Saturday, September 17, 2011

QDFA vs. 310-degree Beverage - A Skywave Comparison

The results I reported a few days ago regarding the QDFA's good F/B ratio vs. the 310-degree beverage was for groundwave reception. Now DX-ers are more interested in skywaves than groundwaves, so tonight I set up a test to find out. Although there were a couple of unexpected results (Pori, Finland 963 stronger on the QDFA than the beverage for instance), the general picture is that the QDFA has 5 to 10 dB weaker signals from the back than the beverage, while it has 9-12 dB stronger signals than the 310 beverage from the front. Admittedly though, only NRK Svalbard 1485 and the NDB "BV" 399 were on the "right end" so the results should be treated with some caution. Add to this picture that the netSDR which was running the QDFA is 2-3 dB less sensitive than the Perseus, which ran the 310 beverage. Switching antennas confirmed this. The QDFA had an extra 10 dB of gain in front of it, a Kiwa preamplifier.

Friday, September 16, 2011

Measuring Sensitivity Of Some Cool Receivers

I've been fortunate enough to have a large variety of receivers coming (and going) at my Kongsfjord QTH. Some are/were mine, some were here on a temporary basis. Two of the latter category are the Icom IC-7800 transceiver, and the Icom R-9000 receiver. Both very high end, both in terms of performance and price. Lots of thanks go to Olav Skår, LA9VFA, who let me play with these superb radios.

Of course I never resist the temptation to measure the sensitivity of a receiver. The IC-7800 is stated (as all Icom receivers and transceivers are) to have reduced sensitivity below 1.8 MHz. As far as I know, all their radios produced in recent years are not desenstized on MW. This was confirmed for the IC-7800 as I measured a uniform -106 dBm to -108 dBm (around 1 uV) throughout most of its frequency range (600 to 20,000 kHz).

The R-9000 however is an older model, and it was indeed rather deaf on MW with figures around -92 dBm or 4 uV. On SW, it was marginally less sensitive than the IC-7800.

I finally got round to measure my new netSDR from RF Space too. A uniform -101 dBm or 2 uV was measured from 600 to 20,000 kHz. I had hoped to see it a few dB better, but it equals the SDR-IQ, a bit better than the QS1R and marginally weaker than the Perseus and the Winradio Excalibur (G31DDC). An external preamp would help.

Parameters: Measured with a signal generator while running AM mode with 6 kHz bandwidth, AGC off, and using 30 % modulation of a 400 Hz tone.

Update: OJ wanted pictures of the wonders, so here they are:

Sunday, September 11, 2011

QDFA vs. Beverage - A Groundwave Comparison

Further to the results we reported yesterday:

Today I did an A-B test on my NetSDR a bit prior to 1100 UTC, local noon. The difference between the 310 beverage and the QDFA is interesting. The Noise Floor of the QDFA was 3-4 dB lower than the beverage, when they were both powered by a 10 dB preamplifier/splitter. Interfering stations on the back of the QDFA, such as the Murmansk stations were attenuated by up to 40 dB (657 kHz). 1134 was reduced from -77 dBm to -115 dBm. On 1521 the attenuation was 13 dB.

On daytime we don't have any stations in the "desired direction" to compare, except the Berlevåg NDB "BV" on 399. It was 15 dB weaker on the QDFA than on the beverage. The illustrations below may speak for themselves. The Ingøy AM transmitter on 153 had equal strength on the QDFA and the beverage. It is located due west of here.
Groundwave spectrum 310 degrees beverage

Groundwave spectrum QDFA

What can we conclude from this?

Front to back (F/B) attenuation increases with decreasing frequency.
Groundwave attenuation is stable and very effective.
Judging from last night's comparison, skywave F/B attenuation is less effective, but still significant.

Nothing of this is new knowledge. It is however extremely satisfactory to see that the QDFA works well. Actually, our preliminary conclusion is that the QDFA is more effective with regard to F/B than last year's QDFA was. Maybe there are subtle changes in the setup we were not aware of that did this. One less subtle change we should have done but so far haven't, is to replace the lamp cord feed line with Twinax. Judging from other lamp cord to Twinax replacements, this will only make the QDFA better.

ADXS2 Final Day - And All Antennas Are Up

On Sunday, the 10 participants of ADXS2 had brunch with rye bread, foccacia and king crab leftovers. We soon entered an engaged discussion about the virtues of Bentonite! The Finnish delegation will leave after noon to catch the Finnair flight from Ivalo to Helsinki. I will leave for Vadsø a few hours later, while OJ Sagdahl and TJ Bråtveit will hold the fort until their departure on Monday.

The Saturday was used for erecting and preparing the QDFA antenna. The creator of the QDFA phaser, Dallas Lankford, has made a few modifications mainly to keep its transistors safe during weather-induced static. For a pre-sesason test, we were delighted that NRK Svalbard had a very good signal on 1485.

To show the Finns the virtue of the QDFA, we arranged a comparison between the QDFA and the 310-degree beverage (pointing towards Svalbard). Interestingly but not very surprisingly, the Svalbard station was the strongest in the band on the QDFA, and European stations were attenuated by 15 dB or more compared to the 310 beverage, while Svalbard was only a couple of dB weaker on the QDFA than on the 310. So the net effect of the QDFA vs. the 310 beverage is 10-15 dB front to back attenuation.

With four well-working antennas up, we are now ready to log new stations.

Saturday, September 10, 2011

The 2nd Arctic DX Summit, Sept 9-11 2011

Been a bit too busy to inform about this event, however here is a brief update.

From September 9 to 11 seven Finnish and three Norwegian DX-ers have met at Kongsfjord, Arctic Norway. The ADXS is a social much more than a DX-ing event, so the focus is on radio equipment, good food, good conversations, beverages (the long copper variant), beverages (the wet variant) and outdoor activities in exceptionally mild and calm weather. Below a photo of our Finnish guests. More as it happens.
From left: Antti Altonen, Mika Mäkeläinen, Hannu Tikkanen, Håkan Sundman, Jan-Erik Österholm, Hannu Niilekselä, Roland Sandberg.

Thursday, September 08, 2011

The RFSpace netSDR

A relatively new SDR on the market is the NetSDR which, contrary to most other SDRs, communicate with the PC over a 100 base-T port instead of USB. Another interesting aspect is that while you need remote control software like LogMeIn to control other SDRs over the internet, the netSDR actually transports its full 1.6 MHz waterfall and demodulated audio via a 10 kbps link. It uses straight TCO/IP and ethernet for all communications. In many ways it can be seen as a downscaled version of the SDR-IP.

The NetSDR arrived today, just in time for the Arctic DX Summit. Initial impressions using Spectravue and SDR-RADIO software are very positive. Its footprint is quite a bit bigger (almost double size) than the Perseus and Winradio Excalibur. It has room however for some very interesting hardware to be retrofitted.

More on this after I have explored the radio a bit further, including sensitivity measurements.