Day: March 30, 2014

Now with my antenna cable, 20m of 5D-2V.

First, both channels at the dummy load.

Then, the delay is measured to be -42.80nS at 7.026MHz (T=142.32nS), which means 99.52nS of delay.

Since the physical length of the cable is 20m, the velocity factor is computed to be 0.67, which is quite a reasonable value.

The impedance bridge shows that the SWR value is almost 1.0, because the 20m cable is terminated with a dummy load.

The voltage difference at the impedance bridge and at the dummy load is due to the cable loss of 20m.

The measured loss is around 0.61dB. Note that the loss of 5D-2V cables at 10MHz is 26dB/km or 0.52dB/20m.

If you remove the dummy load and terminate the cable with an open circuit, the impedance bridge shows:

Note: Something is wrong with the green trace. It should be ch1-ch2 signal, but obviously it is not.

gnuplot> load "gnuplot.txt"
Freq [MHz]=7.026
V1=2.426
V2=1.344
Cursor 1=-2.72e-08
Cursor 2=0.0

vratio=0.553998351195383
phase1 [deg]=-68.798592
phase2 [deg]=0.0

abs(gamma)=0.951950606187447
swr=40.623834169527
cz={1.33775338471275, -14.7339917147814}

With the cable length of 20m and the velocity factor of 0.67, we have:

This shows an open circuit at the far end of the cable.

Cable length measurement to check the velocity factor. The RF signal from the transmitter comes from the left, and the electrical length of the cable (cyan) is determined by measuring the signal delay between ch1 (red) and ch2 (yellow).

This is when both ch1 and ch2 probes are at the dummy load. The frequency of the RF signal is 7.026MHz, and there is no delay between the two channels.

This is when the ch1 probe is moved to the left end of the cable under test (cyan) as is shown in the first figure.

The delay of 11.40nS is observed.

The RF signal is now QSYed to 14.052MHz. Of course, the delay is the same whatever the frequency is.

This is with another cable with the same specifications. The delay is now 11.60nS.

Since the delay of 1nS corresponds to the 0.3m of the electrical length, the length of the cable is 3.42m. Since the physical length of the cable is 2.0m, the velocity factor becomes 0.58, which is somewhat different from the the velocity factor of 0.67 of RG-58U cables.