Didier Juges wrote: > > Bruce, > > You have done it again. Now, I have more ideas for interesting > experimentation and still not more time :-) > > I have 3 coax runs going from my ham shack to the top of my tower to > feed the HF (14 to 30 MHz) and two VHF antennas (6m and 2m, or 50 MHz > and 144 MHz). One cable (HF) is regular RG-213, another (6m) is RG-214 > (essentially like RG-213 with double shield), the last one (2m) is > Ultra-flex air dielectric (more like 9913). All 3 runs are close in > length, about 135 feet. I have fed them with the 1 PPS signal and looked > at reflections with the storage scope (that's how I know the length). It > is interesting to see the big reflection when the signal gets to the > antennas. Of course, the antennas are a poor match for the 1 PPS signal > (fortunately), so they are essentially a short circuit. > > I need to take pictures, now that it is relatively cold here (everything > being relative) and take the same pictures in the summer and look for > differences between the 3 coax lines. Out of the 135 feet, 60 feet are > in the air going up the tower, 50 feet are in the ground (in a 4" PVC > pipe), the rest is in the garage and the attic, so temperature is not > well controlled or constant, but it should all vary in the same direction. > > Didier > > The antenna TDR pictures are at the usual place http://www.ko4bb.com/Test_Equipment/Thunderbolt/
I have put a 10dB 50 ohm attenuator at the output of the Thunderbolt to normalize the output impedance. Not perfect (measures 66 ohms I think), but better than feeding directly. I will redo it with a good 50 ohm series resistor. From the 10dB pad, I have a 4 foot coax cable going to a T, with the T plugged into the scope (1 Mohm input) and the other end of the T going to the antenna. The plot of the 2m antenna puzzled me at first, because it seems well matched over a broad range of frequencies. Then I realized that I have a mast mounted preamplifier that is powered through the coax, so the choke and filter for the supply voltage is providing a match at lower frequencies for the feed line. I am still surprised that the delay to 1st reflection is only a little shorter for the 2m antenna than for the 6m antenna (444nS vs. 488nS). The velocity factors are supposed to be 95% and 66% respectively, so I must have a lot more coax on the 2m antenna. Anyway, I have those for reference now, and I will take the data again when it's warmer. Didier KO4BB _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts