Friday, 4 October 2013

Beginner Antennas

Antenna size depends on the frequency you want to hear: higher frequency (bigger number) - smaller antenna.
You need an antenna that receives signals from all directions - called omnidirectional.
Materials make no difference: any type of wire is good, as long as it keeps its shape. Metal coat hangers are popular, multi-stranded or solid makes no difference. I'm a fan of No 10 house grounding wire.
Beginner, antenna, RTL-SDR, DIY, newbie, tips, radio, VHF, UHF, construction, schematics, wire, braid, conductorYou need to connect your antenna to your RTL stick: coax cable is needed, read my hardware guide for details.

Recommended beginner antenna 


A monopole with two radials optimised for 120-130 MHz - three 60 cm (2 foot) elements in a peace sign / Mercedes emblem shape, coax conductor connected to vertical element, braid to two sloping elements.
Beginner, antenna, RTL-SDR, DIY, newbie, tips, radio, VHF, UHF, construction, schematics, wire, braid, conductor
Why? Perfect for Plane-Ground conversations, suitable for taxi/marine/security speech around 160, works for voices around 450 MHz.
Used this antenna for Airplane position signals (1090 MHz) and AIS with success.
Construction details are left to your inventiveness, or...
the following is a quick, ugly and working solution:
Drinks bottle - neck of bottle should be sloping downwards.
Cut a small hole in the plastic and push the two bottom wires on each side through, so the ends come out at the top.
Connect these two wires to the braid of the coax.
Wrap the center conductor around the third wire, then wrap it tightly
with tape.
Tape the third (vertical) wire to the drinks bottle.
Granted, it looks hilarious on pictures, but it gets the job done.
Compared to a commercial discone, the 80 USD Scanmaster, airport info voice sounds the same; because the discone picks up more noise the signal-to-noise ratio (SNR) is the same.
Beginner, antenna, RTL-SDR, DIY, newbie, tips, radio, VHF, UHF, construction, schematics, wire, braid, conductor
A more aesthetically pleasing solution is using screw terminal blocks and wood - this also enables quick disconnecting of antenna elements. The example above is just a proof of concept, demonstrating that household items can be utilised.

Put your antenna outside
Beginner, antenna, RTL-SDR, DIY, newbie, tips, radio, VHF, UHF, construction, schematics, wire, braid, conductor


The illustration on the right shows three scenarios:
1) antenna on windowsill indoors,
2) antenna on windowsill outdoor,
3) antenna 1 metre from window outdoors.
Signal is the height of the peak, red horizontal line added for easier identification.
Lessons learned: 1) Signal 8 dB less indoors - that's a lot. 2) No point placing it 1 m from window, windowsill is fine.
Even if you live in a condo / flat with restrictive antenna policy, place the antenna outside on the windowsill. Flowering plants and the like are good camouflage, and if you tie bird food to the bottom two radials, you get brownie points from local wildlife and neighbors: songbirds are cool.

Telescopic antennas - about 10 dollars

This should be your first ready-made antenna.
The same collapsible antennas used on old TVs and radios are also suitable for radio. Buy a new one with BNC connectors or salvage one from unused equipment. A 60 cm / 2-foot (when extended) with angled joints is perfect.
If you choose BNC connectors, you will need a BNC-SMA and SMA-MCX adapters. With old tv/radio antennas you simply connect the center wire to the antenna by wrapping around it.
Performance is equal to a wire of a same length, in radio terms it is called a monopole.
You may add two radials similarly to the Peace antenna for performance improvements; or buy three telescopic antennas and construct an adjustable version.
A lightweight Travel Kit is a telescopic antenna with adaptors and an RTL stick with USB cord. Note on the above image Clamp-On ferrite and USB extension cord shield mod - do them, read the noise suppression post for more info.

Rubber ducky

Small antennas usually used on handheld transceivers and walkie-talkies are a poor choice for beginners; they are designed to work with handhelds, where built-in electronics optimize performance. However, they work well for airband, for signals 400 MHz and up, and usually provide adequate performance if you're close to the signal source.
For general "all-in" use they are simply too small.

Discone antennas


Most monitoring enthusiasts have one; it is like having an antenna for every frequency between 25 and 1300 MHz. Drawbacks are: expensive (50 dollar and up), look very conspicuous and nearly impossible to camouflage. Noise pickup is significant, too.
Personally, I use the 80 USD Scanmaster mentioned above with great results. For this post I used the 2-foot Peace antenna for a few days and didn't miss my usual signals.
Before you part with your hard-earned money buy a Low-Noise Amplifier (LNA) to be used with your homemade or telescopic antenna - so you can wring out the last ounce of performance from your existing equipment.
An LNA costs 25 USD, see review on this blog.

Tips and tricks


Frequencies in a country are public: Called "frequency allocations" or "frequency band plans", they are an excellent source of information. Google the terms above and you'll get a government document detailing who uses what frequencies. USA frequency allocation chart here, UK here.

Police frequencies: I know you want them. Bad news: most law enforcement / government communications went digital few years ago (you can't hear them), and sorry, I have no experience nor interest in them anyway.

Airplane Chat

Frequencies are around 120-130 MHz, for exact frequencies google "your local airport's name" and "frequencies". Normally four frequencies are given: Approach, Tower, Ground and ATIS.
Approach deals with planes approaching the Airport, Tower handles landing, and as soon wheels touch the ground Ground directs the bird to terminals. Take-off is in reverse. Larger international airports might have more than one frequencies, e.g. Heathrow has four Approach.
ATIS is continuous weather and landing info for planes, useful for weather check in the morning.
Planes are IMHO the best to listen to.

Where is what / Popular frequencies:


The following is just a quick recap for a total newbie. Really, spend about ten minutes with a frequency allocation chart - even if you're a seasoned veteran.

Mode (top, left of screen) to set in SDRSharp is in brackets.

Broadcast radio (WFM): between 90-108 - signals are strong, everything receives this.
Airplanes talk (AM): around 120-130 MHz. - signals are strong, you need a dedicated antenna to hear airplanes far away. The Peace antenna above is ideal for airplanes.
Weather satellites (WFM): 137 MHz - weak signal, special antenna, see guide above.
Marine channels and AIS (NFM): 160 MHz - see guide above.
Taxis and speech (NFM): 160 MHz - an AIS antenna is just as good.
Shopping centre / mall radios (NFM): 440-480 MHz - also radio amateurs.
Airplane positions signals (AM): 1090 MHz - but that's the next post :-)

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As a beginner, you'll be up and running in no time.

Tips and tricks in the book will save time and money, reduce frustration with computer settings and help you build the best antenna system from shortwave to microwave. Detailed and illustrated step-by-step descriptions on easy-to-do antennas, from shortwave to microwave.
Basically all you need to know to enjoy radio.

13 comments:

  1. Hi ! Thanks again for your great tuto. Your blog turns out to be a really good way to start RTLSDR and not only for mariners ;) I really like your practical approach that make everything "real life method".
    Just a question: for Airplane Chat, could you tell us the type ? (AM, NFM, WFM... )
    thanks again and please continue !
    Anthony

    ReplyDelete
  2. Replies
    1. Updated the guides with modes then.
      FOR ALL READERS: Please comment here like Anthony here, unless you point out areas for improvement I can't fix it.

      Delete
  3. Hi Akos,
    thank you for your great posts. As I'm a newbye on SDR radios, I have some (hopefully) trivial questions.

    I have a RT820, a telescopic wideband antenna from 20Mhz to 1.8Ghz (16.2cm) and a spare "bunny ears" antenna (~80 cm).

    The telescopic antenna looks just like the one shown in your post (the visible portion, at least).

    I can receive signals from commercial radios and up (144mhz band, 433, 1090Mhz etc) but if I tune near 30Mhz (Citizen Band is around 27Mhz) I get STRONG broadcast radio interference, so strong I can hear the radio almost perfectly.. Why does this happen? Is there anything I can do to get rid of that interference? I was assuming that since the frequency is in the antenna range there would be no such problem...

    Note that this is not due to electrical interference. I followed your guide on reducing nose and if I plug the stick directly in the usb port or use the cable mod + alu foil no station is audible until the antenna is plugged in.

    The same happens with the bunny-ears antenna and, to my surprise, there seems to be little to no difference between the two antennas over the rest of the frequencies...

    Finally a doubt: why does the metal part of the usb extension cable produces noise while a bunch of alu foil touching the usb port actually reduces noise?

    Thank you very much and sorry for the long post!

    ReplyDelete
  4. Hi Shadow,

    Thanks for your comments. Sorry I've been away testing new antennas :-)

    1. "The telescopic antenna looks just like the one shown in your post (the visible portion, at least)" I think it is like you pull out and the antenna gets bigger. It is too short for 20 MHz, but maybe you can receive something.

    2. Broadcast radio cannot be helped. It is like a big guy shouting near your head - you can not turn it down. You can have filters and so on, but they are complicated, expensive so not worth the trouble.

    3. Alu foil touching the metal part of the USB stick are like the following: to know how high a mountain is, you must know where to measure it from. Alu foil helps the stick knowing where to measure the height of the mountain from.
    You want even better noise reduction use wire around the metal part, longer the better. More wire tells the stick better where is zero, where is the bottom of the mountain.
    If you want to read more google "counterpoise". Same stuff, more technical description.
    If you have more questions fire away, I'm here to help.

    ReplyDelete
  5. Wow, as a newcomer to all this I'm pretty amazed that the bottle antenna works! Well impressed that something I put together in an hour for about £5 can pick up planes from my London balcony. Will be rebuilding it a little more robustly now! Yours was the only *actually* easy to make (and reasonably sized and not requiring any specialist equipment) aerial I could find mentioned online, so thanks for that.

    Is the bottle antenna likely to be better/worse/the same than one the "mag mount" ones you can with a twisty bit in the middle (e.g. http://www.amazon.co.uk/Centre-Loaded-Mount-Scanner-Antenna/dp/B004MYAGAY/ref=pd_cp_ce_0) do you know?

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  6. Hi Tom D,

    Sorry for the late reply, writing THE book on RTL-SDR.
    Antenna you linked will be no better than the bottle antenna, if you want to spend money / improve reception 1) coil excess antenna cables 2) buy ferrites or toroids, check ebay for T130-2, 10 pounds shipped. Use them on coax cables, better than a new antenna.
    Just did some tests, noise reduction is better for money vs results than a new antenna.

    ReplyDelete
  7. Hi, this is a very helpful post!

    Can someone point me in the direction of an ultra-wide-band antenna (not sure if that's what they're called)? Something that can go from about 20mhz to 1.7ghz, like what Shadow mentioned. I couldn't seem to find any consumer version of it with Google. Thanks!

    ReplyDelete
  8. That antenna does not exist, the closest approximation is a discone antenna generally available for 50 dollars and up.
    Please let me point out that you can make a discone by adding receiving elements to the top of the bottle antenna, for the frequency of your interest.

    ReplyDelete
  9. Hi Akos,
    I've received the notifications of the latest posts and it occurred to me that I never thanked you for your reply :)

    Thanks!

    ReplyDelete
  10. Hi Akos, discone antennas are really interesting and while they are expensive to buy, it looks cheap to make one :) Thanks!

    ReplyDelete
  11. Very useful blog! Thanks a lot. Unfortunately I'm not a Kindle user...but let me tell you something: You solved almost all my basic doubts just by reading your blog. Thanks again!

    ReplyDelete