by Paul Ormandy on 24-08-2004  at Media Network

Receivers

O.K, so it's a pretty essential piece of equipment, but what do you need to make all the effort you've put into antennas, finding a location, extra equipment, time off work and away from loved ones worth the while? A receiver that will maximise your chances of hearing those weak, distant signals.

There are numerous figures and specifications provided by manufacturers and test panels which will point you in the right direction towards a DX dream-machine. If were to purchase a receiver for the prime intention of MW DX, the factors I'd be most interested in are:

• Selectivity
  In the crowded MW band where stations are separated by as little as 1kHz, the ability of the receiver to discriminate between adjacent stations and to provide loggable audio is essential. What's more the width of the filter's skirt is as important as its quoted width and is often determined by the type of material the filter is manufactured from. For example, ceramic filters have very wide skirts allowing interference to ingress whereas mechanical or crystal filters provide very sharp skirts. Look for a receiver with a filter of around 2.5kHz at -6dB and a skirt width of less than 5kHz at -60dB (the narrower the -60dB figure in relation to the -6db the better - don't worry too much what that means, just use it as a point of comparison between receivers).
• If you're tossing up options at purchase time, between a VHF converter, external speaker or a sharper filter, take the latter!
• Sensitivity
  Some receivers aren't designed to apply all of their features to MW and sensitivity is often affected by internal attenuation to prevent strong local signals over-loading the receiver. In many receivers the attenuation can be readily set to zero by a front-panel control, or a simple software fix. In others, (e.g. Kenwood R-5000) it's a soldering iron job which, given the complexity of modern receivers, not all will be keen to tackle.
• Preamps
  Generally useful devices that may be handy for giving a weak signal that extra nudge. Pre-amps that do not degrade the signal-to-noise ratio are extremely useful, though often they have been disabled on MW or require a user software fix to be enabled. Check to see if they will work on the MW band or can be adapted to do so.
• Memories
  Some may consider having a receiver with 400 memories as more than sufficient, though for the MW DXer, the ability to program in every MW DX channel in the best mode, with the optimum filter setting etc. is a real bonus. This allows swift tuning between channels which in a strong opening is very handy for analysing the best frequency(s) to monitor.
• Noise Floor
  And there's little point in erecting long antennas, spending heaps on coax, preamplifiers, tuners, baluns etc, if the weak signals you're chasing can't be heard under the receivers internal noise! A simple test to see how noisy a receiver is, remove the antenna and turn the volume right up - should be very quiet, a low-level background noise, ideally the noise level would be near zero and you would hardly notice the volume had been increased. I'm fortunate to have an ultra-quiet 25 year old Drake SPR-4 that has allowed reception of weak signals at loggable levels, which have been buried on colleagues' receivers.
• Hash
  Modern receivers with all their synthesisers, microprocessors and fluorescent displays can produce a fair amount of internal noise.  This can effect the use of indoor loop antennas near the receiver, as they'll pick up the noise radiated from the electronics. Another simple test is to hold a transistor radio about 50 cm from the set and see how much hash it picks up.
• Operating Voltage/Current Consumption
  If battery operation is required (for example when running mobile or on DXpeditions) the amount of power drawn by a receiver will dictate how long a battery will last (and if you can start the car after a nights DX!). The Drake R-8A uses 2 amps when running (i.e. 30 hours operation on a fully charged car battery) though the drop in voltage will see the set turning itself off well before the 30 hours are up and 1 amp switched off on the front panel. The consequences of high battery drain mean that you'll need to be prepared for long stints at the dials by bringing extra batteries or charging between uses.
  Most receivers are designed to work off 12 or 13.8 volts DC though the AOR 7030+ prefers 15 volts for optimum performance, although it functions very well at 12 volts.
• Antenna Switches
  Most receivers come equipped with the facility for 1 x 50 ohm input and 1 x 600 ohm input antenna with front-panel switching. I’d prefer to see at least 3 x 50 ohm inputs to allow ready access to a range of antennas. Strangely, the AOR7030+ doesn’t have an antenna switch  so an external device is necessary.

Best Receivers

If you're looking for a new receiver, there are a number of models currently available that are hailed by DXers as good MW DX machines. Sadly I'm not in the position of having personally used all of these sets, with the exception of the Drake SPR-4 (which I've owned for 15 years), R-8, R-8A and AOR-7030+, though general consensus would be to seriously consider the following:

Drake R8-B Japan Radio NRD-545 AOR 7030+ For 'pre-loved' units, look for: Drake SPR-4, R4B, or R4C Icom ICR-71 Drake R8 Drake R8A Japan Radio NRD-535 Watkins Johnson HF-1000 (if price is no object!) And if mains operation is possible Collins R390A >> users.erols.com/eengineer/new390A.html Racal RA-17 or RA-117 >> recelectronics.demon.co.uk

Another option worth exploring with many radios to make them really top performing sets is to purchase one modified by sherweng dot com/ radios.html >> Sherwood Engineering For not a lot more than the purchase price, Sherwood can 'hot-rod' several of the above.

Headphones

There are several types of headphone worth considering. Headphones manufactured with the hi-fi market in mind reproduce high levels of bass and treble. They give excellent frequency response and provide pleasant listening to high-quality music. Great for music and often suitable for DXing, though there is another option.

A MW DXer generally isn't too interested in high-fidelity, the frequencies covered by the spoken world are more crucial and there are several headphones that emphasise that frequency range. These are manufactured for the amateur radio and DX fraternity by the likes of Kenwood and Icom. They reduce the amount of bass and therefore rumble and reduce the amount of treble, which will soften heterodyne whistles and reduce hiss.

If you are DXing with others, and particularly on a DXpedition, you'll need a pair of enclosed muff phones to prevent what you're listening to annoying others. There are noise-cancelling headphones which form an electronic noise-barrier to prevent external noise affecting your listening. Sony is one manufacturer of these surprisingly effective devices.

Audio Recorders

It's also important that you have a good tape recorder to capture as many of those often fleeting moments of reception as possible. Some DXers leave the recorder running non-stop, others hit the record button as soon as something interesting pops up. 

And recording media now comes in many forms and prices.

Audio Cassette Recorder. No doubt the cheapest option, though a recorder with external line-in (or microphone socket), tape counter and external DC power connection are handy. These range from el-cheapo to the broadcast quality versions used by media journalists.
DAT. Digital Audio Tape offers one of the highest quality portable tape-based recorders. The tapes and recorders are considerably dearer than audio cassettes but this is offset by the wide frequency response and dynamic range
Mini-Disc. Another portable solution using a small CD and also expensive. What’s more, some mini-disc models radiate a fair amount of hash and aren’t suitable in all situations.
MP3. The new kid on the block. No tape or disk is required as the audio is recorded onto a computer chip. These are still in their infancy and the amount of time available for recording is relatively short though I'd expect these units to become one of the tools of the DX-trade before too long.

A little trick when using stereo recorders. Feed the audio from your main receiver into one channel and audio from a second receiver tuned to WWV into the other. That way you’ll be able to have a time-base accessible by playing the tape back through an amplifier with a balance control. (Thanks to Andy Gardner!)

Accessories

There are other items that the MW DXer may find handy in the pursuit of weak signals, particularly on DXpedition when accompanied by colleagues.

When using modern sets with low-impedance inputs connected to high impedance antennas (e.g. Beverages) a balun is required to maximise signal transfer between the two. Baluns are best installed some distance from the receivers and fed with coax to prevent interaction. It is also a good idea to keep antennas separate as they approach the listening site (e,g, not anchored on the same pole) for the same reason.

When you're fortunate enough to have a number of antennas to choose from, an antenna switch will be required. Ideally they should be metal -encased and offer high isolation between antennas to prevent interaction.

On DXpeditions when DXers are sharing an antenna, a splitter is required to provide equal signal levels to all sets. Standard splitters will cause a small signal loss so amplified versions are another option.

And when you've finally found that ultra-quiet environment, a preamplifier may help you drag a signal out of the mud. It is important that the amplifier has a very low level of internal noise otherwise it will also bury the signal. Make sure the preamplifier has excellent signal-to-noise figures. An good unit would have a gain of 10 - 20dB and a noise figure of around 2dB. Also make sure they work down to 0.5mHz (500kHz) as a lot of preamps are designed for above 1.8mHz.

You'll find most of these available from places like Universal Radio, advancedreceiver dot com/ page46.html Advanced Receiver, stridsberg dot com Stridsberg Engineering or Kiwa all in the USA, Wellbrook in the United Kingdom or Paul Ormandy's >> radiodx.com/spdxr/ Baluns.htm
Equinox balun. Home -brewers may wish to check Mark Connelly's WA1ION DX Labs page