Re: LF: First 137 KHz signal

[Stefan Schäfer <schaefer@iup.uni-heidelberg.de>]

Hello Fausto,

That sounds interesting and promising! You will be a strong signal on 137 from Italy i assume.

If you have your TX antenna resonated to 137 kHz and use it as the RX antenna too (which is not always an advantage!) it will be a first step to reduce the strong 567 kHz signal. If you connect the RX after the low pass filter for the PA, there will be an additional reduction. A notch filter should solve the problem! 567 kHz is pretty far away from 137 so you will have no problem to filter the signal before the RX input.

If you want to have informations about matching antennas and all that one should know about TXing on LF, i recommend Rik/ON7YD's excellent website for this: http://www.strobbe.eu/on7yd/136ant/  and some ideas about circuit constructions: http://www.qsl.net/on7yd/136brew.htm

A audio PA will work but is not the optimal solution for LF, i personally think. If you want to transmit CW, QRSS, DFCW, you don't need a linear amp. A switch mode amp will give you high power and has a good efficiency and an easy drive. A class D or E is suitable and relatively easy to build. But of course you can start with the audio amp...

The easyest way to match the antenna is using a coil against ground where a amperemeter is inserted. Just tune the coil for maximum antenna current. This can be done at low power, unmatched. Then you can find a tap on the coil (coax screen at ground and the inner conductor on the tap) where you find 50 Ohm. search for this point when the antenna is in resonance! On the TX side you may use a suitable ferrite one-coil-transformer and find a tap for the 8 Ohm of the PA...

Good luck and hope to see you soon on the band!

Stefan/DK7FC

PS: There are some LF grabbers that show you what is going on on the band, if you want to compare your received signals. Just a few near your location:
http://members.aon.at/grabber/
http://www.alice-dsl.net/df6nm/grabber/Grabber.htm
http://www.iup.uni-heidelberg.de/schaefer_vlf/DK7FC_LF_Grabber.html




Am 27.12.2010 14:09, schrieb Fausto Coletti:

Hello Stefan,

I'm very happy to my first positive rx test on 137 KHz, i have received clear and audible signal of

DB6NT on qrss3 and  IK1HSS on DFCW, Annex the two Argo images.

The antenna is my 80M fullsize delta loop and is not matched to the receiver but i can receive DFC39 at 138,83

at -65 dBm on daylight. In next day i try to install a matched inverted L on my 25 meter tower (that you can see on qrz.com)

with top wire about 30 meter long. The tower is grounded and i have ready to install 120 buried radial between 15 yo 40 meter long

around the tower ( i want use the tower as a vertical for 160m).

My locator is JN54SM about 15 Km north east from Bologna.

I am not ready to transmit at the moment but i hope to will be ready in next month. I'm making in the short time that i have available

a linear transverter and i want use an audio amplifier to obtain the necessary power.

The only doubt that i have is how to match the audio amplifier to the antenna, i have not found literature.

Now i want continue to improve my receiving equipement with new matched antenna and a good low pass filter at the input of the

receiver (i have a local 567 KHz 25 KW transmitter about 4 Km to my home and the level of it at the input of my Perseus is +10dBm !!!

you can imagine the hard work for the unfortunate input stage of the receiver...).

 

73 Fausto IK4NMF

 

----- Original Message -----

From: Stefan Schäfer

To: rsgb_lf_group@blacksheep.org

Sent: Monday, December 27, 2010 3:11 AM

Subject: Re: LF: First 137 KHz signal

Dear Fausto,

Congratulations to your first reception!

Who is the received station? How do you match the antenna to the RX?
What is your locator and will you transmit on 137 kHz too?

73, HNY, Stefan/DK7F

Am 26.12.2010 16:29, schrieb Fausto Coletti:

Eureka!!!  today at 16,15 UTC i have received my first 137 KHz signal on qrss3...

 

two "O" and after a "K" on dfcw.

 

Receiver Perseus with 80 meter long delta loop.

 

Fausto IK4NMF

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