage isn't ideal, even with a sub-optimal groundplane the whip will be far better.

 

I have done them on the likes of Avid Flyer, the only issue is the traditional wire whip is narrowband compared to those D&M sloped Cessna style "broadband" types which use both a thicker rod and carbon resistor in the base to obtain better match across the whole aircraft band. This can be an issue for some aero radios.

 

The slope of a whip is not that critical, ideally the first half should be vertical and it matters little if the top part is raked back sharply or the whole thing is sloped up to 30 deg.

[danny_galaga]

Thanks. I wanted to see what I could do with what I have, and with as little effort as possible 😄

 

It seems to be better than before. As Nev says - it seems to be a dark art!

 

My last test I got a radio check from a guy about 12 nm away, reading 5. He was in a Drifter but flies 737s for a living. He said not to overthink it, my radio seems plenny good enough for our lil planes. So it may not be brilliant, but better than one or two I've heard before so will suffice for now. I haven't fiddled with settings much. Im ashamed to admit I don't even know much about squelch. 

[BurnieM]

Not a dark art just a quarter wave (middle of the frquency range) radiator roughly vertical with a ground plane (you can even get away with a 15cm x 15cm alu square).

You want your quarter wave radiator outside any metal frame.

Do NOT connect your ground plane to your electrical ground.

 

You can even have your radiator pointing down (again outside any metal frame) but your strength will be poor on the ground.

 

[danny_galaga]

Well, as you can see, it's all done now