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2 Meter FM DXing

2 Meter FM DXing. By KH6TY. FM Dxing – what is it?. FM Dxing on 2 meters is just like working distant stations using SSB phone or CW, but using NBFM instead, together with high-sensitivity digital modes

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2 Meter FM DXing

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  1. 2 Meter FM DXing By KH6TY

  2. FM Dxing – what is it? • FM Dxing on 2 meters is just like working distant stations using SSB phone or CW, but using NBFM instead, together with high-sensitivity digital modes • Experiments have shown that, by using a high-sensitivity digital mode, like PSK31, MFSK16, or DominoEX, “If you can work a station using SSB phone, you can work the same station using FM with a sensitive digital mode.”

  3. Why Should You Be Interested in FM Dxing on 2 Meters? • More stations to work, resulting in higher contest scores and additional grid multipliers • The fun of trying something new • Interesting more hams in joining the weak signal VHF community so later they may want to upgrade equipment and be able to use SSB phone • Exploring the potential of using digital modes to extend the range of VHF SSB communications

  4. WHEN USING SSB - NO CROWDING ON THE CALLING FREQUENCY

  5. What Does this mean to the VHF Community? • Half of the FCC-licensed hams have a tech license and probably only work short distances on 2 meters on either FM simplex or on the repeaters • Many of these hams probably have only a 50 watt to 60 watt FM-only transceiver and some kind of vertical antenna

  6. Repeater contacts are not allowed when working toward the ARRL VUCC award, and working short distances on FM simplex will probably never get you there • If only a small percentage of the tech license hams become interested in FM Dxing, it would create a sizeable increase in the VHF weak signal community, more active grid squares, and more Q’s for everyone during VHF contests

  7. What Does it Take to Work 2 Meter DX on FM? • FM-capable transceiver, preferably 50 watts barefoot or driving a brick amplifier if possible • Horizontally-polarized, high gain, yagi or quad antenna with a rotator • Personal computer with free fldigi, Multipsk, or DM780 digital software • Some kind of interface device to connect the computer to the transceiver • Studying VHF propagation

  8. What is special about an interface for FM-only transceivers? • Most FM-only transceivers can only be placed into transmit with the microphone PTT button • The digital mode software can conveniently switch the transmitter in and out of transmit from the keyboard or using the mouse • Some way is needed to emulate the microphone PTT button, and this can be done with an interface • The interface simply replaces the microphone, and digital audio replaces microphone audio

  9. A Homebrew Interface

  10. For homebrewing, a blank PCB is available for $5.00 and a SASE

  11. Tigertronics sells a good commercial interface with digital VOX for around $100 that uses the USB port

  12. What Digital Modes are Recommended? • MFSK16 is a popular and sensitive “ragchew” mode, but requires careful tuning • DominoEX 4 is a little more sensitive mode, and also more tolerant to mistuning and drift • PSK31 is easier to spot in the noise , but appears to be more susceptible to multipath interference, and not as sensitive

  13. How Does it all Work? • In order to reject manmade noise, FM receivers rely on high enough gain to achieve full limiting on an FM signal • The high gain results in a high receiver noise level below limiting • Due to the high noise level, it is not possible to understand stations under the noise level

  14. How Does it all Work? • Digital modes, such as MFSK16 and DominoEx, can continue decoding when the signal level falls as much as 12 dB under the noise level! • This is similar to the advantage of CW over SSB phone • By using sensitive digital modes, it is possible to work far under limiting and quieting • The software uses DSP filtering to narrow the noise window at audio frequencies • The S/N disadvantage of FM compared to SSB appears to be around 3-5 dB, but that is more than made up for by being able to work far under the noise level

  15. Proposed FM Weak Signal Calling Frequency – 145.000 • Within the ARRL 2 Meter bandplan: “ 144.90-145.10 - Weak signal and FM simplex (145.01,03,05,07,09 are widely used for packet)” • Within the SWR bandwidth of most yagi’s used for weak signal SSB and CW • Below widely used packet frequencies • It is suggested that QSO’s be conducted by moving down, below the calling frequency • Suggestions welcome!

  16. See you on 2 Meter FM DX soon! 73, Skip KH6TY FM02 http://kh6ty.home.comcast.net

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