I've managed the installation of leaky feeder inside an underground tunnel system for a large particle accelerator. The leaky feeder we used had carefully sized windows cut in the solid copper shield of a coaxial cable which allowed 700MHz through 1.2GHz frequencies to pass in and out so we could get cell and P25 emergency radio signals into and out of the tunnels. A way better solution than individual antennas that rely on line of sight. Used the RF engineers at this company to design the system for me...they are absolutely the best... http://pbegrp.com/ The Jeeps will push air through the mine and as long as they keep moving they should be OK...but mine safety is a huge deal, and they should never be entered without first being monitored, inspected and cleared by trained personnel. It's sort of like cave diving...if you aren't trained as a cave diver you shouldn't be in the cave. Bonnie's cave radios are very cool...one day I'd like to give 1.8MHz a try inside a hydro plant penstock where our VHF radios are worthless once you're past line of sight.
Thank you for the excellent information. We are going to be going back at some point. I'd like to implement all these items and show everyone how it all works. Thanks for the great excellent instructions.
Hi Doug, here's a pic a couple hundred feet off the main hwy. We kept eating at a place called Alabama Hills Cafe. Great Eggs Benedict!
Yes, from the info we've been getting on the comments we will be doing all the above, air quality monitoring, cave radio, Leaky feeder antenna and all. THanks for the extensive feedback.
RE leaky feeder...that is a permanent installation...in other words, it becomes part of the mine infrastructure, hung throughout with fixed amps, repeaters, filters and duplexers.
I did not realize this was a fixed antenna feed line. Isn't this more of a telegraph line than an antenna. Or is it a true antenna?
Leaky feeder is a lot of things...but it isn't a telegraph line ;-) It's basically a coaxial cable that is designed to radiate and receive very specific RF wavelengths via the specific size of the slots cut in the outer solid copper shield. We then used standard coax to carry the signal up a shaft and out to multi-element Yagis pointed at AT&T, Verizon and P25 towers. We worked very closely with an independent facilitator to make sure our system didn't interfere with the provider's cell signals. We also had to shield some RF sources in the synchrotron that happened to be radiating very closely to the frequencies we were targeting with the leaky feeder (klystrons if I remember correctly). Here's a very topical introduction: https://en.wikipedia.org/wiki/Leaky_feeder BTW...standard, non-leaky coax can also be purposely made to radiate...the vertical feed on my OCF Windom between the balun and the line isolator (common mode current choke) does this purposely, which gives the OCF both horizontal and omni radiation properties. Nice ad-hoc projection screen!
Doug, you should be able to view the pics from others that went to Alabama Hills, here https://www.meetup.com/LandOps/photos/27152366/ that's a photo gallery. Of course you will see ham radio antennas, mesh equipment, but the Alabama Hills just outside Lone Pine was wonderful. Let me know if you can get into the link. It's a meetup.com gallery.
The leaky feeder then is a cable system with portion of the copper shielding cut and removed to allow RF signals to escape, not something that cables were intended to do, but apparently must be done properly in order to work. I'm going to follow the link provided in the comments to review this for our next deep mine exploration.
Hi KO6KL, I don't know of any 185 kHz SSB radios on the market right now. My 185 kHz LowFER transceivers are homebrewed. Although it is not the same as the units I built, here is a published article on how to build a LowFER radio. -Bonnie KQ6XA FCC Rules permitting LowFER Operations: Sec. 15.217 Operation in the band 160-190 kHz. (a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed one watt. (b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters. (c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.
In caves, I found that 2 watts at 7 MHz or 3.9 MHz SSB, with small whip antennas, will go through about a quarter mile to a half-mile of solid rock. Further if one station uses a longer antenna.
Now that's a little more practical sounding than prewiring the leaky feeder with precision RF escape cuts. I'll recommend the teams give this a try. We're going back to reward mine in May. The end of the mine is 1/3 mile from the entrance with about a 500 foot elevation change.
