You can barely see the inverted L running between the two towers in the photo above. In a prior blog, I characterized its receive performance, which was worse than a lower and shorter inverted L, and worse than both my 250′ and my 500′ Beverages. Nevertheless, because this inverted L is already in the air and because it is convenient to the shed where I would house the variometer that I would use to tune it, I decided to make it my initial choice for a transmitting antenna for 630m, realizing that I will want something better in the future.
The antenna is 60 feet (18.29 meters) long, so it is a very small fraction (3%) of a wavelength at 630m. Thus it will need a lot of inductance to bring it to resonance at my desired frequency of 475.2 – 475.4 kHz. This is the RF frequency for signals in the range of 1000-1200Hz on the WSJT-X waterfall when the radio dial frequency is 474.2 kHz.
The BC306A variometer that am using for this project was used on B17 and B24 bombers during World War 2. It has an inductance range of 85 to 1210 uH. A photo of the innards of the BC306A is below:
Changing the inductance of the BC306A is accomplished by first choosing the range of inductance using the switch near the top of the variometer, and then fine tuning the inductance using the rotary control near the bottom of the variometer that rotates the direction of a small coil positioned within the larger main variometer coil. The inductance ranges provided by each of the switch postiions on the BC306A are:
| Switch Position | Minimum Inductance (uH) | Maximum Inductance (uH) |
|---|---|---|
| 1 | shorted between terminals | shorted between terminals |
| 2 | 85 | 303 |
| 3 | 265 | 595 |
| 4 | 550 | 915 |
| 5 | 835 | 1210 |
Below is a picture showing the BC306A attached to the bottom end of the inverted L and the miniVNA Pro hooked up to the variometer. The variometer and the miniVNA Pro combination was used to tune the antenna to resonance by adjusting the variometer range switch and movable coil and looking at the results on a tablet wirelessly connected to the miniVNA Pro by Bluetooth. To the left of the variometer in the photo is the Lenovo Tab 4 Plus tablet that I used to run the BlueVNA (Bluetooth VNA) software that controls the miniVNA and displays its results.
Using the miniVNA is very handy, because you can place it at the antenna and be looking at the results and controlling it from the tablet in another location thanks to Bluetooth communications between the tablet and the miniVNA Pro. I forgot to take a shapshot of the miniVNA Pro’s BlueVNA software display on the tablet’s screen showing the antenna’s characteristics before I started tuning it with the variometer. So the first snapshot I have was taken after I had gotten the resonant frequency, which before I added the variometer had been well over 1 MHz of course, tuned down to 530 kHz. The screenshot showing how things look after the initial tuning to get the resonant frequency down to 530 kHz is below. You can see the yellow SWR curve dipping down from more than 9:1 to about 4:1 at 0.53 MHz. At that same frequency the impedance, Z, dips down to about 185 ohms, and the resistance of the antenna rises to about 185 ohms. The reactance goes from capacitive to inductive at the resonant frequency. The return loss is poor at about 5 dB, consistent with the poor SWR:
As you can see in the screen grab below, some tweaking of the movable coil knob on the BC306A brought the resonant frequency a bit too low, to 430 kHz, with the impdedance and resistance just over 200 ohms and the SWR slightly greater than 4:1 at resonance:
A VERY SLIGHT tweak of the variometer knob back in the other direction to give less inductance brought the resonant frequency to 474 kHz, with the SWR about 4:1, the impedance and resistance both just under 200 ohms, and the return loss about 5 dB. You can see all of this below:
Having declared success with this result, I put the variometer into the shed and rechecked the resonant frequency with the miniVNA Pro and all was still OK, so I hooked the 50 ohm coax from the variometer to the 50 ohm hardline for the run back to the shack and took the miniVNA inside and attached it to the 50 ohm coax right where the radio would ordinarily go during normal use. The length of the coax run affected the match, and so the miniVNA Pro now showed that the resonant frequency was now 464 kHz. The miniVNA Pro also showed that at the resonant frequency the SWR was 1.6:1 and the impedance and resistance were a bit less than 50 ohms, as is seen below:
At this point the battery for the miniVNA Pro died, so I had to power the miniVNA Pro from the USB port of the computer in the shack. Because of the way the charging circuit for the miniVNA Pro is designed, that meant that I could no longer use the Bluetooth connection to the tablet, so in order to continue to use the miniVNA Pro while it was charging I had to switch over from using the BlueVNA software running on the Lenovo tablet to using the jVNA software for the miniVNA running on the computer. I left the miniVNA hooked up to the coaxial cable at the radio position and connected to the computer running the jVNA software. I then went out to the variometer in the shed about 150 feet away and established a VNC connection between my iPhone and the computer running the jVNA software. Therefore, I was able to tune the variometer while standing at the shed at the antenna to again achieve best match at the desired frequency while watching the results produced by the miniVNA in the shack displayed on my iPhone. The image below shows the result. The display produced by the jVNA software is different than the display produced by BlueVNA, but both give much the same information. You can see that resonance is at 475.28 kHz, the return loss RL is -21.06 dB, the impedance Z is 53 ohms, the resistance Rs is 52.2 ohms, and the SWR is 1.19:1:
Clearly there is more work to be done, but this is a good start and a real demonstration of how useful the miniVNA Pro is. Being able to tune the antenna for best-match-at-the-radio-position while standing at the variometer in the shed at the base of the antenna without needing to run inside to view the result of each change in tuning was a great improvement from having run back and forth to complete the tuning process. Without the remote view capability provided by the miniVNA Pro it would have been necessary to tune the variometer hooked to the antenna in the shed at the base of the tower, then run inside to look at the results displayed by the miniVNA Pro at the radio position in the shack, then run back out to the shed to retune the variometer, then run back inside to view the result, continuing this back and forth task until the antenna was tuned properly. As it was, with the miniVNA Pro, the tuning was accomplished in a couple of minutes all while standing at the variometer position in the shed at the base of the antenna. It took much longer to write this short blog than it did to complete the tuning process!
You can read about the miniVNA Pro at http://miniradiosolutions.com/minivna-pro/
Here are 3 web pages on variometers:
http://www.qsl.net/in3otd/variodes.html
http://w5jgv.com/11.7uHy_Delta_Variometer/
https://wg2xka.wordpress.com/the-variometer/
73,
Roger
W3SZ