VOR/GS Antenna

[Steve65E-NC]

Similar problems on both NavComs in my 65E indicate Antenna problem.  I have been advised to replace old cat whisker antenna with new that includes integrated balun.  Any suggestions on replacement or equipment?  Comant replacement says elements not removable, but my install requires element removal to mount old cover.  

[0TreeLemur]

Nav and Comm radios use different antennas.  Your post mentions both.   From your post I gather that your problem is with the Nav side of both radios?   More detail will help others understand your problem.

There are multiple possible points of failure in the Nav antenna system.   Since  you have two radios, there is a splitter that divides the signal from the Nav antenna to both Nav radios and both GS receivers if both your Nav receivers have GS.   That splitter can fail.   Coax connections are another possible failure mode.  Then the antenna.  I wouldn't assume right away that it is your antenna unless you have some reason to believe that it is.

[David Lloyd]

Steve, antennas do fail but just as likely are bad connections or the splitter.  I have a CI 120-4 Comant splitter if you need one.

[Mooney-Shiner]

I recommend using the multimeter to measure the continuity of the antenna wire all the way to the antenna. I tracked VOR cable continuity all the way to the bottom tail section, but couldn’t access the connection to the VOR whiskers antenna. To the board: How do you remove the VOR whiskers from Vertical stabilizer?

[MikeOH]

2 hours ago, ukrsindicat@yahoo.com said:

I recommend using the multimeter to measure the continuity of the antenna wire all the way to the antenna. I tracked VOR cable continuity all the way to the bottom tail section, but couldn’t access the connection to the VOR whiskers antenna. To the board: How do you remove the VOR whiskers from Vertical stabilizer?

If you are just checking DC continuity, just check to the antenna itself (I'm pretty sure the VOR whiskers are bare metal).  If no continuity, and you've already checked to the base of the tail, then you can start taking things apart.  Or, maybe I misread and you've already done this.

[Mooney-Shiner]

7 hours ago, MikeOH said:

If you are just checking DC continuity, just check to the antenna itself (I'm pretty sure the VOR whiskers are bare metal).  If no continuity, and you've already checked to the base of the tail, then you can start taking things apart.  Or, maybe I misread and you've already done this.

I was doing tail connector to antenna bare metal and was getting 5 OHM, not sure if its good.

Sorry for hijacking the the thread, but I hope this will also help the OP.

[Guest]

On 5/30/2022 at 10:38 AM, Steve65E-NC said:

Similar problems on both NavComs in my 65E indicate Antenna problem.  I have been advised to replace old cat whisker antenna with new that includes integrated balun.  Any suggestions on replacement or equipment?  Comant replacement says elements not removable, but my install requires element removal to mount old cover.  

Bypass the antenna splitter and connect directly to the coax pigtail on the individual nav radios to isolate your issue.

Clarence

[MikeOH]

1 hour ago, ukrsindicat@yahoo.com said:

I was doing tail connector to antenna bare metal and was getting 5 OHM, not sure if its good.

Are you measuring from the center conductor to ground, or the outer shell on the connector? The outer should be low, like 5 ohms. Inner to ground should be open; if low, that’s a problem. Inner to antenna should be low. All of these should be done with the coax disconnected at the radio.

Really, a DC check like this is only going to find gross problems. Short or open. Proper feedline/antenna testing requires special test equipment.

[N201MKTurbo]

12 hours ago, ukrsindicat@yahoo.com said:

 How do you remove the VOR whiskers from Vertical stabilizer?

Remove the sheet metal cover and remove the two screws with a right angle screw driver. They may be so rusted that you need to cut the heads off, raise the puck up and cut the screws off. Then work the screws out with pliers. It can be a real PITA.

[Mooney-Shiner]

On 6/6/2022 at 8:40 AM, MikeOH said:

Are you measuring from the center conductor to ground, or the outer shell on the connector? The outer should be low, like 5 ohms. Inner to ground should be open; if low, that’s a problem. Inner to antenna should be low. All of these should be done with the coax disconnected at the radio.

Really, a DC check like this is only going to find gross problems. Short or open. Proper feedline/antenna testing requires special test equipment.

 

Above I connected security camera wire from Lowes to the bottom connector just so I can troubleshoot the line to antenna.

 

Strangely enough, the reading of 5 OHM was indicated when I was measuring the outer contact of antenna wire and the whisker booms of VOR anthena. I was expecting that the inner contact of antenna wire was connected to the antenna booms. Odd...

Again, I'm hopping my questions are also helping the OP and I'm not just high-jacking the thread.  

[N201MKTurbo]

28 minutes ago, MikeOH said:

I'm not sure what I'm seeing.  What is that zip cord with the purple and red ends?  The measurements should be directly to the coax BNC that is exiting the tail.

You are correct that the inner conductor of THAT connector should be low resistance to the antenna.  The outer should have a good connection to the metal airframe.

Not necessarily. It should have good continuity to one of the radiators of the dipole.

[EricJ]

If this installation has the usual "folded cable" balun (old school), which was kind of standard practice for many, many years on VOR antennas, then the outer conductor (shield) should show low resistance to the exposed part of either whisker, so that's okay.    It's also okay that there is high resistance to the center conductor, because in that type of balun it isn't directly connected to anything, the signal is induced by the shield connection in the balun.

It's also okay if the resistance from the shield to aircraft ground is low.    That seems wrong, but it is actually proper for the shield to be connected to the aircraft frame (ground) and the antenna elements, and the center conductor to be floating (not connected) at the antenna.

It sounds like that's what you're describing, if I understand correctly, so you may be okay.  

Edited June 8, 2022 by EricJ

[MikeOH]

7 minutes ago, N201MKTurbo said:

Not necessarily. It should have good continuity to one of the radiators of the dipole.

Ah, good point!  I wasn't thinking dipole. I deleted my post; bad info.

[vik]

Eric is right on the money!

An antenna is a device to match 377 Ohm impedance of the free space (air is close to free space) to 50 Ohm impedance of the RF cable. That balun is needed to convert balanced (antenna) to unbalanced (cable) load. Both are never perfect, but sort of work in a range of frequencies: around 100-120 MHz in our case.

Look carefully at the picture that Eric attached. The balun is tuned to our frequency. Homework for you: what is the wavelength of 110 MHz. The wavelength is denoted as lambda in the picture.

Two shields are electrically connected at the bottom of the loop and connected to the airframe ground. What often happens, the solder used on the wire there gets old and shields loose good connection or/and good connection to the ground. The antenna is dead! Unfortunately, you can not detect it measuring DC resistance. You need a vector network analyzer. You can also visually inspect the cable if you can get to it.

Also, as somebody mentioned, the corroded connectors will cause the impedance mismatch. This will create a reflection point for radio wave. It is possible to loose almost all signal this way as well. So, check and clean all the connectors in the way.

Once you calculate that lambda, you realize that it is pretty long. That means, the wave will not even notice a gap in your wire. Theoretically, you might have zero DC conductivity, but the antenna/cable will still work fine. Welcome to the magic world of RF

Vik

 

[KSMooniac]

3 hours ago, vik said:

Welcome to the magic world of RF

Vik

 

BLACK magic world of RF!  /FIFY

My dad is an RF whiz in the broadcast industry, and absolutely none of it rubbed off on me.  I'm a structures nerd and needed chicken bones and eyes of newt to pass my circuits class for non EE in college.

Thanks for the good explanation.  I'm about to embark on NAV antenna replacement myself to bury in my new wingtips and hopefully it will work if I can follow instructions. 

 

 

 

[carusoam]

7 hours ago, vik said:

Eric is right on the money!

 

 

For this kind of stuff…Eric is always right on the money! 
 

-a-

[Steve65E-NC]

We checked continuity from coax outer to each element.  One was zero.  Pulled antenna and discovered broken #4 bolt that goes from eyelet, through Bakelite, to element.   Started to remove unit, balun, and coax, but decided that was a bad move and that unit could be repaired with it just pulled up and out.  Had to remove a few cable constraints at bottom to get some extra coax length.  Tied a rope on to be sure not to loose end but did not have to go that far.    Replaced broken bolt and lock nuts, inspected balun and assembly, dielectric grease on electrical connections and Anti Seize on mounting bolts, checked continuity, and put it back together.  Flight tested fine.  Diagram of cable assembly presented here was the key and extremely helpful. Thanks.  Balun was tucked into space at top of fin above antenna and went back in easily.  Removing two mounting bolts without messing up coax was futzy but doable with care and box/open end wrench.  Airframe ground at bottom of balun went to the front mounting bolt.  Might be best to tape on a towel or paper to avoid dangling parts scratching fin paint. 

[carusoam]

Great follow-up details Steve!

Thanks for sharing them.

Best regards,

-a-

[hammdo]

Nice! Mine had 2 issues, bad splitter and bad cat whisker antenna.

after 40+ years, it was time…
 

-Don