Another TDR Question--Driving antenna

amdx · 2018-07-26T02:08:53+00:00

ReCap; I bought the 74AC14 TDR here. > I modified the TDR for a higher output impedance to give me a little better resolution, since my antenna will have higher impedance than the standard 50 ohms. I modified it for a 215 ohm output impedance. I'm driving a 253ft wire on the ground with a termination at the end. At the other end I connect the TDR to the antenna wire and to a ground rod. I monitor the squarewave on my scope, and adjust the termination until I get a flat top on my squarewave without a step. I expect the termination resistor will equal the characteristic impedance of my antenna. It measures 213 ohms. But I also did the resistive voltage divider calculations, with my known output impedance (215ohms) and my know drive voltage (5V) and the voltage of the first step (3.3V). I got 417 ohms with that calculation. Problem: 213 ohms does not equal 417 ohms. Well, I just happened to get into measuring ground rod resistance the last few days (at 60Hz FWIW). I tested the ground at both ends using the method on page 24 of this, I got approx, 117 ohms for the ground at each end. So, it makes sense to me that I need to add 117 ohms the the 213 ohms of my termination resistor. That would seem to tell me characteristic impedance of my wire is 330ohms. 213 + 117 = 330 ohms But, again 330 ohms does not equal 417 ohms. Do I need to add the ground resistance of the source end? That would be another 117 ohms bringing the characteristic impedance up to 447 ohms, this is closer to the 417 ohms of the resistive voltage divider calculation I did above. Do I need to add the ground resistance of both ends? Side note: I think my ground resistance is very high, and I have been working at reducing the resistance. I put 3 rods at each end and I added Calcium Chloride to the rods on one end and got it down to 73 ohms. Mikek

Jul 26, 2018 22 Replies

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amdx 7 years ago

My thread has been hijacked! Mikek

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Rob 7 years ago

The typical antenna used for that is a logper. Modest gain, modest directivity, large bandwidth.

To get more gain and directivity, you would typically use a logper as a feed for a dish. However, that is impractical for home use at those frequencies (it would be more or less ok from 1-2 GHz up)

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Wond 7 years ago

Wire bowtie ahead of a sheet reflector is easily made and effective. Twinlead feed tho...

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