Optical RX schematic

Schematic of the G3XBM optical RX

Several people have asked for the schematic of the optical receiver used in the recent "over the horizon" red LED 481THz tests. Here it is. The BPW34 photodiode does not have any bias and is connected directly (in the air) to the gate of the FET. I have tried reverse biasing but it made little noticable difference to sensitivity. The cascode "head" is from KA7OEI and this is followed by an op-amp buffer and gain stage which feeds the PC. There is an additional stage of gain to drive a small crystal earpiece. I am sure the circuit can be further simplified and optimised but it has allowed me to copy a baseband scatter signal at 8.63km by ear and is as sensitive as any receiver I have yet tried.

Successful 481THz NLOS test (8.63km)

8.63km non line-of-sight reception of 481THz optical beacon

Bernie G4HJW set his high power optical beacon running a 1.082kHz tone this evening pointing in the Burwell direction on a non line-of-sight path from his home QTH at Little Wilberham. He then came over to Burwell to compare his RX and my own from a test site on the edge of the village clear of the village lights. Distance was 8.63km.

Both on Bernie's RX kit and my own we got solid copy by ear of the beacon at a distance of around 8.63km NLOS using 100mm optics. Once found, the signal was solid.  I then set up the optics on the tripod and fed the signal into Spectran where the signal was 30dB over noise in a 0.17Hz bandwidth. Listen here for a recording (best played back using Spectran with 0.17Hz bandwidth settings)

G4HJW's beacon TX

There was hardly a cloud in the sky, so this was largely by scatter from the mist/dust in the air. Bernie says signals have been stronger on other nights. Best reception was with the RX aimed just above the horizon, maybe 5-10 degrees at most above.

This was very gratifying as it is the furtherest NLOS signal I have copied and it verified my RX kit is now working at good sensitivity.

When I got back home I did try to copy the beacon by ear out of the bedroom window but the tree cover and lights made copy not possible. However I will try again later in the week with Spectran running and narrow bandwidth.

Failed cloudbounce 481THz optical test (7.6km)

Possible weak 481THz reception at 7.6km NLOS

This evening I tried a 7.6km non line-of-sight (NLOS) path between Burwell and Newmarket Heath with the 0.7W input red LED beacon running QRSS3 on an 820Hz subcarrier (at home) and modified KA7OEI receiver at the RX end, both with 100mm optics. My PC was running Spectran.

The best I can say is reception was very doubtful:  the screen capture above shows a signal at the correct subcarrier frequency in the right direction, but too weak to identify CW characters. The recent 3.6km NLOS test produced good 10dB S/N signals in 0.67Hz bandwidth - much easier.

With a largely clear sky, few clouds and a slight haze, not ideal conditions for cloudbounce testing. This test will be repeated when cloud cover is better and visibility is clearer. QRSS30 would give me another 10dB so this may be worth a go if QRSS3 doesn't work on a second try. There are several variables that make weak signal forward scatter detection difficult: (a) what vertical angle to use at both ends, especially with few clouds (b) amount and height of cloud cover, (c) horizontal direction of aim.  With QRSS3/30 modes it is a case of small aiming increments and wait to see if a signal appears on the screen. It is quite hard work.

The result is disappointing, but there will be plenty of other occasions to repeat this test and do others. All good  fun.

Some 160m AM QRP rig links

Looking around for some ideas for a local natter box, I came across these links for simple AM transmitters and receivers so far:

http://www.gameangler.eu/delboy/m0dad/construcion/poppet_top_band_am_transmitter.htm
http://www.angelfire.com/de/RadioAnarchy/
http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/1.65MHz_AM_QRP_TX.gif
http://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/AM_RX_Superheterodine_NE612.gif

If you are aware of other, simple, ideas for 160m QRP AM rigs please let me know.  I've still to go through my old SPRATs to see what is there and there have also been some ideas in Practical Wireless.

A 160m AM local natter rig

Further to my post a few days ago about possible bands to use for a very local "natter link" over a 3km path between G6ALB and here, my thoughts are turning to 160m, partly for nostalgic reasons. This is where I first started my ham radio operating with a small 400mW AM transmitter from Practical Wireless (Dec 67 I think).

Although 160m is VERY noisy now, especially at night, I figure that 100mW AM will be enough to kill the QRM/QRN over such a short distance. So, I am gathering ideas for something incredibly simple to build.

For the TX the most sensible rig would be a crystal oscillator (or ceramic oscillator) followed by a buffer and series modulated PA using the modulation circuit from the 2m AM Fredbox. On receive, either a regen receiver (operating just out of oscillation) or even an MK484 (like the ZN414) single chip receiver are worth a try.

This would be no DX rig, just a very basic transceiver to get me into the next village and not much further. I'll keep you posted. Incidentally I am unable to do much building right now as my house is in the middle of some redecoration. I can't find anything!

40m regen receiver kit from OK land

OK1CDJ's version of my simple 40m regen receiver

Ondra OK1CDJ has been in touch again recently and he mentioned that he had built a version of my 40m regenerative receiver and is now supplying a kit of parts in the Czech Republic via www.hamshop.cz. There are some nice photos of Ondra's version built into a small tin box.  My original circuit is at https://sites.google.com/site/g3xbmqrp/Home/regen although I admit I was inspired by W2UW who used this circuit before me in his amazing FET-1 transceiver.

Commercial QRP: KX3 or FT818?

CQ Ham Radio Book

As I've said before here,  I just can't understand why an FT817 successor has still not yet appeared. The KX3 pre-release spec (now nearly 12 months old) gives a good idea of what is needed and Yaesu surely could beat Elecraft on price. Also, Yaesu's alternative is likely to win on size and looks.

Here in the UK, once import duty and tax are added, the KX3 will be expensive, so I am still hoping the FT818 will appear soon and be a more cost effective alternative. I love Elecraft kit but not sure that too many over this side of the pond will shell out over £1000 for a commercial QRP radio. An FT818 at, say, £599 would be a more cost effective investment. I say investment as my FT817 is now 11 years old and been the best amateur radio investment I have ever made.

So, I am still on the look-out for some HARD news from Yaesu Japan i.e. specs, price and release dates. If there are any leaks I promise you that the news will appear on this blog as soon as I get wind of anything. And of course, unlike the KX3 I expect an FT818 would be available as soon as an announcement is made.

The image above is linked from http://www.cqpub.co.jp .

Optical cloudbouncing (stage 2)

With my QRSS3 success last weekend over a 3.6km non line-of-sight path at 481THz (red light) I've been looking at other possible NLOS cloudbounce paths that start from my home QTH where I can set up my 100mm optics 0.7W input beacon pointing out of a bedroom window.

There are several different paths out to around 8km away that will be tried shortly.  I need to do a quick check at the RX locations to see that there are no issues e.g. trees very close and that I can safely park the car off the road at night. The tests can't start for a few days as I have grandchildren with us over the weekend. Maybe the first test will be on Tuesday evening.

Simple local ragchewing rigs

The Sixbox 40mW AM transceiver for 6m. Maybe a Fourbox will follow?

The experiments last week by G6ALB and I confirm that to talk over a few kilometres, very little power is needed. Indeed, below 1mW is all that would be needed on any mode, at least on 144MHz. Andrew and I are now thinking about rigs that we can use to simply keep in touch whilst we get on with other QRP tests on bands from VLF to light. One idea is a 4m version of my Fredbox/Sixbox QRP AM transceivers to be called the Fourbox. This would be extremely simple to build and ideal for communication between a couple of stations in adjacent villages. Activity levels on 4m are not that great so we would be unlikely to suffer much interference issues if a super-regen receiver was used as on the original designs. A simple wire dipole in the loft would be fine for an antenna at each end. At 6m I was able to use T37-6 toroids, but at 70MHz I think these would be just beyond their intended design limits. Small airwound coils would probably be OK.

How little power do you need to communicate?

Last night G6ALB and I did a test on 2m to see how little power was needed to communicate between the two of us. Andrew is 3km from me. We both use V2000 triband vertical antennas. We started the test with Andrew using about 0.5mW pep from a simple DSB rig. We then switched to CW (from a decent low leakage signal generator) and G6ALB reduced power in 10dB steps. The lowest level I could still copy his signal was -55dBm which is around 1.8nW. The screenshot above, taken from Spectran, shows his signal at that level.

This sort of power level could be generated by power harvesting, even rectified RF from the HF antenna. Now there's a thought - a free power TX.