| |
G8MNY > TECH 23.03.05 22:14l 116 Lines 5184 Bytes #999 (0) @ WW
BID : 38151_GB7CIP
Read: GUEST
Subj: A Nest of Dipoles for HF
Path: DB0FHN<DB0MRW<DB0WUE<DK0WUE<SP7MGD<RZ6HXA<IK1ZNW<GB7CRV<GB7CIP
Sent: 050323/1947Z @:GB7CIP.#32.GBR.EU #:38151 [Caterham] $:38151_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Sep 04)
Just starting out on HF I'd thought I would try the nest of Dipoles approach.
HALYARDS
I had 2 halyard points one @ 12M on a chimney mounted pole on the house & the
other 50M away 16M up a tree. Each one has a simple pulley that had the bearing
well greased up. (on the really simple open pulleys, I found that a piece of
suitably cut tin plate could be added to each side of the pulley block wheel to
stop the rope fouling & jamming the pulley).
The halyards are threaded up to be continuous loops so that if the aerial
breaks then it can still be lowered. Also for the tree end there is a put up
rope that was placed over a high branch to hall the halyard pulley up.
Polypropylene rope (eg 6mm draw rope) is OK, but does have a short lifetime of
only a few years in the sun before it disintegrates! Never trust it for
dangerous work where people could get hurt!
Tree.....Pulley Spring
: O.......eee.ì----
: : / Insulator
: : /
: : / Pull Down
: : / Loop
: : /
: Tension
Tie Off Weight
TENSIONING
To maintain & control tension I used a 10Kg heavy weight (old transformer). But
I found this was not too effective, because the mass & the pulley friction
would be slow at keeping the tension, resulting in the middle of the aerial
bobbing up & down with the wind. And with this type of aerial that was a real
knotting problem, so I tried adding some long coil springs out of a chest
exerciser (was not mine). I painted these to reduce further rusting & one
mounted each end just before the insulators. This has stopped the bobbing
around as about 0.5M stretch is available with no slow mass to allow for the
wind & tree sway. A bunjee may have similar performance but a short lifetime!
THE AERIAL
Common
Halyard Feed Point Steel 80m Dipole Halyard
....ì---------------------------------ìì-------------------------------ì......
String \..._________--------/³³\--------________.../ Insulator
40M Dipole\......___---'''/³³\'''----____....../
\...__-/ ³³ \-__.../ 20M Dipole
10M'....³³....'
Dipole ³³
³³ 10M of thin
----___ Wire ³³ Balanced
...... String/Rope ³³ 75ê Feeder
³³
8 turns of feeder on Ferrite Ring> CC
ÀÁÄPL239------ 50M UR67 Coax to rig ---->
DIPOLE LENGTHS
The lengths I used were:-
Dipole Length Freq Band Material
Ft M MHz M
125'3" 38.22 3.7 80 Plastic coated Tinned Steel 2 mm dia.
68'0" 20.750 7.1 & 21 40 & 15 Plastic covered Copper wire 2.5mm(mains)
32'11" 10.028 14.22 20 Plastic covered Copper wire 1.5mm
16'3" 4.968 28.7 10 Plastic covered Copper wire 1.5mm
As there is a lot of force on at the central insulator it needs to be designed
to handle it. I used as large 60A connector block (screws well greased up!)
with a strain collar above, that the 80 & 40M dipole had to pass threw. The
smaller dipoles & feeder entered the connector block from below. All wires were
bared & went under both screws of the connector block.
In the 80M dipole I put 2 twisted loops about 3M before the start of the 40m
dipoles for the string to attach. Each smaller dipole is suspended below with
2 pieces of string with slip knots so that the hang can be adjusted.
The dipole ends have to be well spaced from the above dipole, so the 10M one
ends up more like an inverted V.
SWR tuning for each band is to cut the length of that dipole for wanted
frequency. With some extensive trial & error, it is possible to get usable
match on all bands. The interaction of the aerials is minimised if the spacing
is even.
FEEDER
I used 10M of 75ê thin balanced feeder to a ferrite choke balun & then 50M of
UR67 along a fence to the shack. The weight of the coax is as it goes up to the
balun & feeder is supported by another string from a side tree, this reduces to
a minimum the dipole central load.
Dipoles -ìì-
³³
\\
³³ Side
\\ ;Tree
³³ Balun._:
\\___ /°/ \
-----/ \ _____Coax
Drip Point
As the balanced feeder is just long enough to reach the ground, that is
convenient place to put a plug & socket just after the choke balun. This
enabled testing & garden use.
IN USE
I find it works without an ATU fairly well with the SWR typically less that 2:1
on the useful parts of the bands, but an ATU would help with the higher SWR at
the ends of some of the bands. The loss due to 2:1 SWR assuming your rig is OK
with it (most are) may be much less than the addition of an ATU, and there is
no need for retuning per freq or band just operate!
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
