RF frequency counter and power meter - antenna vs direct connection - Page 1
Forum section: electronics / testgear

author: Hobby73
author_rank: Regular Contributor
timestamp: April 14, 2016, 10:29:20 pm
content: I want to measure frequency and signal strength for small, low-power transmitters in the range of 300 MHz to 2.4GHz. These transmitters don't have an RF output jack and in some cases their antenna is mounted internally within their enclosures. The most convenient method of measurement would be an RF power meter with an antenna.I'm looking at this "value model" handheld RF signal meter. One of the reviewers states it is quite accurate when connected to an RF transmitter directly via cable. However, the same reviewer claims it is not accurate when measuring by antenna. In fact, he says the antenna is "non-functional" and for "show and tell". In other words, he's saying the measurement capability via antenna is worthless!http://www.amazon.com/dp/B00ECU5B2QIs this challenge of getting accurate RF measurements via antenna typical for all signal meters? Are there better solutions for non-contact measurements?

author: uncle_bob
author_rank: Unknown
timestamp: April 15, 2016, 12:52:37 am
content: HiTo get an accurate reading over the entire 300 MHz to 2.4 GHz you wold need something like a horn antenna or a log periodic. Both are highly directional. One typical setup puts the radio on a stand in a field and then runs the detector / antenna through an arc above the radio. You then run a rotary table to get the other axis on the radio. Needless to say this is out of the "cheap and easy" category by several zeros and a couple of comas.The easier approach is to decide on a specific frequency, say 327.83 MHz. You then cut a dipole for that frequency and play with tripods as mounts. The dipole should be useful over a 10% bandwidth. Calibration can be from a second dipole driven with a signal generator. It takes some fiddling to eliminate reflections and coupling issues. It's still at least 1,000X cheaper than doing it the right way.Bob

author: Hobby73
author_rank: Regular Contributor
timestamp: April 15, 2016, 01:36:12 am
content: Bob, thanks for your reply. Fashioning an antenna to match the frequency sounds like a tedious and imprecise solution. But maybe that's the best that can be achieved for non-contact measurement. Your advice matches what the Amazon reviewer said. No wonder all the examples I see of RF measurement have a direct connection!So I guess I need to follow industry best practice and make a direct connection to the transmitter. Since my transmitters don't have an RF output plug/jack, can I simply wrap a wire around the transmitter's antenna? Or do I need to make connections to the contact points where the antenna is attached to the transmitter's PCB board?Update: I just realized that I need to measure the signal strength, which results from both the transmitter's output power and its antenna design. So I'm back to performing a measurement by antenna. I just found this ImmersionRC unit which costs more, ~$150 USD, but has better reviews.http://www.immersionrc.com/fpv-products/rfpowermeter/

author: uncle_bob
author_rank: Unknown
timestamp: April 15, 2016, 11:56:22 pm
content: Quote from: Hobby73 on April 15, 2016, 01:36:12 amHiI first tested this sort of thing back in the 1970's. A direct connection destroys a bunch of things you need to test. There is no real "shortcut" in this kind of testing. You either buy the broadband antenna or you make your own.Bob