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.

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