THIS IS NOT A "PLUG-AND-PLAY" ANTENNA !!!
Tuning this small, short boom Yagi is critical. The dimensions calculated from MININEC proved to be only approximate because of numerous factors including the parasitic capacitance introduced at the split element insulators. The calculated inductance for the various loading coils was also only approximate and effected element tuning.
AT VE6WZ, THE CRANK-UP TOWER MAKES TUNING EXPERIMENTAL ANTENNAS SIMPLE. THE ANTENNA CAN BE REACHED WHILE STANDING ON THE ROOF OF THE HOUSE WHEN THE TOWER IS CRANKED DOWN. TUNING THIS 2 EL YAGI REQUIRED MANY ITERATIONS OF ADJUSTMENT. ON A TOWER, A CRANE WITH A BUCKET WOULD BE REQUIRED
1.) Use MININEC to calculate the reflector tuning at 3.795 MHz: In the final modeling program, the driven element is opened (insert a reactance of -9999 ohms) insert the source at the reflector and calculate the resonant point of the reflector. (with the VE6WZ design, this was 3.794 MHz)
2.) On the tower, open the driven element (no coax feed) and feed the reflector element. Excite the element and tune it to the required resonance. The VE6WZ Yagi was built with numerous adjustable telescopic tubing joints on both sides of the loading coils to facilitate these adjustments. Small aluminum wire capacity hats could also be used down element from the coils if required.
3.) Remove the feed to the reflector and close the element.
4.) Feed the driver and tune the element for approximately 3.795. Do not adjust the hairpin for best SWR yet, since the final reflector tuning greatly effects the feedpoint impedance because of mutual coupling.
5.) Field test for max F/B frequency: Locate a receiver with an S-meter a few hundred feet or more off the back of the yagi. Transmit 10w or so into the Yagi at 5 kHz intervals from 3840 to 3760 and record the S-readings. Use a transceiver with a tuner to compensate for the high SWR and maintain the same power output. Plot the values. The point of maximum F/B will show up as the frequency with the lowest S-reading. Adjust the reflector element (shorter or longer) and test again to tune it to obtain the maximum F/B at 3795.
6.) Retune the driven element for resonance at 3795 and adjust the hairpin coil for minimum SWR by compressing or expanding the coil. (There may be a need to repeat this procedure a few times since the reflector tuning can also change when the driven element is re-tuned....even adjusting the hairpin can effect the reflector tuning because of mutual coupling).
7.) Check the range of the band-switching inductors to confirm correct coverage. Adjust as necessary.
8.) Field test as above the F/B tuning in the CW section of the band. As required adjust the CW tuning inductor (expand or compress the coil) on the reflector to obtain the best F/B on the CW operating center frequency.
|At left is an example EXCEL spreadsheet of the
S-readings taken at VE6WZ while tuning the Yagi for 3795. After this
test the driven element was re-tuned for 1:1 SWR at 3797. The chart
below shows the next test....notice that the reflector tuning moved
slightly, so another fine-tuning was required.
The minimum S-reading off the back at 3797 was S-6.3
The maximum S-reading from the front at 3797 was S-9.3
Assuming 6 dB per s-unit this would be around 18 dB F/B.
These reading where recorded using the IC-756 PRO II in the car about 1200 meters away from the tower.
|EXCEL chart showing S-meter readings (F/B
analysis) and SWR together.
After this test, the reflector was re-tuned to move the peak F/B up about 7 kHz (shorten the element slightly). Alternatively, the driven element could have been retuned to move the 1:1 SWR down to 3792 or so.
SWR BANDWIDTH AND REFLECTOR TUNING
The SWR bandwidth of the Yagi is very sensitive to element tuning. The chart at right shows how the bandwidth becomes very narrow if the reflector is tuned for max F/B ABOVE the center operating (resonant) frequency. For greater bandwidth (but a little less gain) tune the reflector for max F/B below the design (1:1 SWR) frequency.
This effect has been modeled with MININEC but also confirmed with tests on the tower.
The Yagi can be fine-tuned by just observing the bandwidth......if the bandwidth is too narrow, then lengthen the reflector a bit. Conversely....if the bandwidth appears too broad, then the tuning is likely incorrect. This effect is the result of how closely coupled the elements are.
The chart below shows how the bandwidth varies depending on the reflector (F/B) tuning. In all cases the driver is tuned for 1:1 at 3795. (modeled with MININEC)
|This is one GREAT benefit of a crank-up tower.......how easy it is to tune things up. By rotating the Yagi, access is gained to all parts of the antenna while standing on the roof of the house.|