A Homebrew RF Detector For Pennies - Hat Tip AD5X

Like many amateur radio operators, I'm always interested to learn more about antennas.

For some time, I've been wondering what happens when I send a signal down to my lower corner-fed, vertically polarised delta loop for 20m.  Does it, in fact, radiate vertical waves?  What is the pattern of the radiation?  How true are those internet claims that say corner feeding is 'bad' compared to a 1/4 wave down from the apex? 

Sadly, I'm not an electronics whizz, so I had to rely on the excellent, clear work of AD5X, who has published his circuit online, but is intermittently unavailable.  Luckily, an interrogation of the web archive pulled up the document so you can see a copy below, which is reproduced with Phil, AD5X's permission (6/1/12).  The wiring schematic is fine, but the list of parts omits the capacitor, so make sure you include that!  Also, given the ubiquity of high-intensity LEDs in hand torches, garden solar lights and so on, you'll probably find plenty lying around in broken products that you can recycle.
RF Detector RevA

I spent 30 minutes putting this thing together after sourcing all the materials on e-bay for about £10 altogether.  Even for an electronics ignoramus like me, this very simple box of tricks worked the first time I took it out, and it's surprisingly sensitive - it picks up RF from a 5W input from at least 20 metres away, and a stronger input is obviously detectable from a much greater distance than that - very useful for radiation pattern determination.

On the first night of testing, the little LED lit up to show me where the RF was going.  In keeping with a subjective assessment of the antenna's outstanding performance on SSB and PSK, and an assessment using WSPR, I found with the RF detector that the radiation is strong even at very low angles indeed.  What's more, holding the detecting antennas horizontally revealed only the merest weak hint of horizontal waves in certain locations, and holding them vertically showed a beautifully strong, omnidirectional vertical field - all very much in keeping with modelling.

Heavily metal-laden ground at my QTH makes it very difficult, but interesting to model antenna performance.

So, corner feeding cannot, on the evidence of contacts made and now the RF detector, be consequentially deleterious to the vertical radiation, because the field is physically shown to contain essentially no horizontal component.  Models with a 1/4-wave-down-from-apex-feed arrangement shows an identical, weak horizontal element to the field, at about -27dB.  If I move my detector about 2 m from the antenna, that horizontal component is not detectable, so that's nicely in agreement with the numerical value.

But unlike the model, the LED lights up strongly at slightly negative angles - about 4 degrees - to the base wire of the delta loop (we live on a ridge about 65m above the surroundings).  I have since replaced the LED with a 50uA FSD ammeter, to give at least a relative numerical indication of field strength, if one can see the meter when the box is held aloft.  A pot is also required to adjust the amazing sensitivity of the circuit.