Sunday, August 14, 2011

J Street Radio 1610: How to Take a Simple Project Completely Over the Top

This is how to take a simple project completely over the top.
As mentioned in my last post, I built an AM transmitter that allowed me to play music on the old Zenith from an audio source like an mp3, CD, etc.  After some research I picked the SSTRAN 3000.  It took an evening to build the kit.  I’m very impressed with the audio.  It sounded like the commercial broadcast stations.  In all the research I did on transmitters I learned about FCC Part 15 regulations.   Part 15 allows anyone, without a broadcast license, to transmit on the AM or FM broadcast bands under certain restrictions.  It’s the same type of system used by drive-in theaters, road information stations and people who play Christmas music from their house to accompany their lights.  It turns out that there are a lot of “microbroadcasters” out there.  The transmitter instructions included a modification for a base-loaded antenna which gives the maximum amount of coverage while staying within the regulations.  I had most of the materials in my shop already, so what the hell.
Shannon White of BJ electric in Midland was kind enough to provide me with some 16ga magnet wire which was the most difficult part to find.  Everything else came from local hardware stores or my scrap pile.  I followed the instructions for the modified antenna shown on SSTRAN’s website and followed some hints I found from various other sites.  The result was a strong signal for about a half mile and some satisfactory signals as far as 1.25 miles. 
I put together some music with some station ID in between and called it “J Street Radio 1610.”  Tune in if you’re in the “Old Midland” area.  The best reception is between Garfield, Cuthbert, Neely and C Street.  Power lines seem to randomly help and harm the signal and I’m sure all the trees in the area aren’t helping.  I've heard the signal as far as Midland College and the hospital
I’ve read about other low-power stations and some in big cities have quite a cult following and run a mix of live and automated formats.  I really don’t want to invest too much time in this, but it’s an interesting pursuit.  It takes about 30 minutes at the computer to put together a loop of interesting materials and mix in an ID message here and there.
I don’t expect much to come of this but if nothing else I have a nice strong signal around my house where we can hear anything we want from an AM radio!  
I got a lot of good tips from other people’s blogs, so here are some notes for anyone looking into a radio project who finds this page on a search engine:
1.       My antenna was built from SSTRAN’s modified in-line coil plans.  The main antenna section is ¾” and the base is 1” copper pipe.  This is said to increase bandwith and it will certainly stand up to the west Texas wind better than the smaller stock.  I recommend cutting the clamping notches as wide as 1/8” with an abrasive wheel.  This made a much more solid connection than a hacksaw kerf.

2.       I used 13 random length radials to 15’.  I used scrap Romex and didn’t remove the insulation.  I did encounter a buzz when connected to the ground.  I’m eventually going to add ground rods for my ham radio antenna.  That may solve the problem.  The antenna base is about 10’ off the ground on top of my shop.

3.       When I adjusted the gain, modulation and compression EXACTLY as the manual says, my range increased.

4.       There’s little data about the voltages across T1 and T2.  All the manual says is to keep it under 13v.  With the wire antenna and even the base-loaded antenna UNGRONDED the voltage was 6v max.  When connected to a good ground, voltage went to 13 and the range shot out to what I described above.  Ground is everything! 
5.       Instead of soldering connectors to the audio and power, I soldered lead-outs for them.  It saved money and gives a better connection anyway.  Since I had plenty of lamp cord and 4-conductor telephone wire, all I was out was a 1.8” audio plug.

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