# COMMUNITY : FORUMS : BURNING QUESTIONS

## An Transmitted RF-powered LED WARNING light?

The Situation:
I am a disabled Veteran with too much time to fill.  So, I volunteer at the Sheriff's Office doing light clerical work and a little target shooting, but my first love is HAM radio.  It is a GREAT hobby!  It is not without its hazards, though.  My main transmitting antenna is a ground-mounted vertical in the back yard.  We have six children with four still at home and while my kids know to NEVER touch my antennas due to the danger of electrical shock and nasty RF burns, their friends in the neighborhood may not fully comprehend the hazard.

The Challenge (if you choose to accept it):
So, what I'd like to do is to have a circuit out on the post supporting the antenna that would light LEDs, (preferably, four), whenever I key the mike - AND - have the entire circuit use the power from my own transmission... no power supply or batteries!   It would also simply be a cool thing to have!

The Specs:
This is a vertical for the HF bands, from 7MHz to 30MHz and I typically transmit at 100 watts.  The circuit would be at a distance of no more than six inches from the active element.  I have seen crystal radio circuits in the past that used received RF to power a LED but don't remember where.

sort by: active | newest | oldest
tiger125064 years ago
Mmmm... Let's do a simple sanity check.

Typical red LED forward voltage: 2V
Typical red LED forward current: 20 mA

X 4 = 80 mA

Power needed = 2V * 80 mA = 160 mW

You claim that you are broadcasting at 100 W.

So a (terrible) but simple estimate of the surface area needed for a pickup coil at 6 inches...

100 W 160 mW
---------------- = ---------------
4*pi*(6 in)^2 4*pi*r^2

Solving for the missing radius results in a (bad) estimate of a quarter inch loop.
But then again, all estimates in RF are bad, IMHO. Some of the things that make this bad are assuming orthogonal full field of a Hertzian dipole, which just isn't real.

However, such a small figure (of a quarter inch) makes me think that just trying something will quickly give you an empirically discovered circuit.

Try this: Make a loop of wire a couple inches in diameter, wrap it around 30 turns or so, and connect the ends to an LED through a 1k resistor.

You might be pleasantly surprised to find that works. Or not. If not, try lower resistance, more turns, greater diameter of wire coil, in that order.

Cheers.

Cyberscann544 years ago
a wireless warning light is easy with the power of the antenna should have no issue getting power within 20ft I would also think of adding a capacitive discharge sensor with an alarm to go off when people get close to the antennas with the help of this site hackaday and google you should have no issue figuring out how to do it
Wouldn't you try to fence it off and have warning signs? You might want to post a pic of your antenna, is it the guy wires that need to be isolated so they don't touch it or is your antenna like a zip line the kids just want to jump on? How low to to the ground do you have it? Stealthy so the homeowner's association doesn't jump on you? You open yourself up to a big liability by mounting it without restricted access.
4 years ago
+1

I think the rule of KIS applies here. Keep It Simple! Build a fence around the antenna tower and be done with it. If the kids are likely to touch it or play too close to it despite your warnings then what is a set of LEDs going to do? Its a great idea but put up a fence first.
Bama-Bull (author)  mpilchfamily4 years ago
It is getting a fence next week - as soon as Lowe's gets the timbers here - as that is the best way to go,  However, I still want to pursue the RF powered transmission light.  Most of my radio time is at night and it would be a little kick to see the LED fire up when I key the mike knowing it is powered only from the RF from my antenna.

The antenna, or rather, 'antennas', are free standing verticals with NO guy lines at all.  One is a Hustler 4BTV multi-trap for the 10, 15, 20, and 40 Meter bands while the other is a home-made, copper-pipe vertical for the 17 Meter band only. (Though it is a real performer on 12M and 15M as well. I should post the description of how it was made.)  The home made one was first and served alone for a while.  Now they are mounted on opposite sides of the same 4x4 inch post and fed from a common point.  There is a 10 inch separation between the antennas with the 4x4 between them,  Oh, they are mounted 8 inches from ground level.  There is no HOA here to deal with here at all, (I'm a lucky fellow in that respect!)

If anyone can propose a circuit of the light idea I'll give it a go!
4 years ago
I don't know if this would help. I once had a friend (Snowman) who attached a small flourescent lamp to his car cb antenna. At normal output it would not fire. But if he kicked his linear amp in it would light right up. As I recall, he could shut down the linear and the light would stay lit. No wires - No power.
Kiteman4 years ago
That's a great idea, but LEDs are often hard to see in daylight.

Maybe have it triggered by RF, but with more or brighter LEDs powered from a rechargeable battery topped up by a solar cell?