Antennas

The antenna strategy focuses on simple and reliable resonant vertical antennas.
On 80, 40, and 30m RX-Beverages are foreseen, perhaps some loops.
4NEC2 is used to design the antennas and simulate/forecast their behavior ‘on site’.

Eucalyptus and/or bamboo poles (the higher the better) are envisaged as antenna masts.

80m: T-antenna (12 – 15m high) with sloping top loads matched against Zp (‘Y’).
Either strips of chicken fence (said to be ‘available’) or resonant elevated radials
serve as ground system. Note: 80m was not activated during this trip.

40m: Deltaloop (fed ~¼λ from the apex) or a ¼λ vertical with at least
two elevated radials (preferably more).

30m: Vertical dipole array (VDA) when the poles are high enough, otherwise a deltaloop.

Advantages of a deltaloop are: minimum of material, no critical ground system, can be forced towards Z = 200 + j0 Ω to obtain a (broad band) match with a 4:1-balun, good radiation pattern (take off angle is a few degrees lower than a ¼λ vertical above same ground), and very simple (mechanical) construction.
Having built several 40m deltaloops in various places/conditions before, this antenna works very well for DX and is built within no time.

Below a ¼λ vertical with 4 elevated resonant radials and a properly fed deltaloop above identical ground are modelled. (click on image to enlarge)
deltaloop-vs-vertical-40m

20 – 10m: Vertical dipoles arrays for each band (click on images below to enlarge in new tabs).
DSCN9989a  DSCN9563c

6m: Many requests for 50 MHz have been received.
To manage expectations in advance: if… I’m able to build a Yagi, I give it a try.
Hartmut DG7YBN, kindly designed a non critical 5el.Yagi especially for me : -)
Note: due to lack of propagation not activated during this trip.

RX-Beverages (depending on the QRN) are foreseen.
Negotiated lengths are 80m (2λ @40m) as the local population feared the wires
running in the field, and in/next/through to their village.
Will experiment there with ‘open Bevs’ (‘bi directional’) or terminate them,
provided there’s enough space to compensate ‘lost’ directions (i.e. 8 Bevs)
Directional loops may be alternatives (e.g. K9AY, FO0AAA, TX3A etc..)

Malawi great circle map (tnx to NS6T) with VDA horizontal radiation pattern (click on image to enlarge)

7qnl-azimuth-4

One thought on “Antennas

  1. Very nice dxpedition and web site. I like the absence of ‘gung-ho’ machoism and the extremely sensible reliance on delta loop antennas – perhaps the best simple wire antenna possible.

    Good luck with the operation, and thanks for the rare 7Q QSO, despite a rather poor 12m band!

Leave a Reply to John Rowlands Cancel reply

Your email address will not be published. Required fields are marked *

Help

WordPress theme: Kippis 1.15