0 – My SOTA Antenna

I was asked several times (even in CW over the air) what kind of antenna I was using and this post shall give an answer.

My requirements for a SOTA antenna are as follows:
– TRX with automatic antenna tuner (in my case Elecraft KX3 with KXAT3)
– multiband from 60 m to 10 m
– instant QSY without leaving the operating position
– simple to construct and to errect (no duct tape, no Velcro)
– survives high winds on a mountain top

For all my SOTA activations I used two different antennas. Initially I started off with a 2x 22 ft long doublet (2x 6.7 m), a CQ553 450 Ohm feeder and a 4:1 or 1:1 or no balun at the TRX. I soon found out that not all mountains in HB9 offer enough space to deploy a doublet and therefore I switched to a 43 ft (13.1 m) Inverted-L with a few “short” radials. There are many articles on the web about the performance of a 43 ft long vertical antenna. It’s a good performer on the lower bands (40 – 15 m) but turns into a cloud warmer (high elevation angle) on higher bands (12 and 10 m). I already had a 7 m long SOTApole and I wondered what would happen if I use a 6 m long vertical wire section and pulled the remaining 7.1 m to the side.

Construction details:
Antenna:
13.1 m stranded copper wire
4 mm banana plug
Acrylic glass (Plexiglas) insulator at the end
10 m reepschnur and 1 peg
BNC to 4 mm adaptor (KX3 comes with a BNC connector)
Radials:
Several “short” radials (5 x 4 m in my case) with stackable 4 mm banana plugs. Somewhere on the Internet you can read if you are using only a few radials use “short” ones.
Pole:
7 m SOTApole with top antenna insulator and guying kit (rarely used)

With MMANA-GAL basic v. 3.0.0.31 I ran a few simulations against my Inverted-L (Ground: Real, Dielectric: 13 and Conduct: 5 mS/m)
On all bands (even 6 m) this antenna offers omni-directional pattern and low take-off angle.

Geometry of the Inverted-L

Results for the calculations. All R+jX can easily be tuned by e.g. KXAT3.

40 m: 3D Far Field Plot

30 m: 3D Far Field Plot

20 m: 3D Far Field Plot

17 m: 3D Far Field Plot

15 m: 3D Far Field Plot

12 m: 3D Far Field Plot

10 m: 3D Far Field Plot

6 m: 3D Far Field Plot – even 6m looks good.

Following pictures show a few details on the construction of my 13.1m long Inverted-L and setup on SOTA mountain tops.

4 mm banana plug, 13.1m stranded wire, acrylic glass insulator and 10m reepschnur (3mm diameter).

4 mm banana plug. I prefer a screw-version and wire-end sleeve (Aderendhülse) for the stranded wire.

The end of the 13.1 m wire. On the left a top antenna insulator (slide it over the top section of the pole), in the middle a lustre terminal (Lüsterklemme) followed by an acrylic glass insulator and 10 m reepschnur (I use a figure of eight (Achterknoten)).

Move some of the wire insulation into the wire-end sleeve (Anderendhülse). It will survive vibrations much longer.

5 “short” radials on a wire winder. After the activation simply take all 5 plugs as a stack and wind them onto the winder.

The stackable 4 mm plug I had on hand.

HB/AI-012: After the cow tasted one radial I replaced them inside the fence 🙂

HB/OW-024: Kind of elevated radials.

HB/NW-018: Most common setup on a SOTA hill.

HB/VS-102: Luxury setup – wooden cross as second support for the Inverted-L …

HB/VS-102: … and a table for the shack and handrail as SOTApole support.

GW/SW-001: Inverted-L dancing in high winds. Pull the wire in the direction of the wind.

GW/SW-002: In snowy conditions use high visible wires and mark/paint alloy pegs as well. Did you ever search for alloy pegs in snow …

GW/SW-014: Another wintery setup. Instead of Inverted-L a kind of Inverted-V is also possible.

How does the Inverted-L perform?
I used this antenna on more than 100 activations and I never failed to qualify a summit. It survived high winds in GW and winter conditions in GW and HB9. DX wise I worked VK, HS, K, VE, UA9, … and all over Europe in CW using max. 10 Watts. So far I didn’t perform a side by side comparison between an Inverded-V and -L. Depending on the ground (soil, granite, …) the impedance of the antenna changes but engaging the ATU TUNE button on the KX3 for a 2nd time led to a low SWR.

HF Terrain Analysis (HFTA)

HFTA is part of the ARRL Antenna Book and let you simulate take-off angels of horizontal (no verticals are supported) polarised antennas against your terrain. I was using terrain profile of my Contest-QTH direction 30 degrees for the analysis.

This is the terrain profile QTF 030 with my antenna in the top left corner and 6 m above ground. Red line: my actual terrain and light blue I moved the antenna 100 m back on flat terrain.

20 m: blue line shows a dipole 6 m above ground over flat terrain. The red line shows the same dipole over the terrain shown in the first picture. The green resp. light blue line show what happens if I move the dipole 5 m, resp. 10 m back.

20 m: now I move the antenna 50 m, resp. 100 m back. For take-off angles below 10 degrees I still see an advantage against flat ground, above 10 degrees my terrain behaves like flat ground.

Now the same simulation on 40 m instead of 20 m. Here you can clearly see the benefit of a hill top QTH. BTW: 6.5 % of all QSO within Europe are using a take-off angle of only 2 degrees!

40 m: and again what happens if I move my dipole 50 m, resp. 100 m from the drop off.

Inverted-L against AIM 4170C

I setup my SOTA Inverted-L at my Contest-QTH and used the AIM 4170C to measure the impedance of the antenna.

13.15 m long Inverted-L with 5 x 4 m radials on the ground.

Just below the SOTApole the AIM 4170C is located and is connected to an USR-WIFI232-602 which acts as a RS-232 to WLAN converter. The software application for the AIM 4170C runs on the PC conveniently sitting in the shack.

Freq(MHz)	Rs	Xs	Zmag
5.35	41	-118	125
7.03	63	177	188
10.12	2’451	3’132	3’977
14.06	61	-412	416
18.08	208	150	257
21.06	1’324	-138	1’331
24.90	147	-344	374
28.06	214	27	216
50.10	135	-6	135

These value can be matched easily with the KXAT3 tuner inside the KX3.

to do:
– side-by-side comparison between Inverted-V and -L