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Activation report DM/BW-846, Burg Hohenzollern

Hi everyone,

I just completed the activation report of DM/BW-846. It can be found on my Blog:

As always it was lots of fun. Thanks for the QSOs and cu from the next one.

73, Roman - DL3TU

It is not unusual that the KX3ATU can also match an antenna structure consisting of an impedance coupler and a radiator of this length to 60m without an explicit counterpoise. The same will also be possible for other frequencies in the range of 3.5-30 MHz, hi.

However the lengths of your trapped 20/30/40m EFHW appear somewhat unusual for 30 and 40m. Based on these current lengths, the resonances are believed to be at about 9.8 and 6.3 MHz, respectively, which is much too low.

Could it be that the integration of the traps was not done correctly as follows:
With the impedance coupler connected directly to the antenna analyzer perform the following steps:

  1. Cut the 20m radiator to the 20m resonance of your choice
  2. Resonate the 30m radiator by cutting only the 30m “tail”
  3. Resonate the 40m radiator by cutting only the 40m “tail”

When performing the trap integration in this way, you will end up with radiator lengths of approximately 12.5-12.6m for 10.1 MHz and 16.7-16.8m for 7 MHz. The shortening is a “side effect” of the traps.

Have fun with your EFHW!

Hello Heinz,

thanks for your hints!
It is also my experience that the KX3ATU will match almost any wire or wire/ L-C- combination as long as it is not extremly short. The radiation pattern - of course - is another story but on 60m I did not expect working stations outside a circle of, say 300…400 km radius.

Regarding my EFHW I noticed that the wire length (especially on 40m) is somwhat different from other designs, e.g. Pedro’s, CT1DBS, whose design served as an inspiration for my own EFHW.
The wire length was cut in the way you described. Tuning the length of 20m, then adding the 30m trap and “tail”, checking the impact on 20m. Next, tuning 30m at the 30m “tail”. Adding the 40m trap and “tail”, etc.

While tuning the EFHW I found that the heigth of the wire- ends above ground has a big impact on IZI. Especially on 40m, where IZI would drop as low as 500 Ohms (instead of ~ 3000 Ohms) if both wires end approx. 1m above ground. This effect is less pronounced on 30m and almost invisible on 20m.

May I ask what your typical setup looks like?
Thanks for the discussion.

Roman - DL3TU


A welcome feature of resonant half-wave antennas is that the high-impedance (non-radiating) ends are very insensitive to adjacent objects including ground proximity.
Therefore, I can not confirm your assumption that the impedance decreases strongly with decreasing height of the antenna ends.

On the contrary, your measurement of an impedance of about 500 ohms confirms well that the voltage node is unfortunately not located at the feed point of the 40m EFHW antenna wire. In the case of your antenna it is obvious that the wire length for the trapped 40m EFHW is slightly more than 2m too long.
But wait with cut off, as noted in my previous post the wire length is also somewhat too long for the trapped 30m EFHW. So adjust the 30m “tail” first.

My typical 20/30/40m EFHW setup differs only slightly from one summit to the other:
The antenna attachment point in about 5-6m height (6m mast), arrangement as inv-L or inv-V, apex angle not below 90°, feed-point (impedance coupler) at the height of a walking stick. The height of the far end depending on the possibility of attachment, but at least at 1.5-2m.

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I’d like to add the following in order to close the case:

After discussing the design with HB9BCB (offline for easier communication) I made the following changes to my antenna:

  1. Attach the Unun to the GRP- pole at a heigt of approx. 1.5m (before it was lying on the ground. With this measure our setups look very similar from a mechanical point of view)
  2. Tune the wire length for minimal SWR (this is what HB9BCB did, while I was aiming for XL = XC = 0. Unfortunately both frequencies can differ several 100 kHz on 40m.)

The wire length for both designs is now similar. The remaining deviations are due to different L- values of the inductors used in the Traps.

SWR on 20m is 1.15 : 1 and on 30m 1.5 : 1. On 40m it’s slightly higher: 1.9 : 1, because IZI is in the range of 30 Ohms. This behaviour is still not fully understood and requires deeper investigation.

The updated article can be found here: https://dl3tu.wordpress.com/2017/04/24/efhw-dipole-with-traps/

Thanks, Heinz for discussing!