The parts are in and the testing has begun. I'm posting preliminary information to give others a chance to chime in. So, this all began with the realization that my FM Transmitter was probably a little too powerful. I've had it for years, way before I got into the lights hobby. I only used it once or twice a year for outdoor movies and for that, it was only on for about 2hrs. Now, with the lights, it going to be on for hours everyday for a couple of months. With a signal as strong as it is I started to think about attenuation. After doing some research, I figured I would try to make my own attenuator vs. just buying a pre-made one, which by the way, seem really expensive for what they are.
My plan was to try different attenuators and just walk/drive around getting basic range measurements. Nothing fancy, I can't afford the test equipment. Here is a page to help with the calculations (
https://daycounter.com/Calculators/). Under the "RF" section is the PI and T-pad Attenuator Calculator. Putting in various dB values and using 50 ohms for characteristic impedance (that's my antennae), I got a bunch of resistor values to use. Limited by my supply and what the local electronics store had, I can make 5, 10 and 25 dB attenuators.
Parts:
20" TNC Male to TNC Female nut bulkhead RG58 cable (ebay)
HAMMOND 1411B ENCLOSURE, UTILITY, ALUMINIUM, GRAY (local store) (found on amazon, but pricey)
1 Watt resistors (my transmitter is .5W max, so doubled for safety)
1/2" Hole for bulkhead connector
5/32" Hole for antennae cable
Finished design with antennae attached.
Yes there is a cover, but then you wouldn't be able to see that awesome electronic work inside.
Final (semi-final) design
Pi Attenutaor design, currently set for 25dB
Ground braid is attached to bare metal of box to prevent stray signals
Not fancy, but it does actually work
Still need to attach zip-ties for cable strain relief
What I've found in my design is that I cut the cord on the bulkhead side a little too short. Unfortunately, when inserting the antennae, it pushes the female connector into the cable and doesn't make a good connection. There isn't enough friction to keep the inner conductor in place, oops. Careful pliers work allowed me to push the conductor and connector back out. Certainly, not built for constant dis/connect use.
Results so far:
My transmitter, on low setting, is set for .1W and that gets me about 1/2 mile range on a good day. Not the greatest signal at that range, but the radio will still pickup spots and reflections. First, I tried a 5dB setup but that didn't seem to have any effect. I believe weather and atmospheric conditions actually made the signal go further, approx 2000ft+. Upto about 2500ft+ the signal was spotty static but still, barely, present. Next, I tried the other end of the spectrum and went straight to 25dB. Great results as far as knocking the signal down. I went from 2000ft+ to about 300ft. At about 200 ft, I start to get static and at 400+ I lose the signal. This is actually what I was looking for but, the downfall is that the signal might be just a little too weak. Driving up and down my street, things like my mail box or the tree in front of my house causes static. That's only about 100ft away. Un-obstructed signals are actually quite clear. I have parts to make a 10dB but, I think that's not enough, so I'm going to order some resistors and try to find a "happy" medium. I'm thinking the 15dB - 20dB range will work for me. In this experiment, I'm not going for full on regulatory standards, but I'm not looking to draw unwanted attention either by having my own broadcast radio station. My philosophy is "out of sight, out of mind." Don't think that I'm some sort of electrical engineer, believe me, I'm not. I just like to experiment, play, break things and try to learn something along the way. So anything you do is your own liability to you and your equipment. I just hope that this will help contribute to the Christmas lights hobby, keep everybody safe and legal (to a certain extent) and allow this hobby to thrive for many years.