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1. 07:58 AM - Re: Re: RF Antenna Help (Robert L. Nuckolls, III)
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| Subject: | Re: RF Antenna Help |
At 10:38 AM 11/10/2023, you wrote:
>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@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. >
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In the interest of creative evolution
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on physics and good practice.
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