Using LEDs at RF

[HUGH]

I'm familiar with using LEDs as RF indicators with a diode as rectifier and a capacitor to filter. On thinking of fitting LEDs for forward and reflected current to my little QRP (5W) ATU I found at least 2 circuits that used the RF direct so gave it a try.

It doesn't work as hoped and the LEDs are dim, becoming progressively dimmer as frequency increases. I have used LEDs on ac mains with a suitable series resistor but, without a diode rectifier, their lifespan is much reduced.

Anyone looked at this direct coupling idea?

[HUGH]

I'll sort of answer my own question: There must be some newer LEDs that work OK at RF but I just added two 1N4148 diodes and two 10nF capacitors and it works. I should know by now not to assume every circuit on the internet will work.

[N8YX]

The issue is one of power density and duty cycle.

Think of DC as being 100%. For a constant applied RF power in watts, density will decrease as frequency increases. DC Pulse Width Modulation varies the brightness of LEDs via the same effect.

As per your post above, what you need is a combination integrator and limiter circuit. A couple of large, low ESR electolytics can perform the integration function and you can current-limit the LEDs via resistors.

[hamguy]

I would suspect the capacitance of the LED as being a more important factor in the decrease in brightness at higher frequencies. I would guess typical LEDs are have more C across the junction than one might first imagine. Of course, as noted, rectifying the RF with a couple of 1N4148s eliminates the effect of the C of the LED junction.