Dec 16, 2009, 10:14 PM
Joined Jun 2006
Tuning your antenna to the exact frequency
Note that this step is generally uneccessary if you used a caliper and were very precise on your measurements. However it is great for assurance that all is working properly.
Now that you have built your antenna, it is time to tune it in to your frequency. There are multiple instruments out there that can help you with this, however I’m going to assume that most people don’t have them and don’t want to fork out the money for them. So these tests will be done with only the TX and RX and a multimeter.
RSSI Method (most accurate, but more difficult)
How to test:
This test is best done outside. You are going to need to locate the RSSI (received signal strength indicator) pin on your RX. This will tell you the strength of the received signal. It is also known as AGC. It is often pin #8 on the receiver module (counting from the bottom side moving towards the antenna output). it has an output varying between about 5V for full signal to .1V for lost signal.
Separate the TX and RX by the farthest distance possible. 100 yards (or meters) is plenty. Even 100 feet should be ok. Connect power up to your TX with the camera , GPS and all equipment (you don’t need to power up the airplane) connected. Connect a known good antenna to the TX. Make sure both antennas are in the same orientation. If you have a directional antenna on your RX, aim the antenna away from the TX but keep it in the same polarization (ie both vertical or both horizontal). Make sure you have a clear video signal. If not, either bring the antennas closer together or aim your RX more toward your TX.
Hook up you multimeter so that it is as far away as possible from the receiver. When taking readings, you want to be as far away as possible from your multimeter. You’ll want to stand in the exact same place each time you take a reading and do not disturb anything on the receiving side. So it is best to set up your recieving station and then not touch it until the test is complete. It is also a good idea to have a pair of binoculars to take the reading even farther away from the RX to get the most accurate measurement.
Note your RSSI voltage with your control antennas. Write this number down. This will be your control reference number. Ideally this should be about 1.5-2 volts or so, but up to 3 is acceptable. If it is too high your antennas are too close and are seeing too high a signal strength, thus you will not be getting the most accurate readings.
Now replace the antenna with your antenna that you just made keeping it in the same orientation as the receiving antenna. The ends should be between ½” and ¾” longer than your calculated quarter wave. Again record your RSSI voltage as well as the antenna length. Try to be as accurate as possible. Then go out and trim 1/16” (2mm) (or less if you want more accuracy) off of either end of the TX antenna. Again record your RSSI voltage. Repeat this process until your antenna is ¾” shorter than your calculated quarter wavelength or until you notice a significant drop in RSSI voltage. Remember to stand in the same place as far away as possible from the RX and not between the TX and RX.
Graph the RSSI voltage VS antenna length via a spreadsheet program. The peak point on the voltage is your best antenna length. Somewhere in this process, your peak RSSI voltage should be very close to if not greater than the RSSI voltage you recorded on the control antenna if you used a 2dbi control antenna. Rebuild your antenna trimming the ends to this exact length. You can verify that it has peak strength only if you didn't disturb your test set up. If you disturbed your test set up in this process, the RSSI readings will be different.
Current measurement (easier)
Another way to tune your antenna is to watch the current in your VTx transmitter. When the current is at a minimum, the antenna is the correct length. When tuning this way, be sure the antenna is as far away from anything metallic as possible. I recommend putting it on an empty wooden table in the middle of the room. Below are the currents I measured for a 910MHz Vee on a 500mW transmitter:
Note that the calculations on the first page of this thread state that 2.98" is optimal length for the elements.
3.55" - 278mA
3.41" - 274mA
3.36" - 265mA
3.27" - 258mA
3.22" - 251mA
3.165" - 241mA
3.12" - 233mA
3.075" - 230mA
3.045" - 225mA
3.008" - 219mA
2.97" - 214mA (Calculated resonant length)
2.95" - 211mA
2,92" - 213mA
2.89" - 213mA
2.85" - 214mA
2.80" - 219mA
Note that the numbers stabilize, then start climbing. When the numbers begin to climb stop trimming and your antenna is ready.
You now have a tuned antenna! Go fly.
Last edited by IBCrazy; Oct 22, 2010 at 10:30 PM.
Reason: Expained the how of testing a bit more