The International Cessna 170 Association

Gary,
I'll try to explain. As mentioned in the beginning of this message stream, the the purpose of the Balun is to help electrically match the antenna (which is a balance load because both sides are the same) to the coax cable (which is not a balanced load). The name "Balun" is the two words "Balanced" and "UnBalanced" combined to make one word. The idea of using a Balun on the airplane is that the net effect is that it helps the antenna receive better in all directions when mounted on the airplane. IS THIS REALLY NEEDED?? Probably not if using a newer solid state radio because the VOR receivers are normally more than sensitive enough to pick up signals while in the air but if using an older radio, the Balun might be needed to give the radio a little higher signal strength in all directions.

So how does the Balun do its job? On the antenna side of the Balun (Balanced section) the two leads are driven 180 degrees apart by the loop section of the Balun. The loop is 1/4 wave down and then 1/4 wave up to give a total delay (phase rotation) of 1/2 wave (or 180 degrees). The actual physical length of the loop is reduced by about 20% because the signals inside of the coax are slowed down by that much. The net effect acts sort of like a transformer in that the unbalanced coax cable coming from the radio is transformed from a unbalanced load to a balanced load and one side is rotated 180 degrees and then connected to the antenna. If I had a blackboard and you where here, I think I could do a much better job of explaining. By the way, I think in your 3rd drawing, there is a type-o. I am not sure the 3rd drawing would work. I used the 4th drawing because that was what was in the airplane originally and it is easier to make because you do not have to solder at the bottom of the loop.

Jim Collins

Jim, you did a fantastic job with that explanation.

Jim,
Thanks for the explanation.

Jim Collins wrote:By the way, I think in your 3rd drawing, there is a type-o. I am not sure the 3rd drawing would work.

By the explanation and pictures Bob Nuckolls gives in the Aero Electric link on the last page I don't think the 3rd drawing is a typo. In fact he says,

Understand that this BALUN puts a DC dead short across the end of your receiver's feedline coax so don't be alarmed if you put the ohmmeter or continuity tester to this piece of coax and it tests "shorted".

How that works with a dead short is beyond me.

n2582d wrote: How that works with a dead short is beyond me.

That's because a dead short at the end of a quarter-wave-long piece of coaxial cable "looks" like an open circuit at the driving end. An electromagnetic wave arriving at a short circuit is forced to be all current and zero voltage. But the wave energy reflects back up the cable (because there's nothing to absorb it), and after traveling back a quarter wave has shifted in phase by 90 degrees to be all voltage and no current. It's a sine wave, which goes from peak to zero or zero to peak in 90 degrees. So the driver "sees" an open circuit, same as if the stub of coax wasn't there.

Jim Collins wrote:Thanks to all,
I got the VOR antenna fixed. I had to take off the rudder and then take off the Vertical stabilizer cap. I found the Balun's weight was hanging off of the antenna connection and after all the years it fatigued and broke off. So nothing was connected to the antennas. I made a new Balun with fresh coax cable and reconnected everything and now (according to my antenna meter) it works just like new. The Balun that was in the plane was a little different from the others shown in this message stream but it works just the same and is a little easier to make. The loop section is 1/4 wave long which equals about 50cm. Photo below

Jim, that's not the right balun to use. What you have is called a "4:1 half-wave coax balun", and causes impedance mismatch between the antenna and feedline. It acts as a voltage doubler, and transforms the apparent antenna impedance down by a factor of 4. We really want a 1:1 balun because a dipole is a good-enough match to a 50 ohm line.

But in this application it doesn't really matter, because we have a great surplus of received power and a good receiver, so your VOR will work. Arguably, we don't really need a balun at all. It's really a requirement for transmitters, where we care about return currents flowing on the feedline shield and reflected power.

I got around all this by installing an antenna with a built-in broadband balun, which works better than any tuned coax balun.

DaveF wrote:

n2582d wrote: How that works with a dead short is beyond me.

That's because a dead short at the end of a quarter-wave-long piece of coaxial cable "looks" like an open circuit at the driving end. An electromagnetic wave arriving at a short circuit is forced to be all current and zero voltage. But the wave energy reflects back up the cable (because there's nothing to absorb it), and after traveling back a quarter wave has shifted in phase by 90 degrees to be all voltage and no current. It's a sine wave, which goes from peak to zero or zero to peak in 90 degrees. So the driver "sees" an open circuit, same as if the stub of coax wasn't there.

We're not going to be tested on this, are we?

Gary, sence you read this far you passed the test!

Dave F, Thank you. I looked closer at the Balun that came with my plane and that I duplicated and you are right. The voltage does get doubled and that changes the impedance by a factor of 4 (Bummer). I might have been better off not using one at all. With all of the trouble I went through taking the rudder apart to fit it, I am not about to take it apart again to fix it again. Anyway, I found during my test flights that my VOR range is over 100 miles, so I am not too worried. I think my advice to others would be not to use a Balun at all and just simply connect the coax right to the antenna and be done with it. Also,to add a strain relief at the end of the cable to prevent future breakage (which I did).