Interesting Ideas: The “Passive Repeater”
While futzing around the Internet yesterday, I came across an entry from “Joe’s Hobby Electronics” blog. He had a problem with getting a cell phone signal in his home. According to Joe, his “…village is in a bit of a dip and the population density just isn’t really high enough to warrant better coverage.” According to his post, it is illegal to use cell phone repeaters in the UK (and while doing followup research, apparently most if not all of these repeaters are illegal in the US).
In an attempt to legally overcome his signal issue, Joe is in the process of setting up a “passive repeater” (he calls it a “Waveguide” but that term means something different in the US). The theory is simple — you put up a directional antenna aimed at the cell tower and another omnidirectional antenna in your home. Both antennas need to be tuned as close as possible to the target frequency and connected by the shortest possible length of the best possible feedline that you can get. Antenna tuning and quality feedline is critical because this system is entirely passive — there are no amplifiers or powered components in the system at all. Here’s how it works:
- The outside antenna is tuned to the exact frequency that you want to repeat (or as damned close as possible). If the antenna is resonant on that frequency, it will have the most efficiency possible.
- The outside antenna is connected to the inside antenna using the shortest possible length of high-quality low-loss feedline. Because you are re-transmitting received signals (which are very tiny) you want to lose as little of the signal as possible.
- The inside antenna is also resonant but it need not be directional as long as your device (in Joe’s case, a cellphone) is relatively close. That’s not saying it doesn’t need to be directional, but it depends on the application.
This system works both ways (as long as your receive and transmit frequencies are relatively close) and should work just about anywhere in the spectrum. An archived eHam forum thread about using a passive repeater to improve VHF/UHF handheld coverage in one’s home is an interesting read. Mark Brueggemann posts: “I had great success with a passive repeater years ago when I lived in a house with a basement. My workshop was down there and I couldn’t so much as hear the repeater, much less get into it with an HT. Using a coax switch in the hamshack I could connect my main station’s 2M yagi to a run of coax down into the basement, which was terminated with a wire quarter wave groundplane. The signal went from barely breaking squelch to full scale, and I could hit the repeater with the HT anywhere in the basement.”
When I put my UHF/VHF dual band antenna up I’ll do a simple experiment. From my home just northeast of Fort Worth I can’t hit the Dallas Amateur Radio Club repeater from inside my home. When the antenna goes up, I’ll put a half-wave 2 meter whip on the shack end of the coax and see if it is enough to improve the coverage of the HT.
(Published from DFW, Texas)