Andy,

Pretty much any antenna tuning unit (ATU) will work to enable you to use
this antenna on 80 metres. Obviously efficiency will suffer, but you will
get a signal out. The antenna needs extra length or more inductance to bring
it to resonance on 80 metres.

You could experiment with an old metal variable capacitor (around 350 pf)
and a two inch diameter coil with about 12 or 15 turns and a fly lead to
short out the turns. Old mains wiring cable is a good source of wire for
this. Connecting this up in the coax feed in an 'L' configuration will allow
a surprising range of frequencies to be brought to resonance on your
antenna. Check out Google for some ideas on how to wire it up. A metal air
spaced variable should be good for powers up to a hundred watts without
arcing problems.

There will probably be a lot of articles about an unbalanced feed causing
the coax to radiate. Winding the coax into half a dozen 6-inch turns
fastened with cable ties at the feed point of the dipole will help avoid
this problem.

Or, you could add a couple of coils, one each side of the dipole centre to
increase the physical wire length in the air and bridge out the coils to
work at 20 metres, but this would involve raising and lowering the antenna.
You could use a couple of relays to perform the job, but it all starts to
get rather more complex than simply using an ATU.

Regards

Mike G0ULI

"Andy" wrote in message
...
Hi,

I have a 40 meter centre feed dipole cut for 40 and feed direct with 50
Ohm coax. The house end is connected to the centre of the coax and the
pole end is connected to the braid. Seems to work best this way round.
The antenna is fixed tight to the house and the other end is tight to the
metal pole in a straight-line, diagonal across the garden, both ends are
insulated. The garden is about 64 feet give or take a couple of feet.

I cannot drop the ends down because of the house and metal pole at the end
of the garden being so close. I can only feed the antenna with coax. The
neighbours are OK with coax but not the ribbon feeder hanging down and I
don't want to upset them if I can help it.

My question. What are my options to enable me to operate 80 and 40m on
this same antenna wire?


73 Andy