The MHF-1A true Multiband H.F. off centre fed wire antenna With
these benefits:1. Covers 160 Mts - 10 Mts, Top Band to Ten, 1.8 -
30 MHz - including Broadcast Bands. 2 . No A.T.U. Required on
30mts, 20mts, 17mts, 15mts, 12mts, 10mts. 3. Wire antenna can be
bent to almost any shape to fit available space. 4. No ferrite
needed. 5. Can be slung out of a window - which is how I made mine.
6. Totally forgiving construction techniques allowed. 7. Fed with
50 ohm coax to keep local RFI to a minimum and the rig happy. 8.
Can be used 'end fed' style or dipole style layout. How can such a
simple antenna acheive all this ? For the ham bands, the feedpoint
impedance for the given dimensions of the antenna wire will,
broadly speaking, be around 200-300 ohms. The antenna is fed at
this convenient point using a broadband, air spaced UNUN or
unbalanced to unbalanced transformer. The unun transforms the
impedance to one suitable for modern coax fed rigs. What does this
thing look like ? Here is the schematic diagram:
Whats that coil near the middle ? This is the unbalanced to
unbalanced broadband impedance transformer, or UNUN. There is no
ferrite to cause losses. It is a bifiliar winding airspaced on a 1
inch former. 13 turns of speaker wire will do nicely. I wound my
balun on a cardboard tube from the centre of a cooking foil roll.
Wire used is not critical, the thicker the wire the more the watts
it'll handle. I used speaker wire at 24/0.2 gauge ( 24 strands 0.2
dia wires) does'nt even get warm here at 50 Watts CW mode. The
winding details are given below:
Here's a photo of my own coil - its just speaker wire on a
cardboard former wrapped in electricians tape:
The MFH-1 antenna can be used sucessfully on topband, 80 and 40
with the use of an atu. No atu required 30 mts - 10 mts ! Although
I sometimes use my ATU to fine tune the last ounce of energy from
the rig PA ! How did you get all that wire in such a small space ?
Heres my layout below. The two wires can be strung out in anyway
that suits. Dipole layout would be ideal, but 'end fed' style would
work too, I know as thats how mines laid out. In fact for true
portability how about one run of speaker wire for the whole thing ?
Just strip back the excess length on one of the speaker cables
!
Dimensions. One leg at 25.9 metres. The other leg is at 18.3
metres. These two lengths of wire can be laid out dipole fashion,
for those with acreage, or run parallel to each other, for the 'end
fed' style, or laid out anyway that suits. I followed the contour
of my yard, out the top window, sloping down to the yard fence at
6', across, around, and back up for the long leg ! I kept a 2 - 3
feet space between my wires. Does it work ? 20 Watts qrp - 17 mtr
band, first run:
A note on the coax feeder. It is possible to create a RF choke
to further reduce RFI. I did this by making an air spaced coil of 6
inch diameter and 8 turns of the coax cable itself as seen in the
picture: The choke sits at the antenna end of the coax run.
Good luck, enjoy, and good dx !de g0ier op Brian Feb 2010
