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Radio broadcast cache

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I'm not sure if this has been done before, but just an idea I thought I'd throw out there to see if it'd be allowed.


The idea is for someone to drive up to a certain (known) location of a mystery cache, and then through some clues they'll be asked to tune into a certain FM frequency on their radio, which will contain a clip reading out the real cache co-ordinates.


Would this sort of idea pass a review?

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I can't speak to whether it would pass a review or not, but I can tell you that it has been done. In the radio cache that I did, you drove to the radio location, where the coded coordinates for the next leg of the cache were being broadcast.


I have thought about placing a cache like this myself, but obtaining a decent radio transmitter was outside my budget. I did some experimenting with a cheap FM transmitter that was hooked to a discarded iPod set on loop. What I found was that the frequency drifted so much that it would be a real pain to tweak it every day or two. I also found that operating the transmitter from the house mains introduced a very loud hum in the broadcast. I put together a little filter circuit but was never able to overcome the hum. In my research I found that you'd have to spend around $100 US or more to get a PLL transmitter with a good power supply. That was more than I was prepared to spend for a cache. These kinds of transmitters are marketed to folks who do elaborate Christmas light displays that are synchronized to music. $100-200 would not be expensive for something like this.


I understand that some folks have used the talking home transmitters that were all the rage with real estate agents several years ago, but those units are extremely unreliable. I actually purchased one cheaply on eBay, but never got it to run more than a day or two. Another cacher I know has had the same experience with these.


BTW, the relevant FCC info you need is "Part 15." Google that and you'll get all the info you need about legal compliance.

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Cache 9 From Outer Space is my radio cache.


I use a lower power FM transmitter and an mp3 player. I have access to 110v power, so I used ac/dc wall-warts to power everything.


There's an FCC page on lower power transmitters. Big thing is to not broadcast on a frequency that is in use in your area, and not transmit on too high a power*.





* I am not a lawyer or telecom expert, so check the rules, regs and laws yourself.

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There's one exactly like this in Minneapolis. I won't put the cache name here because it could be a spoiler for anyone that hasn't found it (figuring out the frequency is part of the puzzle and yes, it's an Unknown cache type).


If you really want to know, PM me and I'll send you the cache link

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We had two near me in the Chicago suburbs for about two years from 2006-2008 (GCVP66 and GCT9EW). One was where a local celebrity on the WGN radio program caught the bug, and helped a local record the coordinates. The transmitter was in the house of the cache owner. All you had to do was drive up and listen on the frequency for a message from the celebrity - he would give the coordinates. The other one transmitted the coordinates in morse code.

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This is the transmitter I use. Decent ranger and clarity. I bout it assembled. I can solder, but for only a few bucks more, I'll let someone else do the hard work.


That is the one that I tried as well. YMMV, I guess! I know BBWolf had excellent results, but I didn't!


If I were to try again, I'd consider this one instead: http://www.canakit.com/pll-stereo-fm-transmitter-kit-ck555-uk555.html It has a crystal to prevent frequency drift, and it looks to have a better filtered power supply.

Edited by Mom-n-Andy
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I can't speak to whether it would pass a review or not, but I can tell you that it has been done. In the radio cache that I did, you drove to the radio location, where the coded coordinates for the next leg of the cache were being broadcast.



I know where Mom-n-Andy live, so we're probably talking about the same cache. It was archived in 2009. I'm sure this type of cache is still fine, and I think it's a great idea. Just kidding, by the way, I mean I know what City they live in, and the cache was about 25 miles South of there.

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I just set one up a little over a month ago and people seem to be enjoying it.


My cache: What's the Frequency, Kenneth?


I use one of the "Talking House" transmitters mentioned above. They are pretty cheap on eBay I paid about $80 total for two of them new in the box, but I've seen used ones go for even less. Mine has not had any problems so far running non-stop for over a month, but I have a spare if it craps out.


The FCC rules for unlicensed AM broadcasts are less strict than with FM - the "Talking House' transmitter has pretty decent range as a result - I've been able to hear the broadcast from several blocks away on a good day.


[Edit - didn't realize the talking house was already mentioned]

Edited by mattopia
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