FM DXing more info please.

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PaulZA

314 posts

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#107319 9-Aug-2012 11:15

Hi there guys.

I've been reading up on the subject of DXing FM stations. I was watching a video when a guy from Adelaide, was listening to ZM from Palmerston North, and The Breeze from Auckland with some super sensitive Tuner and antenna setup.

DXing looks like a hobby that doesn't cost the earth, and if it really doesn't it's not a bad hobby for me to do in the day.

Just wondering, are their any guys on Geekzone who do that sort of thing, and are able to tell me where to get the cheapest gear available that will do the job?

Also say I did get that setup, what Out of range transmissions would I be likely to receive, and from where?

Thanks

kiwitrc

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#669988 9-Aug-2012 11:19

You could just get an app for a smartphone and listen :)

aros71

122 posts

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#669997 9-Aug-2012 11:44

Ordinarily FM is limited to line-of-sight propagation owing to the fact that - at VHF frequencies, the signal is not reflected back from the upper atmosphere.

Around the Earth in the upper atmosphere is a layer (or actually, several layers) of charged particles called the ionosphere. The different layers of the ionosphere reflect different parts of the radio spectrum depending on the time of day (the sun has a part to play in this too... the charged particles get their charge from it and lose them at different rates at night...)

The ionosphere is what makes intercontinental "shortwave" transmission possible - stick a transmitter in Australia and a receiver in China and give it a few bounces along the way and the signal is received in China.

There are certain frequencies or bands of frequencies that are reflected, some strongly, others not so.

Ordinarily the VHF waves of TV and FM transmissions aren't reflected, which is great because you can re-use the same FM frequency in two places without interference provided they're far enough away from each other.

However, at times approaching the peak of the 11-year sunspot cycle, the ionosphere gets gets charged and starts doing unpredictable things. Sometimes it starts reflecting transmissions from great distances - think of it like a wormhole on Star Trek - except it carries radio waves only :D

The Technical term for this is "Sporadic E" propagation (because it's the E layer in the ionopshere that's responsible) but most radio operators just call it "skip".

When the "skip" is running, you don't need any fancy equipment to receive distant transmissions, a headphone radio will do it... conversely, if the skip is not happening, there is no radio and antenna system on Earth that will allow you to receive an NZ FM transmission in Australia.

So the long and short of this little lecture is - you don't need any fancy equipment, other than lots of time of course... bit like watching the weather really!

Adam Rosner

gzt

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#670027 9-Aug-2012 12:40

PaulZA: I was watching a video when a guy from Adelaide, was listening to ZM from Palmerston North, and The Breeze from Auckland with some super sensitive Tuner and antenna setup.

*Amazed*

B1GGLZ

1961 posts

Uber Geek
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#670149 9-Aug-2012 15:52

Most Freeview cards for your PC have FM radio tuner included.
Just plug one in, set it up then put a decent antenna on your roof. Probably not as sensitive as a decent receiver though.
Check out the Radio/Electronics section (and related magazines) in your local library for antennae and how to build.
You could also get into Amateur Radio very easily. The exam is dead easy now with no Morse requirement. Check with your local Amateur Radio Club (google NZMAA). Most new guys do a weekend course, pass the exam and get their licence in the one weekend. You'll find radio gear on Trademe but Amateur Radio Transmitting gear can only be sold to Licened Amatuers.
Opens up the whole world of Radio Dx.
Cheapest source of good receivers would be places like HRO (Ham Radio Outlet) in USA. Also places in Oz and Hong Kong.