Archive for radio

House FM Transmitter

Posted in Electronics, house tech, technology with tags on February 20, 2011 by marksun

To get some sound around the house,  I bought   a HLLY CZH-05A FM transmitter  that seemed to have a practical and workable design with power to cover the whole house.  In fact at 500 mw power output, I was a little concerned about too much power… more on that later.   After a delay for Chinese New Years, it arrived.  No instructions. No matter.  I hooked it up and it worked great and did indeed cover the whole house.   I took the radio outside  and still crystal clear.  Then I walked out to the mailbox about 200 feet away and still had great sound.  I took a walk around the neighborhood and could still hear my radio station. So far so good as far as being able to hear it around the house.  However there are other considerations, like legality.

With a power rating of 500 mw (1/2 watt) the transmitter is not a high power blowtorch but not exactly a low power device either.  The FCC Part 15 regulation for low power FM band transmitters sets a field strength limit of 250 uV/m @ 3 meters.   Approximately 7 billion people, including myself,  don’t own field strength meters. Fortunately,  there seems to be a consensus on the Internet that this field strength corresponds to  a  range of about 200 feet.  From the standpoint of power,  FM transmitters  with 10-20 mw output are touted as FCC Part 15 compliant.

With my IPOD hooked up and the antenna extended about 30″, I did a range check using an FM frequency well removed from commercial statements to avoid interference.   I tuned the car radio  to the transmitter and drove up the hill in my neighborhood.  I could hear my radio transmitter at the top of the hill a mile from home.   I drove down the hill where the road crosses the freeway a mile away and still had a strong signal.  I kept on going and while there were places where the signal faded or became noisy,  it was still strong in some areas a couple of miles from home.   In the car over a mile  from home, the radio sounded pretty good,  comparable in quality and strength to  commercial FM stations, and louder than some of the weaker stations.

Next,  I range tested the system with the antenna collapsed to about 7″.   In this test, the short antenna effectively limited the range to less than 100 meters or an area of  maybe four football fields around the house.  Still, I would guess that this configuration is still of borderline legality.  Trouble is the transmitter gets hot.

What about not using an antenna?  Here the problem is that the antenna is part of the transmitter circuit.  Without one, there is no load to dissipate the transmitter power and the components of the transmitter overheat, possibly cooking the transmitter.

As far as legality in the US is concerned for an unlicensed FM transmitter, the antenna combined with the transmitter is the key.  To get low power output and minimal range from a radio transmitter, the requirement for an antenna  is to minimize radiation but still provide an electrical load for the transmitter output circuit.  How about  dummy loads,  used to test transmitters, and attach one to the antenna output connector in place of the real antenna.   It seemed like the place to start.  Dummy loads are very simple, especially for a low power transmitter.  In practice there is some radiation from a dummy load.  One could expect short range  without the transmitter overheating.   I went to Radio Shack, bought a packet of 100 ohm 1/2 watt carbon resisters and a connector (BNC) to construct a dummy load.  Two 100 ohm 1/2 watt  resistors in parallel equal 50 ohms and can dissipate 1 watt of power.  I soldered these across the leads of connector to make the load and attached the BNC connector to the antenna jack of the transmitter.

I’ve been using the transmitter like this for a few days and the dummy load does what it is supposed to do. The signal is clear only around the house and immediate vicinity.  Outside the house, the signal fades to noise within 100 feet or so.

The resistors get warm to the touch.  To help keep the temperature down, I glued on a small aluminum heat sink I had laying around, which soaks the heat from the resistors.

The transmitter itself is  a 500 mw HLLY CZH-05A.  The radio seems to be nicely made.  Sound is great.  HLLY manufactures FM transmitters with power output from 500 mw to 20 watts. The higher power (e.g. 5 watt)  HLLY transmitters have some notoriety on the internet as electromagnetic interference (EMI) emitters.

In the end, we have a super short range whole house FM radio station.  The sound is great and I’ve found a new use for FM radios.

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