An Inverted L For HF and
Matching Unit For 40 and 80m
I moved to a new QTH in Reading early in 2010, the house has a small garden which is only 35 feet long and
after trying various ideas I decided to build an Inverted 'L' fed from the base of the vertical section. The vertical section utilises 16 feet of a telescopic aluminium HF antenna with a UHF socket at the base (see image) and is mounted thee feet off the ground. Quarter wave counterpoises for all bands from 80m to 6m are attached to the base mounting bracket. The horizontal section of the antenna is 20/02 PVC covered wire attached to the top of the vertical section and runs to the house.
(Click on images for a larger version.)
The overall length of the antenna is approximately 51 feet and the amateur bands from 20m to 6m are easily matched with a standing wave ratio of 1:1 at the rig using the built in automatic matching unit in my Kenwood TS480SAT. The length of the antenna results in a SWR on 40m and 80m that the Kenwood cannot handle so an external antenna matching unit was required.
After some experimentation I decided to build a Pi network with a tapped coil for 80m and 40m. The matching circuit can be switched out so that the TS480 connects directly through to the antenna socket.
A fan cooled dummy load can also be switched into circuit, either directly or via the matching network. The dummy load and fan are relay operated so a connection to a 12 volt DC power source is required to use it.
1. Select 80 or 40m.
2. On receive adjust C2 for Maximum signal, C2 gives a wide peak on 40m, not so wide on 80 but the matching unit easily peaks on both bands. Try adjusting C1, it will probably have no discernible effect on receive and is best left half open.
3. Initially check the standing wave ratio on low power, it will be around 1:1 on 40m and 3:1 or less on 80. Now adjust C2 for minimum SWR, adjusting C1 will have a small effect.
4. SWR on 40m should be 1:1 and around 1.2:1 on 80m.
The combination of the inverted L and antenna matching unit allows me to operate from 6m down to 80, obviously on 80m the antenna is not very efficient but I am able to work all over the UK on that band. The combination works well all over Europe on 40m, probably further if I was up at the right time!
From 20m up the AMU is switched out of circuit and the antenna works very well on all bands although I have not yet had a QSO on 6m. The TS480 auto matching unit brings the SWR down to 1:1 on all bands.