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. 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.
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.
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 🙂
– side-by-side comparison between Inverted-V and -L