Reflex Receivers

In the days when semiconductors were very expensive parts, the reflex receiver was a very popular design for AM broadcast and simple amateur AM receivers. The basic idea is to use a single transistor is several different ways: as an RF amplifier, possibly a detector and then finally as an audio amplifier by clever feeding of the RF and AF signal back around the transistor. Even today they are fun circuits to try. For example, see the schematic at http://www.ke3ij.com/reflex.htm . Rick has a number of novel circuits on his site.

http://www.ke3ij.com/reflex.gif

In Rick's circuit, Q1 first acts as an RF amplifier. The 1N4148 rectifies the signal which is again applied to the base of Q1, which is now used as an AF amplifier stage. Although adaquate level is available on the collector of Q1 to drive a high impedance earpiece, Rick added another low cost stage to drive an 8 ohm speaker.

Another example of a reflex circuit is at http://www.sm0vpo.com/rx/reflex_rx.htm . Perhaps the most famous reflex circuits came from Sir Douglas Hall in a series of circuits in Radio Constructor back in the 1960s and early 1970s. Some of these are collected at http://www.spontaflex.free-online.co.uk/ . Sir Douglas was a UK overseas diplomat and a very nice gentleman. One of my old school friends lived in the same Devon village and visited him to try some of his prototypes.

Speech processors

Earlier today someone asked me to forward them some links I had on speech processors. This reminded me of a VERY effective speech clipper I built many years ago for a 2m AM QRP transmitter. The audio pre-amp had 2 stages of 6dB/octave pre-emphasis followed by very hard clipping and an active low pass filter to keep >2.5kHz audio levels to very low levels. The signal sounded remarkably punchy and allowed this little 100mW AM transmitter to punch well above its weight. It sounded like a rig running several watts. At some point, after we move, I'd like to replicate the design and give it a go again. It was based around the audio stages I used in the Pye PF8 PMR handheld but with value changes to increase the gain and add the second pre-emphasis stage. Double pre-emphasis meant that there was less clipping at lower audio frequencies and more at the higher end. Compressors are meant to be better, but honestly this little circuit took some beating.

Satellite tracking app (free)

The always interesting Southgate ARS News page has a link to a free app by Tom W9KE for iPhones, iPads and iPod Touch devices to allow tracking in real time of satellite passes. The app has a nice user interface and looks to be extremely useful if you enjoy tracking satellites and the ISS as well as want to work through some of the amateur radio satellites up there.  The App is available from https://itunes.apple.com/us/app/satellite-explorer-pro/id669039200. The only thing the app seems to miss is a list of the amateur satellite frequencies (up/down) and their modes of operation. I guess one has to look this up somewhere else. 

Thunderstorms and Es

Sporadic-E (Es) is an exciting propagation mechanism that allows contacts on the HF and VHF bands out to around 2000km regularly (with multi-hop Es MUCH greater ranges can be covered) in the April-Sept period in the Northern Hemisphere. There is another smaller peak around Dec/Jan too and propagation by Es can occur at others times of year, but much less frequently. Exactly what creates good Es conditions is open to debate still but some believe thunderstorm activity in the troposphere can impact it.

http://images.blitzortung.org/

A way of checking on thunderstorm activity is to look at the map at http://www.blitzortung.org/Webpages/index.php?lang=en .  Right now there are plenty of thunderstorms over northern Europe with lots over the UK earlier, spreading northwards. Looking at the map one might expect good conditions towards Italy and Greece on Es if whatever rises from the tropo region reaches the E layer mid-path.

My internet saga continues - positive outcome on cable at new QTH

Yesterday I reported that at the new QTH our new home was the only one on the close that was unable to get fibre optic broadband, phone and TV services from the provider I currently have used for over 12 years: Virgin Media.

Well, today their field engineer surveyed the road and has confirmed that we WILL be able to get cable services at our new place after all. They were very quick checking this - I only asked yesterday - and it looks like everything will be ready for when we move mid-August.

KX3 purchase?

For the last 6 months I have been saving the pennies in my amateur radio kitty towards the purchase of a new transceiver for general use at the new QTH. I am still very tempted to splash out on a new KX3 from Elecraft, despite the high UK cost when fully loaded with auto-ATU, narrow roofing filter, battery pack, charger and mic. As I tend to hang on to gear for a LONG time, this may be a good investment. All the www.eHam.com reviews are very good and it does appear to be a first class radio. What bothers me though is that for the same price I could buy 2 FT817 transceivers and still have change! My main interest is e weak signal mode experimentation and I am not sure that the KX3 is an ideal radio for use as a base rig for this application: there seems to be a need for lots of cables to connect to a PC, whereas the FT817 is easy using a Signalink USB interface.

As was the case when pocket calculators came on the market in the 1970s, the default position here is to "wait and see", which means I buy nothing and hope that a clear winner becomes obvious in the months ahead. In the 1970s I ended up not buying a scientific calculator at all, HI.

Ending operations at this QTH

Nearly the end of an era (37 years) at this QTH. I will resist taking down the antennas until the last moment in a few weeks from now, but already my mind is turning to the new QTH.

Would you believe it, but every house in Mill Close, Burwell is able to get Virgin Media apart from ours! Even next door they have it.  I contacted Virgin Media who have agreed to send out a field engineer to check why this is. The CATV box on the pavement is about 4m from our drive. It is not the end of the world: I can revert to Sky (or BT) phones, TV and broadband and get over 10M download speeds but I will wait until the VM engineer feeds back his findings as it would be preferable to stick with our current provider. Virgin's broadband is VERY reliable and fast.

I now have a firm plan on where to erect the VHF/UHF antennas. The earth electrode antenna for VLF/LF/MF will be installed in the next few weeks. I have still to decide on the location of HF wire antennas in a way that is "sleuth" i.e. low profile and not obvious. Our new patio is right outside my new shack, so operating in the garden should be very easy and fun.

Going batty

We are spending a few days with our son and family near Canterbury where he lives in an 18th century cottage not far from the school where he teaches. In the last few months their home has become an hotel for a colony of tiny pipistrelle bats who have taken up residence under the centuries old tiles on the side of the cottage. A bat expert estimated there are at least 300 living there.

This evening, as the light faded, we watched as they  flew out from what appeared like impossibly small gaps in the tiles. Bats are a protected species, so they are not allowed to get rid of them, but apart from their tiny droppings they are not a nuisance. It was quite amazing watching them fly off for food this evening.

IC-7100 UK price: £1249.99 from Martin Lynch

The IC-7100 multi-mode multi-band 100W radio is now available for £1249.99 including VAT from Martin Lynch. Although not yet in stock this is the price advertised, which is some £500 less than I was expecting. Considering it includes all the HF bands, 6, 4, 2 and 70cms this is not bad value. I just hope the touch screen is not tacky and unreliable. See http://www.hamradio.co.uk/amateur-radio-main-equipment-mobile-radio-icom-mobile-radio/icom/icom-ic-7100-pd-5020.php .

New VLF tests from Germany

News from Germany of an upcoming VLF test at 8.970kHz using earth electrodes as the TX antenna with an ultra-stable Spectrum Lab derived signal.

Hello Group,

 

We are planning some VLF-activity at 8970Hz during a meeting of  electronic hobbyists nearby  Kassel / Germany (JO40vr) on this weekend.  The plan is to run a VLF-PA between some ground electrodes. Due to other activities we have no exact time plan, when to start. Depending on internet access i will inform you about the details. 

 

Thanks for looking for our signal in advance.

 

73s

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