This morning, I got the following e-mail from a friend of mine who is a member of the University of Michigan Amateur Radio Club. He got a request from a researcher at the university, who wrote:
We put archival acoustic and motion sensing tags on marine mammals. The tags record for about a day and then release from the animals. A VHF transmitter (transmitting in the 220 MHz range) is built into the tag, along with the antenna. We then use directional antennas to ‘home in’ on the tag in the ocean for recovery and data offload.
I am looking for equipment on campus that I could use to tune the antennas I am making and would like to be able to measure things like standing wave ratio and transmission frequency.
My friend wrote back:
Unfortunately 220 MHz is a band that isn’t very popular, so there aren’t as many “tools” available. That means even if you find an HT or mobile unit that will transmit in that band, you won’t have very much luck finding an SWR meter for it. I think your best bet is an antenna analyzer.
He then mentioned a couple of models that covered the 220 MHz band.
Next, he forwarded the e-mail exchange to me and another club member, asking for any guidance that we might be able to give. The first thing that occurred to me, of course, is question T7C08 from the Technician Class question pool. It asks, “What instrument other than an SWR meter could you use to determine if a feed line and antenna are properly matched?”
The answer is a directional wattmeter. By making both forward and reflected power measurements, you can calculate the SWR with the following equation:
I suggested that they make these measurements with a Bird 43 wattmeter, with the appropriate plug-in. Bird even has an application note, “VSWR Measurements Using In-Line Power Meters” on how to do this. This application note not only discusses the theory behind this equation, but also some of the practical aspects of making this measurement, including how the accuracy of a power meter affects the uncertainty of SWR measurement.
This sounds like a very interesting application, and I’m interested in finding out more about it. It’s a good example of how an amateur radio license could be of use to young people interested in pursuing a career in science or technology, even if that career isn’t electrical engineering.
Jeff, WB4WXD, just e-mailed me with the following:
This looks really useful. I’ve just printed off one for myself.
TK5EP mentions that he created the slide rule with a program called Galva, designed by F5BU. Here’s the link:
http://www.f5bu.fr/wp/?page_id=13#Galva_EN
The program is free for non-commercial use (well, technically it’s postcard-ware), and looks pretty cool.
That does look like a cool program. Not only does it allow you to design circular slide rules like this, but also analog meter scales.
Bird has an SWR nomograph. It’s included in their manuals, and available as a plastic-covered standalone. I have one around someplace, and you can get ones from Webster (they show up at all the local SE MI/OH hamfests.
The Bird, though, may not be sensitive enough for fish tags, which are likely in the millwatt range. Kanga US is selling a milliwatt power meter kit, the W7ZOI:
http://www.kangaus.com/content/w7zoi-power-meter
SWR and power meters, though, won’t tell you what the mismatch is, in terms of complex impedance. If someone is trying to tune an antenna, it would be very helpful to know if it is capacitively or inductively reactive, or if it is only off by a resistive impedance, i.e. the antenna is resonant, but the feed point impedance is off. A resonant vertical monopole, for instance, is also never 50 ohms at resonance.
A small, handheld analyzer that covers the 220 MHz band that shows +-Rx, in addition to magnitude of impedance would be really useful. In addition, network analyzers have their own continuous wave signal source, which may be handier than trying to arrange to put an SWR or power meter inline between the fish tag and the antenna, and then arrange for the fish tag to transmit continuously long enough to get a proper reading.
I was also wondering how much power these transmitters output. I was guessing that they’d be higher power than millwatts. Such low power transmitters would be very difficult to recover, I’d guess.
http://www.f5bu.fr/wp/?page_id=13#Galva_EN
This link tried to put a virus on my computer and did turn off my internet.
After turning off the internet and back on it did ncome back.
THAT IS SORRY!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!