Primarily for general aviation discussion, but other aviation topics are also welcome.
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By seanxair
FLYER Club Member  FLYER Club Member
#1592789
VRB_20kt wrote:Interference may be caused by the engine, but is often a result of poor earthing somewhere - either an earth loop or a relatively high resistance path to earth.


Have checked all the earth connections and remade. How do I identify an earth loop? TIA
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By Flying_john
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#1592802
+1 for that aerial, just use to hang your socks on !

Either vertical or a few degrees raked back as has been said. Also check the length it looks a little short, but scale is difficult to judge. From memory a quarter wave is about 57cm/22inches.
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By VRB_20kt
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#1592812
Earth loops are the bain of audio engineers and often aren't easy to resolve. In principle you need a single point that everything is bonded to with a zero resistance path. In real life it's normal to settle for a "tree" arrangement where there is a single path back to the single point. Problems arise when there are two (or more) paths back to the earth point since a small voltage difference can arise between the two paths and this gets amplified by the high gain amplifiers in things such as mixing desks, radios and the like.
What you're looking for in low-level audio circuits is effective screening - which requires a low resistance path to earth from enclosures and a single earth path - which tends to increase the earth resistance.
I'm not an RF engineer and I don't know how (for instance) the screen of the 50 ohm aerial lead doesn't form an earth loop with the aircraft's metal skin that often forms the ground plane for an aerial. From an audio perspective one would hope that there was some break and it may well be that there is a transformer in the RF feed to prevent that very problem (I'm sure there are others here who know for sure).
The practical steps in checking an aircraft radio installation are quite limited. Verify that there is a good earth back to the battery (take care that you know which side of the battery is earthed); check that the case is properly earthed; check that the case isn't earthed through the panel as well as through the ground lead. Hopefully someone will be along in a minute who knows the ins and outs of earthing on the RF side.
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By jamespearce
#1592860
PB wrote:It's hard to find earth-loops once thay have ben installed. It's not something you can easilly check with a multi-meter for instance. Good design and installation really is the key.

Not wishing to be rude to whoever carried out the raido installation, but the antenna offers us a clue that perhaps maybe they weren't an expert, so a good look round might be in order. You may need to strip back some insulation or lift out the headset audio sockets to see what's going on.

On a simple aircraft like your Foxbat, the best approach is proably a simple visual inspection of the wiring. I'd start by looking at the headset wires.

• Are the microphone and headset signals each carried in separate screened cables?
• Is the screen connected to the airframe at one end or at both ends (both ends is a ground-loop!)? Modern Garmin radios have a very effective 'ground block' on the back of the connector. All the audio grouds should connect to this with as short wires as possible. And not be connected anywhere else.
• Are the microphone and headset connectors insulated from the airframe by plastic top-hat washers (if not, you will in all probabality have a ground loop).
• Are the PTT switches each connected by two wires all the way back to the radio? And are those wires screened? With the screen connected only at the radio end? Whlist you clerarly cant 'hear' the PTT line, it is a posible source of noise conduction into the radio.
• Are all the ground wires including the engine earth-strap fixed to a single earth point? If you have earths dotted about all around the airframe, you almost certainly have ground loops.
• Is the radio power supply ground taken back to the single ground point?

Nine times out of ten, sorting all the above will fix the problem.


The clarity and range of the radio is superb except for my particular directional issue.
My recent (8.33) radio installation was thoroughly inspected (as required), and particular attention was paid to earth, ground and screening integrity with some rerouting and improved termination points employed.
Tricky stuff this electronics ! I personally discovered that aircraft earth and radio ground are not the same thing although some installations will still work without separation of these two states of negative source. Several changes were made, not least of which was the radio power supply with an uprating of the wire gauge on a separated circuit with a dedicated trip. I am surprised that the aerial drew no comment from the very experienced inspector though.
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By Flying_john
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#1593221
Designed for mounting on the underside of an aircraft, providing excellent air-to-ground coverage. Bent configuration makes this antenna suitable for helicopters and low-wing aircraft."


They may be excellent for ground clearance but this aerial is a poor substitute for a proper vertical aerial. Amongst other things it will be strangely polarised, and the further you fold the aerial back on itself the more the terminating impedance will rise. Furthermore the adjacent steel tube of the airframe will become a part of the aerial system , strengthening the signal in one direction, reducing it in another, distorting the normal doughnut radiation pattern.

As for ground loops its not something you normally encounter in terminating aerial feeders. They are grounded at both ends, if not the impedance of the cable will alter and the outer screen will become part of the aerial itself. This fact is used sometimes to cheaply construct other types of aerials where precise lengths of inners and outers of co-ax cable are connected alternately together to make "co-linear" and other types of aerial.

A simple home aerial for VHF may be made by simply stripping back a piece of co-ax and folding back the braid back over the outside of the cable leaving the inner core as one half of a dipole and the braid as the other half with the whole thing effectivly being centre fed. The length of the braid and inner form the total lenght of the aerial and hence its frequency of operation. Often the whole thing may be put in a piece of plastic tube , filled with builders expanding foam and hey presto an outside aerial !

The comment earlier about top hat washers for headset sockets, normally applies just to the mic socket where you want isolation from airframe ground at the socket. The headset received audio socket is normally grounded at both ends. It is the mic input that has a high gain amplifier in the radio circuitry and the signals are in the millivolt range whereas the receive audio signals are 1000 times bigger and are less susceptible to interfering signals.

But as said before, good practice is the order of the day for all your radio and intercom wiring and is best left to someone experienced in this field as use of inapropriate cables or poor routing or grounding will give rise to problems.
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By Kemble Pitts
#1593233
Agree that the antenna is best the more vertical it is, that is so that it matches the ground station's antenna which will also be vertically polarised.

However, it is a (much touted and believed) myth that the antenna needs to be electrically bonded to the ground plane. At these frequencies the antenna will work perfectly well if the antenna is electrically isolated from the ground plane. This was explained to me some years ago by an antenna specialist at HR Smith Ltd (specialist aircraft antenna designers/manufacturers at Leominster, Salop).

This will, no doubt, raise howls from some avionics chaps but, for confirmation just check the cork gaskets that are supplied with some VHF aircraft antennae.
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By Flying_john
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#1593252
But even with a cork gasket the antennae is grounded through the co-ax braid thence into the connector plug to the grounded frame of the aircraft when metal.

When composite or wooden structures are used for the fuselage the foil or wires used to form the ground plane are "grounded" to the co-ax braid or to the same earth point that is connected to the radio chassis - hence the term Ground Plane. Anything else would just be an uncoupled metal plane.

"
In antenna theory, a ground plane is a conducting surface large in comparison to the wavelength, such as the Earth, which is connected to the transmitter's ground wire and serves as a reflecting surface for radio waves"
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By flyingeeza
#1593278
Kemble Pitts wrote:Agree that the antenna is best the more vertical it is, that is so that it matches the ground station's antenna which will also be vertically polarised.

However, it is a (much touted and believed) myth that the antenna needs to be electrically bonded to the ground plane. At these frequencies the antenna will work perfectly well if the antenna is electrically isolated from the ground plane. This was explained to me some years ago by an antenna specialist at HR Smith Ltd (specialist aircraft antenna designers/manufacturers at Leominster, Salop).

This will, no doubt, raise howls from some avionics chaps but, for confirmation just check the cork gaskets that are supplied with some VHF aircraft antennae.


Those "isolated" aerials sometimes have a built-in coil at the base which acts as the ground plane.
RF is propagated by a potential difference, so you do always need a ground reference for any current to flow, which in turn generates the RF field.
Sometimes the ground is an airframe, sometimes it's a wire or a plate, or even a coil, but it's always an electrical conductor.
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By Flying_john
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#1593287
Those "isolated" aerials sometimes have a built-in coil at the base which acts as the ground plane.


Never heard the coil called a ground plane before. The coils are there for impedance matching and resonating where you want to use a rod length less than ideal. This also alters the radiating polar diagram.

Image

This base loading inductance is in series with the coax inner connection, with the other end connected to the base of the vertical radiator. The coax feeder's shield is connected only to the antenna's ground-plane. This means that the antenna requires the presence of an effective ground plane at the operating frequency & cannot be operated successfully without one.
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By Paul_Sengupta
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#1593296
Rather than trying to straighten your existing antenna, maybe it would be worth buying some sort of cheap second hand straighter one and substituting it. Or fashioning one from a coat hanger!

Regarding ground plane, what the ground plane is for is to replicate the other half of the half wave dipole for a quarter wave resonator/whip. If you have a half wave antenna then you don't need a ground plane. This is normally centre fed due to impedance, but it could be end fed with a matching network (J-pole/Slim Jim like) or stub to sort out the impedance. This all becomes difficult to mount on an aeroplane though, so a quarter wave with ground plane is the usual choice.
By cockney steve
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#1593312
Please Sir ! I have been paying attention, honest.. :oops:

How does the forgoing reconcile with a handheld's "rubber- duck" ? The said aerial has the standard Co-ax. connector , but the set itself is much shorter than the projecting aerial, so I don't see how the chassis can act as a half of a dipole.

Anyone?