Shortwave Antennas Page 14

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Understanding Shortwave Antennas: Page 14

Add-On Gadgets

Like every other hobby, shortwave listening attracts sundry gadgets. Some of these are quite useful, some are not, and some are for specialty purposes only.

Active Antennas, Preamps, Prescalers

These connect between the antenna and the radio.

With an “active” antenna, the idea is to take the usual 5-foot telescopic whip and add a very sensitive amplifier — thus adding another input stage to the radio, which is, presumably, more sensitive than the radio itself. The ads promise that they will work as well as a 60-foot longwire. Baloney! Modern portable shortwave radios, like Sony, Grundig, and Sangean, are as sensitive as any active antenna. Therefore, adding an active antenna will not improve anything. In other words, do active antenna makers have access to more sensitive transistors than the radio manufacturers?

However, if you do buy an active antenna, or preamplifier, make sure it has a “helical” filter, and relatively impressive “intermodulation suppression” specifications. This simply means that it will feed a relatively clean signal to the radio, not a jumbled bunch of amplified garbage from nearby stations.

A preamp is simply a way to make your existing antenna “active.” Same as above. May be indicated for very old radios with poor sensitivity.

A prescaler is a sort of antenna tuning device. You tune your radio to the desired frequency, then you tune the prescaler to peak the signal. These are good devices for improving the selectivity and signal-to-noise ratio, and are particularly helpful of cheap radios. Most prescalers also include a preamp, but some are passive.

My recommendations: Ignore active antennas. Put up some wire. Ignore preamps. Get a modern radio which has all the preamp built-in that you’ll ever need. Get a passive prescaler or random-wire tuner if needed. MFJ makes a nice collection of such things. Look for their ads in radio magazines, and order a catalog. www.mfjenterprises.com.

Audio Filters

These process the audio (speaker) output in some desireable way. You must plug them in between the radio’s speaker (earphone) jack, and an external speaker (or headphones) which you have to purchase separately.

Audio filters allow you to “narrow” the bandwidth of the audio — like turning a “Tone” control way down, but with more sophisticated control. It is a nice add-on, if you listen to CW (morse code), or if you regularly get interference from stations on nearby frequencies (like 5 kHz away) from your favorite stations. You can filter out the “hetrodyne” squeal — but they will not filter out electrical noise. Be not deceived!

If you get an audio filter, make sure it covers what you want to do, like shortwave broadcast clean-up, or single-sideband, or CW (morse code) copy. Each of these requires different types of filtering. Some units have it all in one box, and of course, cost more. Ask an Amateur (Ham) radio operator — we Hams read the radio magazines and keep up with these things.

Computer Interfaces, Radioteletype and Fax Demodulators

These connect to the speaker output of the radio, and usually to the serial input of a computer. They are used to receive and decode CW, radioteletype (many varieties), and weather fax transmissions.

My recommendations: The cheap ones ($100) are garbage, so don’t waste your money. The better ones which cost several thousand dollars work very well. You need to have a real need-to-know (like surveillance work) to justify the expense. Even then, most of the signals are encrypted, and it’s illegal to intercept them.

2003 Update: Computer Sound-card Software

Lots of “digital” modes can now be demodulated (and transmitted, if you have the license) using only a computer with a sound card and the appropriate software. See my web article Getting Started in Digital Modes for info and many links.

What to Avoid
  • Don’t mess with “active” antennas, unless you are trying to get shortwave in the car or from motel rooms.
  • Don’t mess with multiband, analog dial-type radios. You know the ones that have 21 shortwave bands, AM, FM, TV sound, etc. Get a good digitally-tuned, continuous-coverage (150-30000 kHz) shortwave, in the $150 to $400 range. Get one with “BFO” for sideband and CW listening. The Sangean 818 is the best radio made for less than $200 and I highly recommend it. C-Crane Co. is the best source.
  • Don’t mess with automotive radio shortwave converters or other such rinky-dink shortwave receiver kits. They all have a very limited frequency range, typically 1 or 2 megahertz — and the useful shortwave band is is about 20 megahertz wide! Gimmicks to get you money…
  • Don’t waste money on “wall plug antennas” that promose to use your whole house wiring as a great antenna. They work no better than simply connecting to the ground screw in the center of the outlet plate.
  • Don’t blow $150 on fancy multiband trap-dipole antennas. Traps waste power. A $30 multiwire ribbon dipole works better. Don’t waste $75 on a simple inverted Vee dipole which you can build yourself for $20 or less. Don’t blow $135 on a name-brand discone antenna when you can build one for a few bucks.

Continued…

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