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RF Antenna Help

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jluckey(at)pacbell.net
Guest

Posted: Fri Nov 10, 2023 8:39 am Post subject: RF Antenna Help

Don,

Your explanation helps me understand some of the practical considerations with respect to coax antenna lines.  Thank you. (RF has always been kind of a mysterious subject for me)

Next question:



As you can see from the photos, the center conductor is un-insulated and if I install the C-ring, they will touch - that would be bad.  Can I install a very short length of the center conductor insulation over that exposed center conductor?

I'm thinking that I could cut a ~3/16" piece of insulation (from the core of a length of RG-400), cut a slit in it axially, and press it over the exposed center conductor.  And then, to keep it from possibly slipping off due to vibration (I don't think it will, but just to be safe), can a put a drop of CA super glue on it?  Would that work?

TIA

-Jeff



                  

                                                                    On Friday, November 10, 2023 at 08:19:11 AM PST, Don Pansier <dpansier1(at)new.rr.com> wrote:                 

                 



                 



                 The C shaped device is there to prevent an Impedance Bump. Impedance Bumps can cause high VSWR, weak return loss, and poor system performance. 



Coaxial transmission lines rely on a ratio between the diameter of the center conductor and the ID of the outer conductor, in this case the ratio is sized for 50 ohms impedance. 



Due to size limitations of the connector, it would be very difficult to make the connection from the coax to the center pin of the connector without a removable section. Reinstalling the C shaped section returns the transmission line back to the correct ratio.



Impedance Bumps in Coaxial cables can also be caused by pinching, crushing and bending beyond the min radius,  all changing the ratio between the ID and the OD of the transmission line.

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Nov 11, 2023 7:58 am Post subject: RF Antenna Help

At 10:38 AM 11/10/2023, you wrote:

  	  Quote: 	
		  Don,



 Your explanation helps me understand some of the practical considerations with respect to coax antenna lines.  Thank you. (RF has always been kind of a mysterious subject for me)



 Next question:

 As you can see from the photos, the center conductor is un-insulated and if I install the C-ring, they will touch - that would be bad.  Can I install a very short length of the center conductor insulation over that exposed center conductor?



 I'm thinking that I could cut a ~3/16" piece of insulation (from the core of a length of RG-400), cut a slit in it axially, and press it over the exposed center conductor.  And then, to keep it from possibly slipping off due to vibration (I don't think it will, but just to be safe), can a put a drop of CA super glue on it?  Would that work?



 TIA



 -Jeff



 



 On Friday, November 10, 2023 at 08:19:11 AM PST, Don Pansier <dpansier1(at)new.rr.com> wrote: 



 

 The C shaped device is there to prevent an Impedance Bump. Impedance Bumps can cause high VSWR, weak return loss, and poor system performance. 



 Coaxial transmission lines rely on a ratio between the diameter of the center conductor and the ID of the outer conductor, in this case the ratio is sized for 50 ohms impedance. 

 Due to size limitations of the connector, it would be very difficult to make the connection from the coax to the center pin of the connector without a removable section. Reinstalling the C shaped section returns the transmission line back to the correct ratio.



 Impedance Bumps in Coaxial cables can also be caused by pinching, crushing and bending beyond the min radius,  all changing the ratio between the ID and the OD of the transmission line. 	


   Yeahhhh . . . sort of. The practical effects of 'bumps', 'kinks'

   and sharp bends in perhaps 20' of coax are worries only

   an academic could appreciate.  These are things easily detected

   and measured in a lab with sophisticated equipment.



   But can one demonstrate effects of these anomalies while cruising

   at 10K in your RV? Not so much.



   Consider the attached drawings that illustrate variations in

   coax cable terminations. In Figure 1, we see two techniques

   commonly found in countless production aircraft. (a) crimp

   some terminals on the ends of the conductors and (b) install

   a righteous coax connector.  In Figure 2 we see one of Bob

   Archer's famous, wing-tip VOR antennas tailored to RV

   aircraft. Note that it features crude coax termination as in 1(A).



   However, Bob's design also features a matching section

   intended to optimize impedance matching between the antenna

   and feedline. In Figure 3, we see a VOR antenna that was used

   on hundreds of thousands of aircraft for decades. It too uses

   terminals-on-conductors ('bumpy') and no attempt to optimize

   impedance matching or correct for conditions posed by

   connecting an 'unbalanced' feedline (coax) directly to

   a 'balanced' antenna (dipole).



   Hmmmm . . . if one scans these two antennas with a network

   analyzer or time domain reflectometer, I suspect that the

   Archer antenna would present 'nicer' characteristics

   than the legacy 'rabbit ears' used for a century or so.



   However, in terms of practical performance, it would not

   surprise me to discover that the whiskers outperform

   the wing-tip antenna in terms of receiver sensitivity

   as measured in a circle around the airplane.



   Now, does that bode ill for Mr. Archer's brainchild?

   I suspect not . . . there are many examples flying

   today. We're not hearing/reading of operator-owners

   replacing them with 'something better'.



   I cite this as one of countless examples of variations

   in performance among similar systems. The market-place

   question is, does Bob's antenna perform adequately to

   the pilot's needs?  The answer to that must be 'yes'.

   This in spite of theoretical anomalies (like a 'bumpy'

   coax termination) having no demonstrable shortcoming in practice.



 

   The little c-ring under discussion in this thread

   is one such example. Your market-place question is,

   'would I KNOW that it were or were not present by

   observing stuff on my panel? You've noticed that it might

   prove unhandy should it not hold proper position

   within the connector and cause a short.



   I'd pitch the thing and not give it another thought. 



  



 

 

   Bob . . .



                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================

  

    In the interest of creative evolution

    of the-best-we-know-how-to-do based

    on physics and good practice.

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jluckey(at)pacbell.net
Guest

Posted: Sun Nov 12, 2023 8:54 am Post subject: RF Antenna Help

Thanks Bob for taking the time to explain and to Don also...



I ended up doing as I described in my previous post and it seems satisfactory.  (I did that work before seeing BobN's post)

I have not transmitted with the radio yet.  That will come next week.



                  

                                                                    On Saturday, November 11, 2023 at 08:07:42 AM PST, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:                 

                 



                 



                  At 10:38 AM 11/10/2023, you wrote:  	  Quote: 	
		  Don, Your explanation helps me understand some of the practical considerations with respect to coax antenna lines.  Thank you. (RF has always been kind of a mysterious subject for me) Next question: As you can see from the photos, the center conductor is un-insulated and if I install the C-ring, they will touch - that would be bad.  Can I install a very short length of the center conductor insulation over that exposed center conductor? I'm thinking that I could cut a ~3/16" piece of insulation (from the core of a length of RG-400), cut a slit in it axially, and press it over the exposed center conductor.  And then, to keep it from possibly slipping off due to vibration (I don't think it will, but just to be safe), can a put a drop of CA super glue on it?  Would that work? TIA -Jeff  On Friday, November 10, 2023 at 08:19:11 AM PST, Don Pansier <dpansier1(at)new.rr.com> wrote:   The C shaped device is there to prevent an Impedance Bump. Impedance Bumps can cause high VSWR, weak return loss, and poor system performance.  Coaxial transmission lines rely on a ratio between the diameter of the center conductor and the ID of the outer conductor, in this case the ratio is sized for 50 ohms impedance.  Due to size limitations of the connector, it would be very difficult to make the connection from the coax to the center pin of the connector without a removable section. Reinstalling the C shaped section returns the transmission line back to the correct ratio. Impedance Bumps in Coaxial cables can also be caused by pinching, crushing and bending beyond the min radius,  all changing the ratio between the ID and the OD of the transmission line. 	


   Yeahhhh . . . sort of. The practical effects of 'bumps', 'kinks'   and sharp bends in perhaps 20' of coax are worries only   an academic could appreciate.  These are things easily detected   and measured in a lab with sophisticated equipment.   But can one demonstrate effects of these anomalies while cruising   at 10K in your RV? Not so much.   Consider the attached drawings that illustrate variations in   coax cable terminations. In Figure 1, we see two techniques   commonly found in countless production aircraft. (a) crimp   some terminals on the ends of the conductors and (b) install   a righteous coax connector.  In Figure 2 we see one of Bob   Archer's famous, wing-tip VOR antennas tailored to RV   aircraft. Note that it features crude coax termination as in 1(A).   However, Bob's design also features a matching section   intended to optimize impedance matching between the antenna   and feedline. In Figure 3, we see a VOR antenna that was used   on hundreds of thousands of aircraft for decades. It too uses   terminals-on-conductors ('bumpy') and no attempt to optimize   impedance matching or correct for conditions posed by   connecting an 'unbalanced' feedline (coax) directly to   a 'balanced' antenna (dipole).   Hmmmm . . . if one scans these two antennas with a network   analyzer or time domain reflectometer, I suspect that the   Archer antenna would present 'nicer' characteristics   than the legacy 'rabbit ears' used for a century or so.   However, in terms of practical performance, it would not   surprise me to discover that the whiskers outperform   the wing-tip antenna in terms of receiver sensitivity   as measured in a circle around the airplane.   Now, does that bode ill for Mr. Archer's brainchild?   I suspect not . . . there are many examples flying   today. We're not hearing/reading of operator-owners   replacing them with 'something better'.   I cite this as one of countless examples of variations   in performance among similar systems. The market-place   question is, does Bob's antenna perform adequately to   the pilot's needs?  The answer to that must be 'yes'.   This in spite of theoretical anomalies (like a 'bumpy'   coax termination) having no demonstrable shortcoming in practice.    The little c-ring under discussion in this thread   is one such example. Your market-place question is,   'would I KNOW that it were or were not present by   observing stuff on my panel? You've noticed that it might   prove unhandy should it not hold proper position   within the connector and cause a short.   I'd pitch the thing and not give it another thought.     

    Bob . . .                    ////                    (o o)     ===========o00o=(_)=o00o=========     < Go ahead, make my day . . .   >     < show me where I'm wrong.      >     =================================      In the interest of creative evolution    of the-best-we-know-how-to-do based    on physics and good practice.

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