August 16/17 2010
I have installed my 40m vertical antenna (see: http://www.gbanttow.nl/).
This time, I placed it just on the edge of our garden, using the iron fence that runs underneath as a counterpoise. The antenna is fed against both this asymmetric fence and the iron pipe it is mounted to. This pipe is stuck in the wet clay for about 60cm (2 ft). Please notice the T200-wound choke oin the Aircel-7 coax-feedline.
Using my AIM 4170 vector impedance analyser (see: http://www.eham.net/reviews/detail/6258) I quickly ran a test from the shack. A very much non-textbook effect shows up immediately: the graph shows multiple minima and maxima?! My best guess is that this effect is caused by the asymmetric lay-out of the iron fence, i.e. this fence runs about 60m (180ft) to one side and 20m (60ft) to the other side.
I also expect this asymetric earth-system lay-out to effect the radiation pattern. More specific, having a non-symmetrical earth-environment will most likely induce lobes with a high angle of radiation.
Will run 4NEC2 simulation to check this assumption.
Despite the "strange"impedance plot, first results are promising. Excellent reports throughout EU.
August 19 2010
The vertical was now only fed against an iron pipe, stuck 2" into the wet clay.
Very remarkably there's hardly any difference showing up in the impedance plot?!
Hum... I'm measuring the antenna-impedance in the shack, so there's a good 60m of Aircel-7 in between my AIM-4170 and the antenna. Let's now check how the impedance of the feedline looks without the antenna attached, leaving it open at the end.
August 20 2010
OK - - lets make things a bit more easy and just do the measurement at the antenna's base .... This looks good! Relatively sharp minimum, though at 6.9215Mc. Vertical seems a bit too long. Rs at frequency of resonance is 56.705. Will check 4NEC2 later to calculate theoretical Rs so as to get an idea about ground loss with this "2feetgroundrod+2radials"ground system. Come back later for more ...
August 24 2010
OK, Yesterday evening I found the time to model the antenna using 4nec2 (see: http://home.ict.nl/~arivoors/). Following to illustrations show the wire model and radiation pattern.
The impedance of the antenna simulated over perfect ground is shown below: note the impedance at Fres = 38.33 Ohms.
This implies the efficiency of the real antenna using only two elevated radials to be 38.33/56.71 = 68 %.
(= theoretical radiation resistance over perfect ground)/(measured resistance, i.e. radiation resistance + ground loss).
Will try to find some time next weekend to measure the antenna using four and maybe six or eight radials and verify this indeed lowers the measured impedance!
August 25 2010
This evening I ran some new tests - based on some new ideas.
One of my primary questions was whether or not my two radials were the proper length, and indeed resonant on the intended frequency of 7.050Mc? I decided to use my analyser to check this, by connecting it in such a way that it measures the impedance of the two radials against the 2feet ground rod. This proved a worthwhile approach - please see the impedance of these two radials against ground in the diagram below.
It is evident the set of two radials is definitely NOT tuned to the desired frequency. Instead, they resonate on about 6Mc. At the intended operating frequency they show an impedance of about 86Ohms, against an impedance of 56.59Ohms at their frequency of resonance.
Next step for me was to determine whether the vertical element itself had the proper length. Did this element resonate on 7.050 or not? To check this, I followed a similar approach and ran a test where the analyser measured the impedance of this element against the 2 feet ground rod earth. As one can see, this vertical element resonates at 6.625Mc (and NOT at 7.050Mc). The antenna using this 2 feet ground rod as an earth shows an impedance of 68.464 Ohms on this resonant frequency.