Friday, 8 July 2016

Is Your Miniwhip Too High?

The Mini-Whip at University of Twente's (Netherlands) Remote Receiver

A recent posting to Yahoo's ndblist Group described an interesting experiment by Dirk Claessens regarding the signal-to-noise ratio (SNR) versus height of his PAØRDT active whip. Dirk's tests were posted on Yahoo's Navtex DXing Group where some further interesting discussion seemed to confirm his findings.

Here is what Dirk discovered, backed-up with his graph data, clearly pointing to the 'ideal height' at his location ... and probably yours as well.

Hi all,

You may recall that to get rid of a source of QRM, my whip is now hanging from a rope-and-pulley system, about 5 .5 mtr from the house, at a height of 7 meter agl.
As the height of the whip can easily be changed, this is an ideal situation to test the behaviour of the whip wrt noise levels, optimal height etc..
I had done this test before some 4 years ago, but not very precise. Time to do it again, and documenting it.
What I also wanted to check, is if there were any noise sources of the own house possibly reaching the whip.
(how far does the "noise bubble" go in function of the height?)

The Perseus was tuned to 549 KHz Deutschlandfunk Nordkirchen, the station closest to 518.
2 markers were set, one to the signal, and a reference marker in a quiet spot nearby on 543 KHz, to get a reading for the noise floor.
The whip was then lowered in "1 meter each minute" steps, readings were taken and written to the marker file.

First the absolute values were plotted.
Note that: blue = noise floor, red = signal, and that the left and right axis scales have identical spans of 19dB, but are shifted, in order to get a compact graph.
We see that for a delta height of 6 meters:
-the noise floor goes up ~8dB, or 8/6 = 1.3 dB/m, almost linearly.
-the signal goes up ~14 dB, or 14/6 = 2.3 dB/m, clearly curved and showing a maximum at ~5..7 m.
The continuous lines are polynomic (2nd degree) regression lines.

The noise on the measurement values seems to increase with decreasing height.  Was this caused by my body standing under the whip, and near to the whip for the lowest measurements??

What really matters of course, is the signal over noise value, this is plotted below:

-Within a narrow 1 dB band, the curve shows a clear optimum in the region of 4..7 m agl, a familiar value often given as optimal by Roelof.
-The measurement was performed during the day and thus with ground wave propagation. As the whip is truly omnidirectional, I cannot see a reason why the behaviour would be different at low angle DX signals.
- At 5.5 meter from the house, the whip seems to be outside of the "noise bubble"

and later, following discussion:

I have just checked the noise floor again at 518 (with no signal present)

Perseus set to 125 kS/s, Span/RBW 25/30.5
Shield grounded: -125 dBm
Not grounded:  -110 dBm

That's a whopping 15 dB difference!
I have also buried the coax ~20cm deep from the grounding point to where it enters the house.

The ideal height was also that recommended by Roelof, PAØRDT, originally and points out that one of antenna-building's most sacred commandments ... "the higher the better", is not always true!

I have often recommended this simple antenna for those looking for a very effective yet low-footprint receiving antenna for use on the LF and MF bands.

Much more information about Roelof's popular miniwhip may be found in previous blog discussions here.

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