Difference in performance/behavior by loading location ?

[Externet]

Hello all.

A 50Ω wideband impedance transmitter [TX] connected to a RG58 coaxial [====] and to an antenna; in two configurations :

TX=R================================================= aerial

TX==================================================R aerial

Where R is a 50 ohm non-inductive resistor loading at the TX output connector; or, placed at the aerial end of the coaxial (in parallel to the RF line.)
What differences to expect in behavior / impedance / SWR / performance / attenuation for each case ?

 

[ahsrabrifat]

The key difference between the two configurations is where the 50Ω termination resistor (R) is placed. In
Configuration 1, the resistor (R) is placed at the transmitter (TX) end. In Configuration 2, The resistor (R) is placed at the antenna end.
In con 1,the transmitter sees a perfectly matched 50Ω load right at its output. However, the coaxial cable will not necessarily see a matched termination at the antenna side. In Configuration 2: the coaxial cable is correctly terminated at its end with 50Ω, meaning it behaves as a matched transmission line. The transmitter sees whatever impedance is reflected back from the antenna system.

 

[Externet]

Thanks. Yes, as shown. A 50Ω R at one end of the coaxial or at the other end.
In configuration 1 the transmitter 'sees' a 50Ω coaxial in parallel with 50Ω, not exactly 'perfectly matched' , or is it ?
In configuration 2 the coaxial 'sees' the antenna is in parallel with 50Ω, not exactly 'correctly terminated' , or Is it ?
What are the effects of each ?

 

[FvM]

Neither configuration makes sense. Transmitter should be more or less matched to transmission line, additional resistor according to config 1 kills the matching. Effect of config 2 depends on antenna impedance. Ideally antenna has 50 ohm impedance, in this case it's already matched to coaxial cable and a resistor causes mismatch and signal attenuation.

 

[volker@muehlhaus]

What are the effects of each ?

In case 2, the line is matched to the transmitter output impedance, and the length of the line doesn't matter. No impedance transformation here. The transmitter will "see" the resistor in parallel to the antenna impedance.

In case 1, the antenna impedance will be transformed to some value that depends on line length and wave length, so it varies with frequency. This frequency dependent (transformed) value will be in paralell to the resistor.

So in case 1, line length matters.
In case 2 it doesn't matter.

 

[vfone]

A simulation using a short 1m RG58 coax cable, shows that S21=-0.25dB and S11=-25dB.
50 ohms resistors placed either at the input or at the output, degrades S21=-3.8dB and S11=-10dB.

Using a long 20m RG58 without 50 ohms resistors, S21=-4.9dB and S11=-30dB.
The same 20m RG58 coax cable, with 50 ohms resistors placed at its ends, shows that the resistors (wherever they are placed) degrades the transmission line insertion loss S21=-8.2dB. Return loss S11 is better with 50 ohms resistor placed at the output compared to the input (-22dB vs -10dB).

In all cases a 50 ohms antenna was assumed at the output, and frequency of 50MHz.

 

[volker@muehlhaus]

20m RG58
frequency of 50MHz

50 Mhz -> wavelength is 6m
20m x factor 0.66 = 30m

In your testcase, the cable length is a multiple of lambda/2, so impedance transformation will give you Zin=Zout
You should try again with cable length lambda/4

--- Updated Sunday at 1:05 PM ---

Let's assume we have an antenna with 200 Ohm input impedance. Cable length is 90° = lambda/4 at 50 MHz.

--- Updated Sunday at 1:15 PM ---

If the resistor is in series (not shunt), it looks like this:

 

[vfone]

The impedance mismatches in coaxial cables, theoretically appears only if the coaxial cable is too long or is improperly terminated.
In our case, when using 50 ohms resistors placed in parallel to the input and to the output, the RG58 coax cable is improperly terminated which cause reflections.

We are doing simulations using ideal coaxial cables. In reality, coaxial cable impedance is more dependent by its length due to cable imperfections. RG58 is a cheap cable, and is not keeping well its impedance vs length. For example an LMR-200 coax cable behaves totally different when is about multiples of wavelength.

 

[volker@muehlhaus]

RG58 is a cheap cable, and is not keeping well its impedance vs length. For example an LMR-200 coax cable behaves totally different when is about multiples of wavelength.

As an RF professional, I don't agree. RG58 is homogenous along its length, so it keeps impedance vs. length.
Everything else is math that can be derived using the proper cable specs.

 

[FvM]

All quantitative parameters in this thread (cable length, frequency, antenna impedance) have been guessed by contributors, no useful info given by original poster.

 

[vfone]

If you read carefully you will see that the original post does not mention the length of the cables, which had to be chosen somehow. And I think it is better to choose them randomly than depending on the wavelength.
As an RF professional, unfortunately, I had very, very bad experiences using RG58 coax cables.