House FM Transmitter

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.


7 Responses to “House FM Transmitter”

  1. marksun Says:

    The transmitter I find doesn’t always power up on the preset frequency. So far my main complaint. I’ve also been using the transmitter with the original antenna collapsed to get a better signal in the house. Maybe this isn’t good for it? Does run a little warm, which means something in the box is getting really warm.

    • marksun Says:

      Radio got stuck on 153mhz. Disconnected entirely from power for a few minutes did nothing, but then multiple power cycles reset it to normal.

      I also have noticed a current on the chassis when I touch it with my finger – not a shock, but a vibration. This turns out to be a 45 vac leakage from the power supply on the +12v, and 25 vac on -12v. I’ve read that some voltage regulators can oscillate causing problems like this. A post on the 7809 for example mentions that for this particular IC the data sheet for the 7809 suggests 0.33 µF from the input to ground and at least 0.1 µF from the output to ground. And the ground lead should be as short as possible. So maybe it’s something like this. No idea if this is the problem with the transmitter losing it’s default startup frequency.

      Trying another 12v power brick that does not seem to have this problem.

      • Sep 17 2011 – stuck again at 153.5 mhz. It had been resetting to 108.0 mhz and not remembering its previous frq for some time. So far have not been able to get it to reset. Are off the air?

  2. GARY O'DATA Says:


  3. Given the performance I see with my HLLY, I figure it is not legal in the U.S. unless the power is reduced.

  4. I thought the HLLY got fried after being stuck at 153 mhz, but after a year in the junk box, I connected it up and it is working again. In retrospect, the single resistor dummy load seemed to result in overheating so that approach to limiting power is out. Seems like it should have worked but while I look into that, I found another and better idea to cap power.

    Fri – I made a 10db pi attenuator circuit to go between the transmitter and antenna. The antenna is fully extended for impedance matching. The result is a reduced range of around 200 feet, and a usable range with great audio of 50 feet. A 10db attenuator cuts the 500mw of power to 50mw. The transmitter remains cool, a good indicator. So far so good, another 6db would cut power down to 25mw, while adding another 10db circuit would take it down to 5mw.

    Sat – In startup, the transmitter came up at 153 mhz but a power cycle or two got it back to 108mhz and working fine.

  5. In the past couple years I learned a bit more about radio transmitters, so thought I’d update. On a whim, I plugged the HLLY in again today – it came up and stuck at 108 mhz. Then after a while and for no particular reason it got unstuck showing 107 mhz. The tuning started to work and it is now transmitting ok in an empty channel.

    As far as emissions go, the power of the transmitter is not the issue. The issue is effective radiated power, so if you use a resistive dummy load like I did a few years back when I started playing around with the HLLY or use attenuation with a lousy antenna, the power will be low and likely legal. For a low emission system, nothing wrong with a resistive dummy load. Just dissipate the heat and you’re good.

    I forgot about the HLLY – I still need to get the TV audio around the house maybe we’re not done yet. It seems to have taken a little while to “warm up”… why that should be a factor who knows – stability seemed to be a long term concern with the HLLY.

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