19 Sept 2010

Why NOT to use QRO on 136kHz

Andy G4JNT was testing a 600W RF WSPR beacon on 137kHz today when he noticed burning from the plastic shed used to house the ATU. When he opened the door he saw the plastic shed and shelf melting and flames everywhere which he quickly extinguished.  LF at high power can be dangerous stuff and one reason why I've restricted my activity to QRP.


Anonymous said...

I was experimenting with a VMOS
topband transmitter and got some nasty RF burns at 6 watts out.
Its a good job the cabinet was some distance from the house.
Its best to stick to low power.


Anonymous said...

So why is RF at LF more dangerous than RF at other frequencies? Something to due with skin effect perhaps?

73's David

Anonymous said...

You require lager loading coils
at VLF to match your attena system
to your transmitter.
If there is a mismatch in the set up and most of the R.F voltage appears across the loading coil
causing it to heat up.
Due to the large inductances at VLF
you will get a large back emf generated.
Think of it acting as a large tesla coil.
Skin effect is more of a problem at higher frequencies and this results in heating your coax up.
Thats way early UHF transmitter used wave gauids.

Anonymous said...

Yikes! Got it, thanks for the explanation.


Anonymous said...

I seem to recall Andy saying that he suspected it was the ali tape that was used to waterproof the enclosure, arcing to some other metalwork in the shed.