| |
G8MNY > TECH 13.03.23 09:06l 208 Lines 9714 Bytes #999 (0) @ WW
BID : 42693_GB7CIP
Read: GUEST
Subj: A Nest of Dipoles for HF
Path: IW8PGT<I3XTY<IZ3LSV<ED1ZAC<GB7CIP
Sent: 230313/0754Z @:GB7CIP.#32.GBR.EURO #:42693 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Dec 21)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
Just starting out on HF & with a large garden, I'd thought I would try the nest
of Dipoles approach (Spider web), rather than the simpler multiband doublet
(e.g. G5RV). The doublets if they have a good Q will in theory have a more
complex & narrow band Zs & different polar diagrams per band than the dipole &
therefore my be a less useful aerial. The ganged up dipoles in a nest should
give much the same performance as a single dipole for each band, as the non-
resonant dipoles will only present a few % loss as each of their Zs should be
much higher.
³ ³\ / ³\ / ³ \ /
³\ / ³ \ / ³ \ / ³ \ /
³ `\_ _/' ³ \_ _/ ³ \ / ³ \ /
Zo´ ""Ä-Ä"" Zo´ "Ä" Zo´ "Ä" Zo´ `-'
ÀÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ ÀÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ ÀÄÄÄÄÄÄÂÄÄÄÄÄÄ ÀÄÄÄÄÄÄÂÄÄÄÄÄ
Fo Fo Fo Fo
DIPOLES M U L T I W A V E L E N G H T S D O U B L E T S
º
º º
/"~"\ /"~"\ /~"\ º /"~\ (~"\ º /"~) _ /~\º /"\ _
| |º| | "-.__\º/__.-" _.Ä.\ ³º³ /.-._ (_\`\.³º³./'/_)
³ )º( ³ _>º<_ (_ :>º<:_ _) _=`>>)º(<<'=_
| |º| | .-"" /º\ ""-. "Ä'/ ³º³ \"-" (_/'/'³º³`\`\_)
\,_./ \._./ \__/ º \__/ (__/ º \__) \_/º\_/
2 LOBES 4 LOBES 6 º LOBES 8 º LOBES
º
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 by falling off the side of the
wheel).
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 one time
pull up rope that was placed over a high branch to hall the halyard pulley up.
Polypropylene rope (e.g. 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
: : / Loose
: : / Pull Down
: : / Loop
: : /
: Tension
Tie Off Weight
INSULATORS
These need to withstand the tension, be as light a possible, be UV stable &
still insulate when wet. In practice with a metre or so of modern synthetic
rope at the ends & pure rain water (not sea spray) end insulators are not
strictly required.
The plastic & ceramic egg type insulator are designed to be in compression with
the tie & wire looped over each other separated by the egg, NOT the tie & wire
looped through each end hole keeping it in tension.
.===-.. ... =:\
/ _ `\\. /'_ """\\\
==////=======) ³ >>\\\\\==== ======/////===) ³ >>\\\\\=======
\ " .// \ " __.///
`====:' `""" =:/
R I G H T W R O N G
Black plastic egg insulators are ideal for the middle of the 80m dipole of this
design where low weight is important & due to the low the Z the insulation
requirements are minimal. Ceramic ones are best used at the ends were their
weight is less important.
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!
Electric Cow fence springs (0.2-2M used around gates) are suitable for
tensioning single light aerials, but heavy ones like this possibly not.
THE AERIAL
Halyard Feed Point Steel 80m Dipole Halyard
úúúúì---------------------------------ìì-------------------------------ìúúúúú
String `ú.._________--------/³³\--------________..ú' Insulator
40m Dipole `'úú...___---'''/³³\'''---___...úú''
`úú..__-/ ³³ \-__..úú' 20m Dipole
''úú.³³.úú''
Dipole ³³
\/---___ Wire ³³ 10m of thin
³³ Balanced
'`úúú... String/Rope ³³ 75ohm 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
In the 80m dipole I put 2 twisted loops about 3m before the start of the 40m
dipoles for the string to attach to. 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 the 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
(string lengths) are even!
THE FEEDPOINT
As there is a lot of force on at the central insulator it needs to be designed
to handle it. I used a black plastic egg insulator on the top 80m dipole & hang
a large 60A connector block (screws & wires well greased up, & with water
sealant on the top!) 5cm below it, make all the wire connections long enough to
go under both screws. With solid wire ends, these can also be bent & hooked
over the connector edges (top & bottom) to add more support.
.===-..
/ _ `\\. 80m Dipole
=====////=======) ³ >>\\\\=======
\\ \ " .//' //
`\\ `===-' //'
`\\ //'
`\\ //' 40m Dipole
=============\\ //=============== ( & 15m)
ÚÁÁÄÁÁ¿\ Water
Well Greased³ 0 0 ³ `Sealant
connector ³ 0 0 ³
ÀÂÂÄÂÂÙ
____====//| |\\=====____ 20m Dipole
====~~~~ ,// || || \\. ~~~~========
//' \\// `\\
___.=='' ³ ³ ``=.__10m Dipole
~~~~ ³ ³ ~~~~
75ohm Feeder
FEEDER
I used 10m of 75ohm thin balanced feeder to a ferrite choke balun & then 50m of
UR67 along a fence to the shack. The weight of the coax is unimportant 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. And also allows water to
drip off the hanging feeder & not go to the bulan.
Dipoles -ìì-
³³
\\
³³ Side
\\ ; Tree
³³ Balun°_: string
\\___ /°/ \
-----/° `\._____________Coax to Shack
Drip Point : Optional
: Earth point
As the balanced feeder is just long enough to reach the ground, that is
convenient place to put a plug & socket just after a normal ferrite ring choke
balun.
Shack Coax
ÚÄÄ¿Cable ended
Well ³ ³ SO239
Greased ÀÂÂÙ
,ÄÁÁÄÄ Ferrite
,/ ||//`\ Ring
Balanced 75ohmÚÄÄÄ¿ /\ /""""\//`\
ÄÄÄÄÄÄÄÄÄÄÄÄ==µ0 0ÆÍͳ==³ ³==³ Bifila
ÄÄÄÄÄÄÄÄÄÄÄÄ==µ0 0ÆÍͳ==³ ³==³ wound
feeder with ÀÄÄÄÙ \==\____/\\ / thick
low drip \//|| \\/' enameled
point. `ÄÄÄÄÄ' wire.
This SO239 connection point enables testing & use from the garden.
IN USE
I find it works without an ATU fairly well. There is no need to tune anthing
across a band or between bands, as the SWR typically less that 2:1 (often 1:1)
on the useful parts of the bands. But an ATU would help with the higher SWR at
the ends of some of the wider bands. The loss due to 2:1 SWR assuming your rig
is OK with it (most are), may be much less than the loss due to the addition of
an typical ATU (0.3-1dB)!
See other bulls "Tree Antenna Sky Hooks", "Delta Loops", "Balloon & Kite
Aerials", "Understanding Trapped Antennas" & "HF ATU & SWR Bridge VC300LP/QT-1"
Why Don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
Read previous mail | Read next mail
| |
