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The Lowdown: Marconi’s Revenge N 42° 43.656 W 077° 55.368 “Marconi’s Revenge” is the debut of medicdave’s radio cache. The cache coordinates are for the location of the 2005 Geocaching Fall Gathering at Letchworth State Park on 2005-10-15. The only way to find the cache is to find the tiny radio transmitter that’s installed inside. On the morning of the event, a radio frequency (channel) will be clearly posted at the above coordinates. Tune your car’s radio to that channel, then drive South from the lodge where the event is located. As you approach the cache, you’ll hear a tone on the radio. Park when you begin hearing the tone, then tune in the same channel on a portable radio such as a walkman or hand-held police scanner. To locate the cache, hold the portable radio close to your chest and stand in one spot while turning, slowly, in a circle. You will hear a weak spot in the signal – at this point the cache is immediately behind you. After walking around and taking a few more readings, you’ll know roughly where the transmitter is. As you move closer, the signal may get too strong, and no weak spot may be heard as you rotate. If you are using a scanner with a detachable antenna, try removing the antenna and then taking more readings. FTF prize: TBD!

It’s been quite a while since I’ve even looked at working on the Radio Cache, aka “Marconi’s Revenge”. This was a project that I originally dreamed up in the spring of 2004. I bought the parts and started assembling things in early 2005, and the project stalled around March ’05.

Life became crowded amidst searching for a house, making future plans and starting a new job in July. As of this writing (October), the house has been purchased and will be moving in during the first week of November.

As I started going through my things and preparing to pack, I came across the box of parts for the Radio Cache. At the same time, the annual Geocaching Fall Gathering at Letchworth state park is fast approaching.

I looked at the amount of work still ahead to make the Radio Cache all that I originally envisioned, and I looked at my free time in which to work on it… And at the big gap between the two! With household projects looming, a new idea popped up: make the Radio Cache an event cache! If it doesn’t need to sit outside permanently, powered only by solar panels, then the elaborate control software I imagined isn’t necessary. If it only needs to work for a day or so at a time, it can simply be powered by batteries.

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So, without further ado, the Radio Cache will make its event debut at the 2005 Geocaching Fall Gathering at Letchworth!

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How it works

The cache transmits a very low power radio signal in the FM broadcast band. This signal can be picked up by any FM receiver – the radio in your car, a common “walkman” type portable radio or any other FM radio that you can find. Since the transmitter is so low in power, it is legal to use without a license from the FCC, and it is possible to locate using simple techniques.

Finding the radio cache requires using a common radio direction finding technique called “bodyblocking”. Bodyblocking requires a portable receiver, like a walkman, a handheld scanner or another battery-powered FM radio that you can carry easily. In order to locate the transmitter, simply stand in one spot and your body around while holding the radio close to your chest and listening to the cache’s signal.

As you turn, you will notice a “null” in the signal – a point where it gets weaker or disappears completely. This is because your body absorbs radio waves, so the null will appear at the point in your rotation when the cache is at your back. Normal radio stations have so much power that you don’t hear this effect. But the radio cache’s transmitter is weak enough to make the effect noticeable.

To locate the transmitter, you simply need to use the bodyblocking technique at several different spots where you can hear the cache’s signal. At each location, you will determine a direction toward the transmitter. After you’ve stopped and “bodyblocked” enough times, all the arrows will seem to be pointing toward the same place … the place where the cache is located!

You may get too close to the transmitter, in which case the signal doesn’t “null” as you turn your body. If you don’t have enough readings to locate the cache at this point, you’ll need to move away from it until you’re able to hear the signal get weaker again. Alternately, some folks (ham radio people especially) may have radios with detachable antennas. If you’re so close to the cache that the signal doesn’t drop out when you bodyblock, you can remove your antenna completely, or attach an attenuator between it and the radio in order to reduce the signal strength.

One more word of advice: Using a pair of headphones will help immensely!

How do I find it at Letchworth?

When I arrive for the day (probably around 8AM), I will place the radio cache and post the frequency (a.k.a. the “channel”) on which to hunt for it. Tune your car radio to this channel, then drive South, farther into Letchworth State Park. You’ll hear static, and perhaps some choppy noise from distant radio stations, until you’re within a few hundred yards of the cache. When you get close, you’ll hear a constant tone on the channel where the cache is transmitting. At this point, park and start bodyblocking with your walkman, handheld radio or scanner!