M0DTS.CO.UK

M0DTS.co.uk

136 / 500 KHz

Recently (March 2007) i have ben thinking about having a listen on some of the LF bands, initially 136KHz and finally i decided to give it a go.

I has a good search around on the net for an easy to build loop for 136KHz and eventually i came across G3LDO's Superloop receiving antenna for 136kHz. This appeared to be most simple loop to build that did not need any matching transformers etc.. which i could not have made without buying some ferrite cores especially..!

I have temporarily built the loop without the PVC tubing as in Peter's design on a 2m cross of 2" by 1" wood (that's all i had!) and just mounted on a stand that is easily rotatable. I fixed four screws in the end of each piece of wood and wrapped the 80m of wire around the outside of the screws forming the 2m square loop. This is the resonant part of the antenna and will need as it says in Peter's design around 1200pF to make it resonant on 137KHz +/- a few. Next a single turn of wire was wrapped over the top of the other loop and this is used as a pick up loop and feeds back into the shack via coax to the radio.

The wire used for the loop needs to be very low resistance over the 80m length and probably greater than 2mm diameter.

The first attempt i had of making the loop did work in a fashion but the resistance of the 80m of 1mm unknown wire that i used was over 40 Ohm! and seriously affected the Q of the antenna, i managed to get some 2.5mm copper stranded wire and re-wound the loop and now the total resistance across the loop is around 0.5 Ohm and the Q is much higher as i noticed when trying to get the main loop re-tuned for 137KHz, it was much more sensitive to a small capacitance change.

Here's a photo of the loop.....hmmm i think i ned to disguise it somehow!

137 loop

I think the next thing to try is a vertical wire antenna and use it to compare with the loop to check performance of antenna and receiver.

For receive i'm using my Icom 706 IIG which is very deaf at 136KHz, i think nearly 30dB down in sensitivity from the hf bands. i had to build a simple pre-amp using a MMIC to bost the gain up to use the radio properly.

The pre-amp is just a MAR-6 with 47nF I/O coupling, 100Ohm resistor to Drain from +5v and a parallel tuned circuit at the input to block out the broadcast signals breakthrough due to the wideband nature of the device, It's simple but it works.

Last night the 8th June after 5 days listening on the band there was some activity...

I heard M0BMU and G3WCB both in IO91 square

Here are some screenshots of the DFCW signals they were transmitting.

M0BMU 136.320KHz

M0BMU

Audio recording of M0BMU's CW ID

G3WCB 137.695KHz

G3WCB

Hopefully there is a lot more to come on this page..... Now time to get a transmitter built to actually have qso with a few stations :-)

First Transmissions

This weekend (16th June 2007) i managed the first transmissions on 137KHz from my cobbled together station!

The antenna i have used is a 'T' antenna with ~7m vertical and 30m top. The vertical is loaded with two coils wound on buckets at the base which are a few mH inductance (not measured yet). The fine tuning of the antenna is done by sliding the buckets on top of one another and the more they slide together the larger the inductance etc.. this reduced the size of the coil considerably. Antenna capacitance is somewhere around 200pF.

The buckets are 30cm diameter and depth with about 100 turns on each of 24/0.2 stranded pvc insulated wire.

The feeder is coupled with a few turns around the large coil near the earthy end.

The earth currently is a singe 3 foot length of 15mm copper tube forced into the ground.

My Transmitter starts with my DDS 'Wobbler' as the signal source, this feeds into the amplifier which is very much the same as this design. Producing over 100W into 50Ohm load with 28V.

136 amp