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.
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)
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.
7 m SOTApole with top antenna insulator and guying kit (rarely used)
With MMANA-GAL basic v. 188.8.131.52 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.
These value can be matched easily with the KXAT3 tuner inside the KX3.
– side-by-side comparison between Inverted-V and -L