Antenna Balun Design

[David_H]

There appear to be several different Balun designs being used to help with unbalanced antenna transmission lines (two designs are shown below). One item that seems rather inconclusive so far is the inclusion of the velocity factor for the coax. It appears as if the coax velocity factor would apply for the design shown in Fig 3-6 below... but not for the folded Pawsey Stub design due to the Balun being outside of the dielectric sheath.

Is there a benefit to using a Pawsey Stub Balun over the one depicted in Fig 3-6? The design in Fig 3-6 leaves the transmission line open and the second purposely shorts the transmission line across the Balun.

Formula: (300/f) x (velocity factor) x (0.25) = 1/4 wavelength Balun length;   f=113 [MHz]

If a velocity factor of 0.69 is used for RG400... calculations lead to approx. 18" for the 1/4 wavelength Balun length.

If a velocity factor of 1 is used (air)... calculations lead to approx. 26" for the 1/4 wavelength Balun length.

Are there any EE's here that can chime in on this one?

[carusoam]

Somebody mentioned the other day... an antenna with an internal balun...

If @Gagarin is back from orbiting the earth, he may be knowledgable on the subject...

@Jerry 5TJ knows tons of avionics detail...

@Baker Avionics

@Jesse Saint

So I invited a few knowledgable people that come to mind...

Best regards,

-a-

 

[Gagarin]

The above balun is for a  narrow frequency range like the VOR/LOC band it precludes the GS band of 330 MHz. Later NAV  antennas uses transformer type baluns for a wide operating frequency range. This allows for the same antenna to be used for GS/LOC/VOR.

Antennas with integrated balun

https://www.aircraftspruce.com/catalog/avpages/comant157p.php?clickkey=528096

Coupler for dual NAV/Coms and single NAV antenna.

https://www.aircraftspruce.com/catalog/avpages/dmH69-1.php

It workout very well on my Vostok 1 capsule

Gagarin in space a must to watch, enjoy it

Jose 

KP4DAC

 

 

Edited November 30, 2019 by Gagarin

[EricJ]

10 hours ago, David_H said:

Are there any EE's here that can chime in on this one?

EE with a few decades experience in wireless communications, and I have to say I have no idea why GA makes this stuff so hard.  Usually a balun can be implemented with a small matching transformer, and is extremely inexpensive, like one of these:

https://www.ebay.com/i/312852631651

or one of these:

https://www.amazon.com/Male-Type-Balun-Matching-Transformer/dp/B01MT7QALQ   - the little black boxy thing.

Both match a 300 ohm antenna to a 50-75 ohm cable.   I don't know what the impedance is of a typical VOR antenna (probably about 300 ohms, like a rabbit ear TV antenna), but, as mentioned, many modern antennas build-in the balun because it is not expensive or difficult to do, facilitates a good rf connector on the assembly, and saves a lot of trouble.   The only reason I can think that stuff like your Fig 3-6 is done (which is very often what is done in little airplanes, at least the old heaps that I've been around), may be because it might be a bit more robust to lightning or something like that.   I suspect the newer antennas with built-in baluns put a breakdown diode or similar protection device in there with a transformer or something.   That said, people have been putting those super-cheap matching transformers on outside TV antennas for many decades with high reliability.

The second drawing you posted appears to be an ambiguous (i.e., slightly misleading) depiction of a Pawsey stub balun.    "Velocity factor" is normally almost never anything you should have to worry about.   As shown in the diagram, just use the freespace wavelength.   It doesn't need to be super-precise.

If you search for "balun" on either aircraftspruce or edmo, you'll get to this:

https://www.aircraftspruce.com/catalog/avpages/rami15-97-6.php

I suspect either getting one of those or making one, like your Fig. 3-6, or getting an antenna with it built-in, are going to be the best bets without making an IA tilt his head at your airplane.  FWIW, the Fig 3-6 you posted is Fig. 3-6 from AC 43.13-2B, so it is, at a minimum, FAA accepted data for installation on an aircraft, and is what is implemented in the aircraftspruce "balun" cable.   Functionally, a little transformer would do the trick, but I'm not certain of the turns ratio it would need to match your VOR antenna.  
 

Edited November 30, 2019 by EricJ

[David_H]

Thanks for the replies @EricJ and @Gagarin.

Eric, you're correct that Fig 3-6 came from AC 43.13-2B. The coax removed was originally constructed in this manner. It had a Balun fabricated with an approximate length of 16" which could possibly correlate to using a velocity factor for the dielectric sheath. AC 43.13-2B makes no mention of using the velocity factor though.

It's also possible that the Balun tuning length was split between the bands for the VOR/GS and the LOC... perhaps somewhere around 190MHz instead of 113MHz. This would also yield a Balun length of approx 16" without using a velocity factor. As Jose stated, the Balun is tuned for a very narrow frequency band. This makes one ask if it would perform very well at any of the operating frequencies since the tuning frequency is so far away from any of the operating bands. Also, does the higher GS frequency band need to be tuned at all since long-range reception isn't necessary for the GS band?

[EricJ]

1 hour ago, David_H said:

Thanks for the replies @EricJ and @Gagarin.

Eric, you're correct that Fig 3-6 came from AC 43.13-2B. The coax removed was originally constructed in this manner. It had a Balun fabricated with an approximate length of 16" which could possibly correlate to using a velocity factor for the dielectric sheath. AC 43.13-2B makes no mention of using the velocity factor though.

It's also possible that the Balun tuning length was split between the bands for the VOR/GS and the LOC... perhaps somewhere around 190MHz instead of 113MHz. This would also yield a Balun length of approx 16" without using a velocity factor. As Jose stated, the Balun is tuned for a very narrow frequency band. This makes one ask if it would perform very well at any of the operating frequencies since the tuning frequency is so far away from any of the operating bands. Also, does the higher GS frequency band need to be tuned at all since long-range reception isn't necessary for the GS band?

A few points:

Using "velocity factor" for baluns is one of those things that has a lot of OWT/religion around it, kind of like "shock cooling" or doing touch-and-goes in a Mooney and stuff like that.   There are really very, very few places where it matters, and I think this isn't one of them. 

I think you're right that it was probably cut to put the band center (of resonance) between the VOR/LOC and GS frequencies.   If you didn't care about GS, it could be a bit longer, but I suspect the difference would be pretty small. 

For things like this "narrow band" is relative.   It's not going to be all that narrow and is still going to let a LOT of stuff through.   While I think you're right that you don't have to worry about GS sensitivity quite as much since GS transmit antennas are very high gain and only need to support a smaller range, giving up sensitivity when you don't need to, especially for something potentially safety-related like a GS, is not usually desirable. 

I suspect that one reason that this style of balun gets used so much is that they've been around since the 30's and 40's so from the FAA's usual perspective are perfectly state-of-the-art and completely sufficient for use.  They do still work and you can point to AC 43.13 to say it's okay, but I'm glad some antenna suppliers are bringing us forward at least a few decades and putting them in the antenna.   Putting a $2 transformer in there would work just fine but would probably be rejected on principle by many IAs. 

[cliffy]

Used to make VOR antennas for the VariEze and I just used a ferrite balun inside the base, about a 1/4" cube.

Worked great and was a "hot' antenna. EAA had an article many years ago about using a series of ferrite rings around the RG-58 cable as a balun. It worked great also. 

The fold back loop shown above is traditional and has been around for decades, No need to get too carried away with it, Build like it shows in 43.13 and move on. 

[David_H]

1 hour ago, cliffy said:

The fold back loop shown above is traditional and has been around for decades, No need to get too carried away with it, Build like it shows in 43.13 and move on. 

Did you use 113MHz for the tuning frequency to calculate the Balun stub length (lambda/4) shown in 43.13? What length do you arrive at?

[cliffy]

I don't remember the length now. I've built several over the last 55 years. All the ones I made in the last 30 years were using the feritte balun and those were 20 years ago. I just know what is shown works (been there done that and its not that critical). Every Cessna came with them 40 years ago. Call a junk yard and ask for one. Put in a BNC connector and have at it. It really isn't rocket science.