This article reports noise bridge impedance measurement of the Bowral 7MHz dipole for local contacts.
The measurements were made using an RX1 Noisebridge and Kenwood R5000 receiver for the null detector.
Fig 1 shows the RX1 Noisebridge.
Measurements were made at the source end of 13m of RG6 (75Ω) transmission line which is part of the impedance matching scheme. The normal common mode current path was preserved to ensure that the measurement process did not unduly disturb the thing being measured.
|f (MHz)||Dial R||Dial X (pF)|
Table 1 shows the raw measurement data, dial R and X every 100kHz from 6.8 to 7.4MHz.
A spreadsheet was prepared to calculate key metrics from the dial readings, and plot the results.
Fig 2 shows the calculated line input impedance and VSWR(50) across the 40m band. There is a little measurement noise, the real phenomena would be a smoother curve. It takes great care to keep measurement error low, the calculated values can be very sensitive to dial readings.
The frequency for minimum VSWR is about 100kHz lower than original tuning, showing the effects of a recent long period of rain. In the summertime, the frequency of lowest VSWR will move up towards 7.1MHz.
Note the very low line loss, less than 0.3dB, for a line that cost $5 for the entire length.
Knowing the characteristics of the transmission line, it is possible to calculate the impedance at the load end from the data above.
Fig 3 above shows the calculated load impedance and VSWR(75) derived from the source end measurements and line characteristics.
This shows the expected R and X vs f for a shortened dipole, resonance in this case is around 6.7MHz (extrapolating the X curve).
The matching scheme employed depends on shortening the dipole to excite a VSWR(75) of 1.5 at the target frequency, then cutting the feed line to a length that delivers and input impedance of 50+j0Ω. As mentioned earlier, the antenna system is detuned by the effects of recent rain, and the frequency where VSWR(75) is 1.5 is about 100kHz low at around 6.95MHz.
The antenna system design, matching scheme and commissioning measurements are reported at Bowral 7MHz dipole for local contacts.
Fig 4 is a reduced screen shot of the spreadsheet developed to support the task. Click on the image for a full sized picture. You may then need to click on the resultant image to expand it to full resolution, albeit possibly with scroll bars.
The humble noise bridge (in this case an RX1) in capable hands can provide a very useful tool for antenna system design projects, though it takes experience and knowledge and a great deal of care to make valid measurements. Augmented by some spreadsheet calculation and presentation tools, the data is clear in guiding design direction.
© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.