I went ahead and built a Cloverleaf antenna, right-hand polarized, for a video transmitter, transmitting @ 1280MHz and 800mW. After about a week, however, the transmitter stopped transmitting. I'm wondering if it's possible that the cloverleaf "overloaded" it somehow? I made it with rather thick, 18AWG wire. From the shield to the pin on the connector, I measure 8 Ohms resistance. If it helps, I'm using a RangeVideo transmitter. Is this a possibility? My only other option is that the antenna broke off in a quadrotor crash, leading to the "no antenna = letting the magic smoke out" situation that the manufacturer warns of, however, I don't remember this happening... Secondly, is there any way to test if my antenna is built "correctly" (whatever that may mean, I'm new to radio transmission and reception) without expensive test equipment? Thanks!
1 Answers
It's possible that the transmitter was damaged by reflected power from a poorly matched antenna. That's the scenario the manufacturer is warning about with running the transmitter with no antenna. Without a well-matched load on the transmitter output, the RF power reflects from wherever the discontinuity is and back into the output of the amplifier. This can cause the amplifier to operate incorrectly or even destroy it.
Antennas look simple, but there's a lot going on electrically, and matching an antenna's impedance to the system impedance at the operating frequency can be a bit of a challenge. Although impedance is measured in Ohms, it is not a resistance, and can't be measured with a DMM. Instead, impedance is the relationship between voltage and current through a circuit, and it depends not only on the resistances in the circuit, but also the capacitances, inducatances, and the measurement frequency.
An RF network analyzer could do the job, but those instruments are indeed quite expensive. You may be able to make friends with someone who has access to one and could help you tune your antenna. For an idea of what's involved, read The Dropout's Guide to PCB Trace Antenna Design.
Amateur radio operators use SWR meters to measure the ratio of the power going towards the antenna to the power reflecting back from it, and make adjustments to minimize the reflected power. While those SWR meters aren't extremely expensive, they are designed for much lower frequencies than 2.4 Ghz, and it is useless to match an antenna at a frequency other than its operating frequency. I've never seen an SWR meter in the microwave range that could reasonably be bought or built by the casual hobbyist.
Other options:
- Buy an antenna that's already well-matched
- Buy a transmitter that is more robust against poorly matched loads.
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