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 40m to 10m
– 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 22ft long doublet (2x 6.7m), 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 43ft (13.1m) Inverted-L with a few “short” radials. There are many articles on the web about the performance of a 43ft long vertical antenna. It’s a good performer on the lower bands (40 – 15m) but turns into a cloud warmer (high elevation angle) on higher bands (12 and 10m). I already had a 7m long SOTApole and I wondered what would happen if I use a 6m long vertical wire section and pulled the remaining 7.1m to the side.

Construction details:
Antenna:
13.1m stranded copper wire
4mm banana plug
Acrylic glass (Plexiglas) insulator at the end
10m reepschnur and 1 peg
BNC to 4mm adaptor (KX3 comes with a BNC connector)
Radials:
Several “short” radials (5x 4m in my case) with stackable 4mm banana plugs. Somewhere on the Internet you can read if you are using only a few radials use “short” ones.
Pole:
7m 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: 5mS/m)
On all bands (even 6m) 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.

40m: 3D Far Field Plot

30m: 3D Far Field Plot

20m: 3D Far Field Plot

17m: 3D Far Field Plot

15m: 3D Far Field Plot

12m: 3D Far Field Plot

10m: 3D Far Field Plot

6m: 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.

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

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

The end of the 13.1m 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 10m 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 4mm 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 6m above ground. Red line: my actual terrain and light blue I moved the antenna 100m back on flat terrain.

20m: blue line shows a dipole 6m 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 5m, resp. 10m back.

20m: now I move the antenna 50m, resp. 100m 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 40m instead of 20m. 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!

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

13.15m long Inverted-L with 5x 4m 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