Line Trap Impedance Calculation

[rogerj1]

Folks,
I'm working on a project which is my first on a DCB scheme using Power Line Carrier equipment. We are replacing the relays, TX/RX unit, and the line tuner, but will be using the existing line trap,, and I have little information on the line trap. I do know from old records that its a GE type CF02A with .265 mH inductance, 230 kV & 1200A rating, and adjustable wide band. Also known is the DCB freq which is 155 kHz. In specifying the new tuning equipment, I have to spec a line side characteristic impeance, which is expected to normally be within 150 to 500 ohms. I do know my line impedance from our ASPEN model (Zpu = .003+j.0294 which is 1.59+j15.55 or 15.62@84 deg ohms on 230 kV base,, 60Hz). My question here is how to produce a value for the Line Characteristic Impedance for the line tuner spec, and I believe it would be as follows:

X = 2*pi*f*L
L = X/(2*pi*f)

given the X of the known line impedance and solving for L @ 155 kHz, I get a value of .02 mH for the line (assuming to leave out the small R contribution here). Combine that with the .265 mH value of the line trap, I get the following for the Characteristic Line Impedance

Line Characteristic Z = 2*pi*155kHZ*(.265 mH + .02 mH) = 277 ohms.

Please confirm or correct me if I am off on my calculations.

Thanks,

 

[wroggent]

I don't really know anything about DCB, but I have only seen characteristic impedance of a transmission line, Z_C, defined as:
Z_C=(z/y)^(1/2)
Where:
z=R+jwL Ohms/meter
y=G+jwC Siemens/meter
and w is the frequency in radians/sec.

So, I think you also need to know the shunt admittance for the line, C. You can probably ignore the conductance, G. Also, I'm not so sure that you would want to include the trap inductance in your impedance calculation.

 

[cranky108]

Why are you trying to be so detailed? Most tuners should be able to adjust for that range. Also use a wide band trap so you don't have to mess with it.

To purchase the tuner, you need to know the coupling capacitor value for the stack.

The tuner and coupling capacitor should form a tuned circuit, with a quality set by the type of tuner you use (not tuner quality, but tuning quality). A single frequency will give you the best quality.

You really don't need to be so percise as you will lose 50% of the transmitter power in the tuner/trap part of the circuit. But it dosen't matter because you also lose 50% of the noise.

The biggest loss you can make for yourself is in the lead between the coupling capacitor and the tuner.

 

[scottf]

You should not include the line trap inductance in the characteristic impedance of the transmission line.

The reason you need to specify it with the line tuner is so that the line tuner can be pre-tuned and tested for you in the factory, specifically, the Impedance Matching Transformer part of the tuner. From a practical perspective, you don't have to be so precise with the line side characteristic impedance, if memory serves, the connection options on most IMTs are 175,210,265,320,450 ohms with a +10% addition possible with a link.

Technically, you'll also need the coax characteristic impedance too. Normally it's 50 or 75 ohms. If you don't list one, the manufacturer will likely assume 75 ohms.

As cranky suggested, single-frequency tuning is likely the best option, as it will give you the best/lowest insertion loss, if you're using one center frequency. It will also be the most cost-effective.

 

[rogerj1]

thanks for discussion so far..
I've realized that i'm not applying the correct formula here. The only reason I've been asked to specify the Line Characteristic Z is because its on the manufacturer’s order form. And I don't really know if it needs to be exact or approximate,,,,just because I have no experience with it. As for characteristic impedance of the line, I've been directed to apply the Z = sqrt(L/C) formula,, and not the actual 60hz line length impedance. The only info I have on the trap is that its an adjustable wide band, .265mH, 1200A & 230 kV rated, and we're only using a single freq at 155 kHz. The coax from the tuner into the control house is RG213 cable at 50 ohms.

thanks again,

 

[scottf]

Sorry I didn't catch that before. Yes, the line tuner needs to know the line CHARACTERISTIC impedance and not the actual impedance. As I listed above, this value can be approximate. It's used to find the closest setting on the impedance matching transformer, which balances the impedance between the line and the coax you're using to minimize power loss.

The line trap should work fine for you, as long as the adjustable range includes 155kHz. It will probably give you a minimum blocking impedance of 400 ohms for a given range, which should be fine for your application.