12 Jan 2011

Short Marconi vertical on 136kHz - forget it!

ATU for 136kHz. Antenna connected to top of ferrite rod
Disappointing results yesterday with a very short Marconi vertical tuned using a ferrite rod loading coil and variable capacitor. ERP was calculated as around 200uW, but in reality the ERP was around 10dB down on the vertical loop (based on WSPR reports locally), so the ERP was around 5uW only. Most of this can be put down to the coil losses and to tiny current actually flowing in the antenna part actually radiating. A much more efficient antenna for the band will require a BIG loading coil outside and more wire in the air with some top loading. I could erect a 6m vertical antenna with a sloping top capacity hat horizontal section with an overall vertical rise of 10-12m. Antenna improvement could increase ERP by >6dB. TX power could be raised by 6dB. Taken together this would make a difference, but it would not meet my QRP ethos.

My conclusions are that an ultra-simple QRP approach to 136kHz TX is just not possible if one is to achieve useful results consistently.

1 comment:

TiƩgui said...

Hello Roger !
Happy New Year !

You schould put the coil on the top of the vertical part of the antenna just before the capacitive hat.

Also, the wire you use is too thin, try with "Litz wire", I'm sure it will work beter. You can find those wires on AM radios on the ferrite antenna coil.

73, from Thierry.