ODARS
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A Three Element Portable Beam for Two meters
Posted: 27 Apr 2020 07:24 AM PDT
http://www.odars.org.uk/a-three-elem...or-two-meters/
By ODARS member Brian Hawes, G2KQ
In this design, the radiating elements are cut from a steel tape measure.
Very thin and light. I have chosen three elements as a reasonable
portability/performance balance.
The overall length which is 500 mm is about half that of a conventional
design.
This design is intended for occasional outdoor field trips. It was not
built as an “erect outdoors and leave for ever” antenna.
If you don’t want the added complications of this short beam, you can still
make a conventional design with steel tape, just for light weight and ease
of construction. Lots of designs on the internet.
The significant feature of this design is the short length.
If the element spacing is reduced, the feed point impedance reduces.
The idea is to close up the spacing until the impedance is 12.5 ohms and
then introduce a 4:1 Guanella transmission line transformer between the
dipole and the feeder.
The antenna as shown in the second picture below weighs about 280gm/10oz
The starting point
Tape measure
Aerial with measuring tape elements
The elements can be folded to take up less space. Trussed up ready to go
out. Springs open when the croc. Clips are removed!
Aerial with measuring tape elements (folded)
3-Element Portable Beam Performance
Measured with a 2-meter feeder on the antenna to a Daiwa reflectometer and
then a 200mm lead to a Baofeng GT-5TP running 5 to 6 Watts
3-Element Portable Beam Performance
Construction notes
I have used the materials I had to hand. A fiberglass tent pole 530 mm long
for the main spar and bits from an old fishing pole for the element
supports. Others will necessarily have to find different answers.
The steel tape measure was a ‘Jumbo’ type, about 25 mm wide edge-to-edge
across the curl.
The steel tape, although very springy, is surprisingly easy to drill with a
normal HSS drill bit. It can be cut to length with ordinary scissors.
The tuning of the dipole is sensitive to even a few mm error. Start with it
10-15 mm oversize on the two elements and reduce each very carefully 2 or 3
mm at a time to tune.
Ready to go out into the fields!
Our intrepid signal searcher takes his construction out for a field test
Does it work?
On my first walk, one mile North of my home village of Cassington, near
Oxford, with open fields around me and about 90m a.s.l., I was able to get
into Reading (RD) and Swindon (WH) repeaters both about twenty miles away.
Contacts commented on my very good signals.
Next walk, I shall try direct calling on 144.5.
It outperforms my MFJ1714 which is IMHO the best telescopic for a hand-held.