Doublet Antenna
In early December 2022, I installed this inverted V doublet.
The location of the centre-fed radiator is highlighted in yellow, and the balanced transmission
line ('feeder') is emphasised in white.

The doublet has a top of 2 x 23 m. The centre feed point is supported at 11
m above ground by a copper beech tree.
The antenna radiator is made of PVC insulated multi-strand tinned copper wire (19
strands of 0.4 mm diameter) and was originally fed with 30 m of 300 ohm twin 'ribbon' feeder.
In April 2024, I replaced the ribbon feeder with
home made open wire
feeder to improve the stability of the match when it rained.
Ribbon
feeder suffers from a change in velocity factor when it gets wet, altering
its electrical length and therefore requiring adjustment of the antenna
matching unit to restore the SWR. Open wire is much more stable when
it rains. No re-tuning of the antenna matching unit is required when
it rains.
Normally, an
MFJ-974HB balanced matching unit is used to do the transform to unbalanced 50
ohms. But good results have also been obtained when using a
home made
switched balun in conjunction with a regular unbalanced antenna
matching unit. Having a balun that is switchable between 1:1 and 4:1
impedance transformation ratios can help to provide a better match with
antenna matching units that have trouble matching a very high, or a very low
antenna impedance.
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