OBSERVATIONS FROM NORWAY                                                           LA2QAA                                                        06 Dec 2009

FM ON LINEAR SATELLITES . LA2QAA's Christmas project.


With the primary aim of encouraging FM satellite aficionados to eventually upgrade to linear birds  I decided to pen an article to remind operators that it's perfectly feasible to use FM rigs to communicate via CW on a linear satellite, thereby allowing them to "get-their-feet-wet" without buying a new rig..

As stated, this is perfectly feasible, providing two simple rules are applied.

1) *DON'T* use too much power ... you know !!! why.

2) Use a clean sine wave signal for the audio oscillator.

The old timers will remember keying the PTT of their FM rigs to send CW.

For the newcomer, this was called MCW - (modulated continuous wave) - that is to say, simple on/off keying by pressing the PTT.

A bit 'chirpy' but perfectly adequate for 'communicating' ... (a key word for 'amateurs').

It's obvious that those Hams who only own FM rigs are *NOT* going to run to the local emporium and purchase the latest all singing, all dancing multimode rig just to try to work CW.

Therefore, we need a way to achieve that using existing equipment.

The only 'extra' items to your normal FM handheld is an audio oscillator fed to the mic input and a reliable method to key the transmitter.

99.999% of today's FM handhelds (and scanners) also have the facility for copying AM signals.

I mention scanners in case, like me, you want to use your FM handheld to TX and tour scanner to RX, that is to say, full duplex ... though judging by the 'zoo' on the FM satellites, very few amateurs have understood the principle of full duplex.

Without delving too deep into the realms of the technology of 'How's' and 'Why's' the scenario is this ... The station who hears your CQ call, if he has his brain in the ON position will switch to CW and reply to you - (if he has his brain in the OFF position he'll mutter something like - "There's something wrong with that idiot's modulation").

Assuming he does reply to you in CW, his signal will be perfectly readable with 'YOUR' rig set to the AM position ... (think! before you speak).

You *WILL* be using your dual band FM handheld set to TX on 70cm FM and RX on 2m AM.

The Doppler will change slightly more on VO-52 than on AO-7, due to orbital height but remember, most handhelds have about 10.000 - (ten thousand) - programmable memories these days ... (at least!).

Now !!!, at last,  you've finally found a use for some of those memories.

Most operators are familiar why one should *NOT* use too much power - particularly
on FM so I won't go there.

The other criteria is a 'clean' oscillator signal that is *NOT* a square wave - (that's the NE555 out, then). You don't !! want a signal that sounds like a regurgitating water buffalo.

The following is an explanation of the system I use - (see circuit diagram and pic).

The 'oscillator' is the audio output from a homebrew electronic keyer that is a clean sine wave. This is fed to the mic via an attenuator (if necessary) since FM rigs don't usually have an external mic gain control.

My IC-E92D is such that to TX, the barrel and the tip of a 2.5mm jack need to be shorted. I achieve this with a small relay mounted inside a 'scavenged' laptop computer power supply - (see pic).

To do the actual switching, I use the 'grounded' PTT signal from the keyer via a PNP transistor switch feeding an NPN relay driver. The PNP transistor also controls the relay's 'delay' via the RC time constant components to alleviate any relay 'chattering'.  The hold-in time can be adjusted with the 10k potentiometer.

The electronic key is built into a spice box purchased at the local hardware store. The paddle is a pair of micro switches 'RIPPED* out of a joystick, albeit, not 'Bencher' class but perfectly adequate.

On high elevation passes it *IS* possible to work VO-52 with low power and a whip -
(your rubber duck does *NOT* belong in this equasion) - so you can work portable CW with nothing more than a handheld FM rig, a keyer and a small box toted in a shoulder bag.

NB.
For aerial recommendations and amount of power etc see elsewhere in OBSERVATIONS.

You *DO* have a pair of headphones in your shoulder bag, right? ... silly question, of course !!! you do - AND !!! you know why, don't cha' ... (feedback).

A bit of *GUMPTION* is required to work low power CW using an FM rig on a LINEAR satellite but IMHO that's what amateur radio is all about ... communicating and experimenting.

Points to ponder.

After checking your dictionary - 'gumption' (Manchester slang) can be loosely translated into- 'operating procedure'.

Contrary to popular belief from some of the self-professed 'experts' - it is *NOT*
forbidden to operate FM on a LINEAR transponder ... however, it should be obvious that one should definitely *NOT* !!!! use 300 watts to a 10 element beam on FM ...

(As seems to be the case on AO-67 and AO-51).

C'mon !!! guys, I was kidding.

The whole object of the exercise here is to encourage people to try something different. It's aimed at those who *CAN'T* afford to splat out 2 years income on
one of the latest, greatest rice boxes ... (like LA2QAA !!!).

I *KNOW* not everybody is a CW fan but in my opinion it takes a little bit of skill to use the mode, and therefore, is a lot more rewarding.

IMI barred - (repeat).

My opinion only , I do *NOT* ask anyone to agree with it. It's merely a suggestion.

Read ... digest ... even if you only mutter ... eejit!.

The plastic cabinet for the electronics was salvaged from a laptop PSU.

Spice Tin keyer.

73 John.   <la2qaa@amsat.org>