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. One good reference is “BCC-Handbuch 4. Auflage” on page 2-13 (only in German available). For 16 radials make them 0.05 lambda long.
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.

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

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.

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.

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.

Update: 20210808
As mentioned by Gerard, VK2IO on https://reflector.sota.org.uk/t/dx-s2s-attempt-easter-saturday-31st-march-2018/17122/42 he added a link with an extra 5 m section to give better 80 m performance. One of my random radial is 6.34 m long which I will reuse as extension for my radiator: 13.1 m + 6.34 m = 19.44 m. That should work fine for 80 m 🙂

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