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G8MNY > TECH 13.12.24 08:29z 140 Lines 6420 Bytes #172 (0) @ WW
BID : 21791_GB7CIP
Read: GUEST
Subj: QRO Dummy Loads
Path: SR1BSZ<EA2RCF<I0OJJ<GB7CIP
Sent: 241213/0823Z @:GB7CIP.#32.GBR.EURO #:21791 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Corrections Jul 09)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
PRINCIPLES
1/ Ideally for a perfect match at all frequencies a dummy load should be a
resistive disk on the end of the coax. Drawback is the low dissipation, but it
is used in low power RF test probes at UHF/SHF.
_____ 3:1 Ä´
Resistive °_____COAX 1:1 Ä´ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>
Disk °_____ SWR ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>F
DC 1M 10M 100M 1G 10G
2/ More practical approach is to make the coax outer the resistive element.
This gives high surface area & relatively high dissipation. Elements are made
for 25W to 1kW. The Z is controlled by the coax inner to outer ratio as in
normal coax (e.g. 2.3 in air for 50ohm), plus the element resistance must equal
the load Z.
³°°°°°°°°°°°°°°°°°°°°°°°°°³ ì Ä´ .
³ ³__ 3:1 Ä´ |^| -"
³ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ___ COAX 1:1 Ä´-------------" "
³ßßßßßßßßßßßßßßßßßßßßßßßßß __ 239 ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>F
³ ³ DC 1M 10M 100M 1G 10G
³°°°°°°°°°°°°°°°°°°°°°°°°°³
But they do suffer from 2 problems. They radiate at higher frequencies as the
voltage on the outer is seen as a 1/4 wave & a good aerial, this also mucks up
the load match. An outer cage reduces some of the radiation, but does not help
with the SWR.
3/ Put the resistive rod inside a tube. This works just like 2/. but it has no
radiation. However it still suffers from miss match as the element becomes
close to 1/4 wave. And with the smaller thermally screened load power
dissipation is a problem.
_________________________ ì Ä´ .
³ ÀÄÄ 3:1 Ä´ _!"-"
³°°°°°°°°°°°°°°°°°°°°°°°°°ÃÄÄ COAX 1:1 Ä´--------------"
³_________________________ÚÄÄ 239 SWR ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>F
DC 1M 10M 100M 1G 10G
4/ By putting the resistive element rod into a conical box, the match can be
maintained to much higher frequencies & the 1/4 wave problem goes away. UHF
100W loads are made this way.
____
___....----""" ³ ì Ä´ .
..---""" ÀÄÄ 3:1 Ä´ ."
³°°°°°°°°°°°°°°°°°°°°°°°°°ÃÄÄ COAX 1:1 Ä´--------------------"-"
""---...___ 50ohmÚÄÄ 'N' SWR ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>F
0ê """"---Ä...____³ DC 1M 10M 100M 1G 10G
The internal load to conical box forms a coax, of the same Z as the load
resistance element has left to go. The cone starting diameter is 2.3 (for 50ê
in air) times the rod diameter, tapering to the same diameters at 0ê. The outer
case may be a heatsink, but the hot resistance inside can easily be damaged by
overloads.
5/ By using the same principle as 4/ for the outer mesh cage cone, & inner coax
centre rod cone with a large high power ceramic HF tube load, can be made to
work OK at UHF! I made one that stands 1kW intermittently at 432MHz & is still
a good load @ 1296MHz.
...__
""--..__
Clip ""--..__ 0ohm
__ ""--.. ì Ä´
³°°°°°°°°°°°°°°°°°°°°°°°°°³ 3:1 Ä´ .³
""-----....._____ ÀÄÄ 1:1 Ä´-----------------"--" "
0ohm _____:::::====ÄÄÄ COAX SWR ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>F
..-----""""" 50ohmÚÄÄ 'N' DC 1M 10M 100M 1G 10G
³°°°°°°°°°°°°°°°°°°°°°°°°°³
"" __..--""
50ohm __..--""
__..--""
"""
The inner cone is made from large coax braid soldered to a thick wire to the
'N' connector, & a clip holds the braid attached on to the outer of the ceramic
tube end. The outer galvanised mesh (1cm holes) cone is bolted to the plate
that has the 'N' socket bolted to it. The tube ground is by soldered on tags.
There is some radiation with a mesh outer, but dissapation is good.
6/ Some designs use overrun load resistors in transformer oil or other cooling
system. Submersed in freely flowing Oil, some resistors improve dissipation 20
times. So small resistors can be used & often several are placed in parallel to
make the load.
COAX 239
Ú\____³_³_³____/¿
³ _ _ ³ ³ ³ _ _ ³TIN ì Ä´
³ _³ ³ ³_ oil³ 3:1 Ä´ ."
³ °° ³ °° ³ 1:1 Ä´----------------"
³ °° ³ °° ³ SWR ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>F
³ °° ³ °° ³ DC 1M 10M 100M 1G 10G
³ °°_³_°° ³
³ 6x 300R ³ 1kW load
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
SWR limited to internal leads & large resistor size.
Main problem with these is loss of oil through seals, or upturned tin etc.
7/ Or you can use modern small Metal Film resistors on low thermal resistance
ceramic material, with thermal conducting past, on a large heatsink.
ßßßßÛ___ 2x
ßßßßÛ ³ 100R ì Ä´
ßßßßÛ°³ ÀÄÄ 3:1 Ä´
ßßßßÛ ÃÄÄÄÄ COAX 1:1 Ä´------------------------
ßßßßÛ°³ ÚÄÄ 'N' SWR ÃÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄÄÄÄÂÄ>
ßßßßÛ___³ 100W DC 1M 10M 100M 1G 10G
ßßßßß
The advantage is the small load size permits good frequency response & fairly
high powers, but over power surges will destroy them.
8/ And finally don't forget that long reel of coax, these also make good dummy
load if the coax loss is high enough & the first few metres are unwound to
dissipate the heat.
_______________________
(_______________________(-< QRO
(_______________) Load
(_______________)
QRP _(_______________) 50M drum
Load <-)_________________) of Coax
I have run >400W on 70cms on test this way without a QRO dummy load. The first
few metres of UR67 coax did get quite hot though & there was some radiation,
but the SWR was as good as you can expect from coax!
On say 13cms, 2m of UR43 may be OK for 50W without a QRP load, as the cable
loss is so high SWR < 1.2, Radiation ??
See my Buls "QRO 1kW HF Metered Dummy Load" & "QRP Power Meter & Dummy Load"
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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