Friday, January 21, 2011

WSPRing on 80

A blog post by Roger, G3XBM, about how far he was able to be received running 5mW of WSPR on 80m using a lash-up antenna prompted me to give QRPP WSPR a try. But before I could start, I needed to make an attenuator. The lowest power my K3 or K2 will go down to is 100mW, but not with very much accuracy. I decided to make a 20dB attenuator which would divide my power by 100, so I could run 1W and get 10mW output.

There are numerous websites that will design an attenuator for you given the working impedance and amount of attenuation required. I was not looking for precision - I would be surprised if the output of either of my radios was within 10% of the selected power anyway - and moreover I wanted to build it now using parts from my junk box not have to send off for some exact values and wait for several days for them to arrive.  My attenuator was built as shown above and gave a 1.2:1 SWR over the HF range which was good enough for my purposes.

I started yesterday evening running 10mW on 80m into my end loaded attic dipole. I received no spots at all initially. I was puzzled as to why I could hear the WSPR audio using the K3 monitor even though the extension speakers I use with the K3 were switched off. I was about to compose an email to Elecraft enquiring whether the last firmware update had the effect of routing the monitor audio to the internal speaker even when an external speaker is connected when I realized the sound was coming from the computer speakers! Windows had decided to renumber the sound cards since I last used WSPR and although the receive channel was still the same and spots were being decoded the transmit audio wasn't going to the radio. Doh!

Some time after I began transmitting I received a couple of spots from G3XLW 478km away in the south of the country. But that was all. I decided to leave it running overnight and received spots from David off and on throughout the night. These were augmented in the morning after 0800z by three spots from M0DDT at a distance of 355km to the south east. Not as impressive as Roger's results. I guess my attic dipole, being very low for an 80m antenna, is a cloud-warmer and sends most of the RF straight up. Roger was loading the feeder of his 10m halo which doesn't sound like much of an 80m antenna but in effect he had a top loaded vertical which probably has more lower angle radiation. That's my theory, anyway.

It was an interesting test and I shall try very low power WSPR again on other bands. However I don't agree with the extremist view held in some circles that people should only use WSPR with such low powers. It's certainly interesting to discover if you can get to the end of the street on 10uW but higher powers of a few watts will reveal propagation to places lower powers won't, information that is likely to be more useful when applied to making contacts using other modes.

4 comments:

Paul - PC4T said...

Hello Julian, indeed it is fun to see how far we can come with low power QRPp, but I agree with you, I change the power output when needed. 5 watt is a bit standard for WSPR, I think. My working power is from 40 watt to 50 milliwatt. I will seldom use more power then 40 watt. The power output depends on band/time/conditions/ etc. I would not limit myself to just QRP only. The same for the different modes. I like it all so now and then. 73 Paul

PE4BAS, Bas said...

Hi Julian, actually max, power I ever used on WSPR is 1 Watt. But had better DX results (even better as with 1 W) with 500mW. Next project will be a attenuator. But first I will try to finish my voicekeyer. I'm very curious what I'll be able to reach with 10mW or even 5mW. 73, Bas

Julian said...

Bert PA1B, who was unable to post a comment, points out that the attenuator would be a better match to 50 ohms if two 120 ohm resistors were used at the input. I agree. Unfortunately the only higher wattage resistors I had were 100 ohms, so that's what I used. But anyone buying parts to make the attemuator should follow Bert's advice and use 2 x 120 ohms.

Bert also points out he has a Power Attenuator Calculator on his website. Unfortunately for me it requires Microsoft Excel, which I don't have, so I couldn't try it.

Tjeerd, PA3GNZ said...

Bas and others.

Yiou can also use the "Power attenuator Calculator" in the free program "Open Office".