Libelle Antenna

I've been having some difficulties with the antenna in my H301 Libelle. Hansjorg Streifeneder claimed that the original antenna had a poorly built balun which had likely failed long ago. I've also been having trouble with the feedline, so I was hoping to just replace both the feedline and antenna. According to Streifender, once the rudder is removed the antenna can be easily accessed and replaced. No cutting neccessary.

I was able to get the rudder off (one through-bolt), but when I had it off there was only about 1.5x1.5' hole into the fin. Through the hole I could see a metallic pole going up the left side of the fin. I assume this is the antenna, but there was so little access to it, I couldn't even see the balun or the feedline, let alone touch the antenna. Anyone ever tried (successfully) to replace the antenna? Is cutting neccessary? With the rudder off, how do you get at it?

Thanks,

Dan ,I had a 201 Libelle with the same problem. Assuming the 301 and 201 are the same , there was a small hole on the right side under the horizontal stabilizer approximately between the rear axle and the underside of the stabilizer on the fuselage side.This was used to tune the antenna. Sometimes the hole is gelcoated over.Unfortunately my balun was rotted out. If you can get another balun that would work you would have to cut out the old one from the outside. The antenna element is bonded to the inside corner of the vertical fin spar so forget removing it. What I eventually did was use a Les Sebald antenna (articles in soaring mag years ago) which is basically thin wire that goes around the vertical fin spar and uses a ceramic balun.If I had it to do over again I would get the original balun and cut the hole

Dan,

I thought I had the same problem recently H201B #75. Ultimately, I was able to trace the problem to a bad BNC connection, so my problem seems to be solved. I talked to a number of people regarding the balun in the fin; the general concensus is that it's too difficult to get to. My next step was to install an antenna in the fuselage adjacent to the O2. I found a folded dipole in Aircraft Spruce and Specialty that measures 26.5' tall by 12' deep - designed to conform to the curve of the fuselage.

Jay

All H-301's and early H-201's have a *tuned antenna*. It consists of a variable condenser and a coil. The adjustment on my 301 is located on the left side under the stab, 12 cm forward of the aft edge of the fin and 8 cm up from the bottom. I cleaned the condenser with electronic contact cleaner and then *tuned* the antenna by placing a *field strength meter* near the antenna. Have your helper hold the mic button down with the radio on 123.3 and then tune the variable condenser for maximum output on the field strength meter. The received signal oscillates in the RC circuit and becomes the first stage of amplification. Don't know how this RC circuit works on transmit, but it works well on both transmit and receive. JJ Sinclair 500 lb. Gorilla, Pat's got me on a diet.

And better lay on the ground as far as possible from the antenna, so as not to detune it too much with your body capacitance.

You mean an 'LC' circuit, don't you? Besides, unless there is an active component (a transistor with power supply - which I don't believe at all) at this point, this part of the circuit does not amplify.

A tuned circuit (coil (L) and capacitor(C)) at the base of a vertical radiator is very likely a coupling network that matches impedances between the feedline and the antenna, to optimize its radiated power by avoiding bad standing-wave-ratios and possible related interference with other equpment. If the impedance match is OK on receive, it is on transmit as well. This kind of circuit is basically a 'reversible transformer'. The only requirement is that the components be designed for the power and VHF voltages present when transmitting.

All this reminds me a really bad case of interference with an electronic vario in a H-201, just before a contest. The quick cure was 1/4 wave antenna behind the cockpit and some ferrite sleeves on the instrument wiring. What puzzled me was the (original?) white 75 ohm plain vanilla TV coax feedline, with a pair of standard plastic TV antenna connectors 3 feet from the tail. 75 ohm is not exactly a happy choice for a vertical and what was the purpose of the splice in the fuselage, at a place you cannot reach? There was no sign of repair anywhere, and the feedline was neatly fastened at regular intervals to the inner fuselage. Could this have been a original design?

Willy VINKEN - OO-ZNY - ON5WV

I never actually saw what's at the feed-point of a Libelle-antenna, but I doubt it could be a 'balun', unless the antenna is some sort of vertical dipole, which I isn't very likely.

An antenna with open ends is inherently a balanced radiator. When opened at the center and fed with a parallel-conductor line, this balance is maintained. If such an antenna is fed at the center through a coaxial line, this balance is upset because one side of the radiator is connected to the shield while the other is connected to the inner conductor. On the side connected to the shield, a current can flow down over the outside of the coaxial line, and the fields thus set up cannot be canceled by the fields from the inner conductor, because the fields inside the line cannot escape through the shielding afforded by the outer conductor. Hence, these antenna currents flowing on the outside of the line will be responsible for unwanted radiation. 'Line radiation' can be prevented by devices designed to 'decouple' the line for antenna currents. They are generally called 'baluns', for 'balanced-unbalanced'. They can be designed so that in addition, they adapt the line impedance to the antenna-impedance if necessary (a 1-to-4 step-up from 75 ohm coax to a 300 ohm dipole for instance), but this is not their primary goal. So far for the 'why' of the device.

Sailplanes are usually equipped with a (unbalanced) coaxial feedline AND an (unbalanced) vertical radiator. So, why would they need a balun?

Willy VINKEN - OO-ZNY - ON5WV

On Tue, 16 Oct 2001 08:18:42 GMT, 'Marc Silverman'

I knew all that Willy, Was just checking to see if you did. I don't know how it works, but it does and not too bad when tuned up. I have strung new RG-58U down a fuselage boom and then slid 1' aluminum sleves coated with epoxy and flox. Let the sleves come to rest about every 6' and they are secured to the fuselage wall when the epoxy dries.

Question for Willy, I know just enough about radio to be dangerous. Having said that, when I built my Crystal Set (1940 or so) all I had was a coil and a crystal (with whisker). If I hooked my ear phones to the antenna, I didn't hear anything, when I hooked them to the other side of the LC circuit, I heard music. Wasn't that a very weak amplifier? I think you said the LC circuit in the Libelle wouldn't work without a power supply. Can you explain without using the big words?

So, that's your secret weapon, JJ! Mine is a fellow-lady-pilot I've been cherishing for years, the only one skinny enough to plunge down my Std. Cirrus fuselage and fit the tubing on the TE-probe base...

Things tend to be safer with transistors these days. I remember a homebrew short-wave power (tube) amplifier that swallowed 400 milliamps at 2500 volts ... Izt nicht für Finger poken und Mitten grabben! Das sightseeren Rubbernecks keepen Hands in das Pockets und watchen blinken Lights!

Done that too. But later... And is was pure amazement. Nowadays, kids consider as granted a credit-card size TV to watch a Grand Prix Down Under. Live.

Nope. Just a passive circuit that was resonant with the radio signal so it could 'select' it, and then rectify it through the galena crystal which acts as a diode. There's no amplifier without an external power source.

No, I didn't say that. The LC circuit always does its (passive) job. An amplifier on the other hand needs an 'active' component (vacuum tube, transistor, opamp..) that requires power.

Let's give it a try..

A coil and a capacitor, connected in parallel, form an LC circuit. It is said to be 'resonant' for one frequency. The larger the coil and/or the capacitor, the lower the frequency (or the larger the wavelenght if you prefer). Something like the string on a guitar.

The behaviour of such a circuit is as follows: when it is fed at one end with radio signals picked up by an antenna, and connected to the 'ground' at the other end, it simply shortcircuits all the antenna signals to the ground, except the signal that happens to match its 'resonant' frequency. Which means that for that signal, you can measure a certain voltage between the ends of the circuit. Actually, an LC circuit has a very low resistance (impedance) -hence the 'short-circuit'- for all non-resonant frequencies. It is a 'filter'.

Once rectified, the tiny energy picked up by the antenna -and selected by the LC- is sufficient to generate an audio signal in the phones. The voice coil is actually brought to live by a very small fraction of the energy radiated by the transmitter.

With good quality parts, you get good selectivity, or a narrow, sharp spike in the 'bandwith curve', which means that adjacent frequencies don't interfere. Progressively, airband receiver design evolved to better selectivity. You pprobably remember when channels where 50 KHz apart (or worse?), then 25, and now 8.333...

Sometimes, we intentionally design circuits with 'poor selectivity', at least over a certain range of frequencies. We want our glider antenna to work for frequencies between 118 and 137 MHz, which is quite a broad 'bandwidth'. One way to achieve this is by using a large diameter tube for the radiating rod, typically 1', rather than a thin wire. Additional LC circuits are designed to 'reject' signals lower than 118 and higher than 137 MHz, to minimize interference from FM radio, lower TV bands a.s.o.

Basically, we want a broad open gate with steep walls on the side (engineers call it 'skirt selectivity'.

Combinations of L and C can be designed as 'bandpass filters' (broad or sharp), and also low-pass filters or high-pass filters.

Well, here we are. And dinner is served. Does this make your fingers get itchy enough to pick up the old soldering iron again, JJ?