[time-nuts] 75Z vs 50Z for GPS receivers

Didier Juges didier at cox.net
Sun Jan 28 19:00:05 EST 2007


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



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