technology, amateur radio, scouting and me

New Antenna System – pt 2, What?

Posted on by • 0 Comments

See https://wp.me/p4spp6-ih for part 1

Garden Plan

I started off by seeing how I could get the best from using the fibreglass mast in the middle of the back fence. Unfortunately, the internal angle would be too small for a dipole erected in a horizontal Vee, so that left me with 13m to play with. Not much.

I could put a 30m dipole in (with the ends drooped) and could then have 20, 17, 15, 12 and 10m dipoles in a fan; but this would be a nightmare to set up, and the weight would probably be too much for the fibreglass mast . Also, conditions are still declining IMO, so I really want 40m (at least).

Luckily, a change in the garden provided me with more options. We’ve just put in a new shed in the bottom SE corner of the garden, and this gave me the idea of mounting a pole on the end of the shack (B on the plan) and another on the end of the shed (C on the plan). Add in a stub mast on the gable end (A) and I now have 36m to play with (albeit in an odd shape). Plus, the middle is roughly above the shack (see plan).

80m Doublet

This led me to the antenna I am actually building: an 80m doublet (with one drooped end) fed with ladderline from the shack. One side runs in a straight line from the gable end (A) to the shack (B). The other runs at right angles to the shed (C) and then back to the house (D). I’m going to model this to see if there are any reasons not to follow this path, but it looks good.

Tuner

The problem of course is that I really need a balanced tuner for this to work. Even without modelling, I foresee that the feedline will be unbalanced because of the odd shape. Putting this straight into an unbalanced tuner would be foolish. However, I do have the SG-230.

What I am doing is to install a 1:1 Guanella (Current) Balun on the end of the ladderline and connect the unbalanced output direct to the SG-230. The Balun should force the ladderline to be balanced and reduce radiation from the ladderline.

Provided the impedance swing is not beyond the capabilities of the SG-230 (and I’ve no clear idea of what those are), this should work. As the tuner will now be “hot” I’m installing an RF Choke on the coax from the tuner to kill any RF that wants to go down the outside of the shield.

The next part will cover building the antenna.