To fight very likely QRN in Africa for 80, 40 and 30m (open) Beverages are envisaged.
Targeted length is 2λ @80m. I plan to roll out four (open) Beverages to obtain 8 ‘directions’.
A Beverage antenna is actually a transmission line above ground. Its impedance
depends on the wire diameter and wire height above ground.
With heights around 1 – 2m and wire diameters of 1 – 2mm the impedance amounts ~500Ω.
When using 50Ω feedline the transformation ratio has to be 10, but 9 is also ok.
It’s not that critical. For 75Ω feedline transformation ratio is around 6.7.
What is more critical is that the received EMF is not dissipated in the transformer itself!
A general rule of thumb is that the reactance of the windings has to be 5x the connected
impedance. I always use 10x, thus XlBev = ~5000Ω, XlRX = ~500Ω for the lowest
frequency, i.e. 3.5 MHz.
I had some binocular cores lying around, with a measured Al = ~0.8 μH.
Stacking three of these cores results in an Al of ~2.4 μH. Rumours tell that stacking
cores improves S/N as relatively ‘more’ copper is ‘shielded’ inside the (binocular) cores.
Since Xl = ωL = 2πfL -> L = Xl/2πf = 5000/(2π*3.5E6) = 227 μH
Further, μH = Al*N² -> N = √227/2.4 = 9.7 -> 9 turns will do for the
secondary (Bev) side, 3 turns for the primary (RX) side.
For 75Ω feedlines 10 turns secondary (Bev side) and 4 turns primary (RX side) suffices.
I used this transformer in conjunction with a 100m long West Beverage during the 2014
CQ160 CW contest, and it delivered me quite a few Caribbean stations/multipliers, which
were barely/not audible on my reversible NW/SE and NE/SW 200m long Bevs.
However, my ‘biggest discovery’ was during the PACC 2014 contest where this
‘West Beverage’ was left open at the far end. It received amazingly well on 40m, and
outperformed the TX-deltaloop (of course) and ’160/80m’ reversible Beverages
in more than a few cases. During the contest it became my default 40m RX-antenna.
Nice contact was with ZL1BVB on 40m. Listen here.
Pictures of the transformers are depicted below (click on images to enlarge in a new tab).
The ‘West’ Beverage (feed point) itself is shown below (click on images to enlarge in a new tab).