Monday, September 07, 2009

The QDFA - A Possible Conclusion

First, let me point out that ground matters. Very poor ground favours the beverage (and other longwire type) antenna. Antenna designs like the EWE and K9AY want good ground. Both the EWE and the K9AY have been tested here, with mediocre results while a good ground K9AY made wonders for me home in Berlevåg. The beverage is literally on "home ground" here in Kongsfjord and would prove to be a worthy opponent to any other antenna design. A beverage in Grayland may not be allowed to reach its potential because the beverage thrives in the lossiest ground.

Let me also point out that the two places where the QDFA has been tested before have much better ground than the rocky, stony morenes in Kongsfjord. Dallas Lankford mentioned that EZNEC simulation did not give any answer as to how the QDFA would perform over lossy ground. Maybe this real-world test does.

Last night we compared the QDFA with two beverages (and even one pointing way off North America). The 310 is my favourite nighttime beverage because of less European QRM than the 340. The latter performs best during daylight hours. So basically, the competition was between the QDFA and the 310. Conditions favoured the eastern part of the USA and Canada. Signal levels were from threshold to "fair".

In short, there was no significant difference. On upper MW, the signal-to-noise levels were roughly the same or a few dB in favour of the QDFA. Readability was mostly the same. What I heard on the QDFA I could also hear on the 310. The reduced sensitivity of the QDFA revealed itself on frequencies like 740 (Ontario) and 590 (Newfoundland) where the 310 had audible and even readable levels (590) and the QDFA couldn't hear any audio. On an undisturbed frequency like 1650 (Ontario) the QDFA and the 310 were identical. The QDFA was less sensitive to Loran C noise though, even if the Loran C station is in its front lobe.

So, is the QDFA a failure? Definitely no. It is probably just placed on the wrong spot, and even then it matches the 310 in performance, at least on the upper half of the MW band. It has a wide front lobe, and it can null a very wide angle off the back. And it is extremely quiet. It performed somewhere along the lines Dallas Lankford had predicted (or feared), given the lossy ground. Since it was an astounding success at Grayland, it should be equally good on a European site with good ground conditions.

The downside is of course that you have to make it yourself and it takes up quite a bit of space (120 meters from end to end). And you need to make sure that the deltas don't collapse if you're in a windy location.


4nradio said...

Hi Bjarne,

Your posts on the QDFA have been very interesting to follow along with. What a difference the rocky terrain and conductivity must be in Kongsfjord compared to rain-soaked Grayland! The only rocks around Grayland are the decorative ones brought in as part of homeowners' landscaping efforts ;^)

You have made a lot of comments about the signal strength differences between the two antennas. I'm wondering about co-channel or adjacent-channel improvements with the QDFA. Which North American channels coincide with--or are very close to--"pest" frequencies from Europe? I know Kongsfjord is further from European powerhouses than we are from our local/regionals in Grayland, but your Euro stations are far more powerful, too.

How is the QDFA at reducing the QRM from these pests in the broad null of the QDFA? At Grayland, that was the beauty of its performance, not the sensitivity of the antenna. (Although I realize you need as much s/n in remote Kongfjord as possible.)

Keep up the fine reporting on your new antenna system, whatever the outcome.

Bjarne Mjelde said...

A comment from Dallas Lankford:
First let me say that I found your comments thouough, detailed, and fair. I have no complaints. Basically you found that the QDFA has virtually no null at Kongsfjord. I am sure we wish that the QDFA had performed better given that you, OJ, and I put a lot of work into this project. But facts are facts and can't be wished away.

I am not sure what is a "good" ground or what is a "bad" ground for a Flag, Delta Flag, EWE, K9AY, or related antenna. This is uncharted territory for me. But based on your experiences with EWE's, K9AY's, and now the QDFA it is difficult to escape the conclusion that EWE's, K9AY's, and now Delta Flags have poor nulls when used above rocky soil like you have at Kongsfjord.

I had a similar experience with the 1500' BOG I took to Quoddy Head. It had great nulls off the "far end," even when unterminated, here in Ruston Louisiana. But at Quoddy Head the 1500' BOG had virtually no null at all. The soil at QH is rocky, probably similar to Kong. But are they the same with respect to nulls of EWE's, et al.? I don't know. The QDFA has not been tested at QH.

The soil at Grayland is obviously very different from the soil at Kongsfjord (or QH). It is saturated with water, probably brackish water (water with some salt content), which should make it a very good ground indeed. But I can't explain why the QDFA "liked" the Grayland ground (or my ground here in North Louisiana) so much better than the Kongsfjord ground. EZNEC simulation predicts no difference. But as I have said before, EZNEC simulation does not always produce the same results as in the real world.

I was not surprised that the QDFA lost the low band sensitivity contest with the beverage at your wonderfully low noise Kongsfjord site. An additional 10 dB or more of QDFA sensitivity (gain) could be obtained by using larger flag (rather than delta flag) antenna elements. But of course, a QDFA without its null would be pointless.

Bjarne Mjelde said...

Guy: The point of my posts was to evaluate the QDFA from a QRM reducing perspective, and to see if the Grayland results could be replicated. So the answer to your question how the QDFA is at reducing pests off the back end is in this post's conclusion: It didn't do that very well.