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G8MNY > TECH 07.09.15 13:13l 172 Lines 7910 Bytes #999 (0) @ WW
BID : 41907_GB7CIP
Read: GUEST
Subj: HF ATU & SWR Bridge MFJ-904H
Path: IW8PGT<IZ3LSV<IW2OHX<UA6ADV<GB7CIP
Sent: 150907/0831Z @:GB7CIP.#32.GBR.EURO #:41907 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Sep 14)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I was given a broken "Deluxe Travel Tuner" from one of my students.
F R O N T R E A R
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ÚÄÄÄÄÄÄÄ¿ Tx Ant Ind ³ ³ -o- (o) (o)ú(o) ³
³³ /~~~\ ³ ÜÜÜÜ ÜÜÜÜ ÜÜÜÜ ³ ³ Earth Bal _By ³
³³|_/_\_|³ Û Û Û Û Û Û ³ ³ ( 0 ) Ü ( 0 ) ³
³ÀÄÄÄÄÄÄÄÙ ßßßß ßßßß ßßßß ³ ³ Ant Tune Tx (ú)³
³ () ³ ³ 12v³
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
It is a small cheaply made unit, the 2 tuning Cs are "preset types" with only 1
bearing, so they will soon short out with wear or a dropping on their knobs!
The L indutance switch shaft had never been greased, & it had seized up solid.
Aluminium on Aluminium is not a good bearing surface!
REPAIR
Removing the L & switch, & putting the shaft in a large vice & applying a spray
3x ferrite rings Switch
Floating __ __ __ .Ü|ÄÄ¿
End stayÞ / \/ \/ \ Ý.Ü| ³ seized
Þ Ý Ý Ý ÝÝ.Ü| ³ÄÄ¿/
ÞÜÜÝ Ý Ý ÝÝ.Ü| ³| ³²²²² shaft
Ý Ý Ý ÝÝ.Ü| ³ÄÄÙ
:Ý Ý Ý ÝÝ.Ü| ³|Earth
ú\__/\__/\__/ Ý.Ü|ÄÄÙ|tag
'''''''''''''' ---/
1 or 2 turns Spacer
per tap Windings
grease (not thin WD40), I was able to free it all up OK.
OTHER PROBLEMS FOUND.
1/ Was the lack of inductance on the 3 ferrite cored L for 160m (not designed
for 160m!) So I added 2 turns to the unconnected switch post (making the full
12 options) & moving the earth connection to it. This helped 160m tuning
without both Cs at Max (not always an option with aerial Z).
2/ I found the 2 min L positions (1 turn each) were too little inductance, so
not of any use on 10m! So I added 2 turns (enammelled copper wire in insulation
as this is the high voltage coil end) after the last switch post to the tuning
Cs so minimum L is now 2 turns. This gave better 160m tuning too. :-)
Both of this gives the 12 steps more usable options from 160m-10m.
3/ Wiring of the 2 tuning caps was incorrect & UNSAFE! The shaft & mounting
nuts with insulation system, were wired at the VERY RF HOT end of the tuned
circuit! I rewired mine like this for, much lower RF voltage on shafts!
Bypass
Switch
³___³ Stator
Ü Ü_ _ _ _ _ Link
\ n \ nÄÄÄÄn n===from The shafts should have been
| =====²===== =====²===== Coil at Tx or Aerial voltage,
| =====²===== =====²===== & never the kV end of the
| =====²=====\ =====²===== power tunned circuit!
| =====²===== | =====²===== Even though there are plastic
| =====²===== | =====²===== front shafts for the knob
\_=====²===== \_=====²===== brass sleeve & screws etc,
RotorÚÁÄÄÄIJÄÄÄÄÁ¿ ÚÁÄÄÄIJÄÄÄÄÁ¿
tag ÀÄÄÄIJÂÄÄÄÄÙ ÀÄÄÄIJÂÄÄÄÄÙ
(moveable) ³²³\insulation/³²³
(position) ² kits ²
Tx Ant
FEATURES
150W max. now 160m to 10m (no sign of satuation @ 100W all bands)
ATU is "T" Type, 12 tapped ferrite inductor, 2 tuning Caps (300V rating? &)
(will flash over @ 100W peaks on 160m into a load).
30W & 300W Forward scale small cross needle power meter.
6W & 60W Reverse power scales, for increased SWR sensitivity.
4:1 BALUN option for balanced aerials (link from wire ant)
12V Meter scale Lamp.
BLOCK DIAGRAM
Rig ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
0)Ä´ DUAL POWER & ÃÄ´ BYPASS/ ÃÄÄÄÄÄÄÄÂÄ(0) 259 Coax
³ SWR METERS ³ ³ TUNE SWITCH ³ ³
ÀÂÄÄÄÄÄÄÂÄÄÄÄÄÄÙ ³ ÚÄÄÄÄÄÄÄÄÄÄÄ¿ ³ ÀÄ(o Wire
Lamp Range ³ ³ ÚÄÄÄÄÄÄÄ¿ ³ ³ ÚÄÄÄÄÄ¿ > link for balanced
>ÄÄÙ ³ ÃÄ´ A.T.U.ÃÄ´ ³ ³BalunÃÄ(o
12V ÀÄÙ ÀÂÄÄÂÄÄÂÙ ÀÄÙ ³_____ÃÄ(o
C C L__________³____>Earth
CIRCUIT
The SWR bridge is quite conventional.
Ferrite ring BYPASS SWITCH
TX(0)ÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>oÄÄÄÄÄÄÄo<ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ(0) 259 ANT
ÄÁÄ ³/ ===== o o ÀÄÄÄÄÄÄÄ(o WIRE
3-10p=== ÚÄÄÄCCCCCÄÄÄ¿ ³/ ³/ |
/ÃÄÄ)ÄÄÄÄÄÙ ³ Tx === === Ant ====== |link for
n22 === ÃÄÄÄ68êÄÄÄÄÄ´ /³ /³ ÚÄCCCCCÄ(o 4:1
ÄÁÄ_³_ _³_ ÀÄÄÄÂÄÄÄ Ferrite ³ ÄÁÄ BALUN
\_/ 4x10n \_/Ge A.T.U ³=======Inductor ÀÄÄÄÄÄÄÄ(o
ÃÄÄ´ÃÄÂÄ´ÃÄÄ´ ÀCCCCCCC¿
100 ³ 100 L /³\ ³
ÃÄÄ´ÃÄÅÄ´ÃÄÄ´ ÀÄÄÂÄÙ
ÚÄÁÄ¿ ÚÄÁÄ¿ ÄÁÄ
100k 50k 100k 10k
PRE PRE PRE PRE
300wÀo oÙ30w 60wÀo oÙ6w
\ \ Meter ³
ÃÄ´ÃÄÄÂÄÄ´ÃÄÄ´ Switch ÚÄÄÂÄÄÄÄÄÄÄÂÄÄ0 EARTH
Forward ³ Reverse ÄÁÄ ³ 10n=== ³
Meter ³ Meter ³ ³
750êÀÄÄÄÄÄÁÄÂÄÄÄÄÙ750ê ÀÄMeterÄÁÄ(ú 12V
ÄÁÄ Lamp ÄÁÄ
CONSTRUCTION
It uses thick 2 U part aluminium case, thick open wire for all internal
connections. N.B. the short screw is beside the meter!!
The toroid ferrite inductor (now) has 12 taps to a 12 way switch.
A single PCB provides for all the component mounting for the SWR bridge.
TUNING UP
The best tuning method is to use the ATU at low power (between 10 & 30W) &
pre-tune it, set the ATU with both Cs @ 50% & adjust the L tap for initial
minimum SWR. Then tweak (rock too & throw) both the Cs for a perfect 1:1 match.
This can take time as they are interdependent & their combined capacitance
tunes the L for the frequency used & on 50ê Tx & Ant Cs will be equal in value.
N.B. always some Ant C & an antenna, failure to have a load can result in
arcing, & possably damage even at quite low powers!
This is kindest tuning method for the ATU & also for minimum QRM to others. A
very briefly re-tweaking of Cs only at full power may be an advantage as the
meters are more accurately (sensitive) with more power.
Both the bypass switch & the ATU variable L, should not be changed if there is
any great power flowing, as the contacts can easily be damaged with arcing!
Aerial Tuning Units are also called Aerial Matching Units, but the best aerial
is one that is already tuned by design (cutting to length) & a good match (tap
point for right Z)!
IN USE
The ferrite balun & tapped ATU inductor, can suffer saturation losses if used
in-appropriately. But in general there is much less loss with a ferrite cored
device, than the much much bigger air spaced equivalents, as there is many
times less wire used & associated copper loss. Typically power loss is only 5%
(0.2dB) @ 100W (100W in 95W out) on any band at any SWR (due to the "T" type
ATU's high Q) which is quite low for a combined ATU & SWR bridge.
If the random wire aerial won't tune up to give 1:1 SWR, then try a different
length wire (shorter or longer by a metre or so).
The "T" type ATU is easy to use, but is not so good at removing Tx harmonics as
the ã (pi) type. But it does an excellent job of protecting the Rx from strong
signals on lower bands (e.g. removing MW Broadcast signals as it is >-18dB for
each frequency halving.)
See also my Tech buls on "HF ATU & SWR Bridge VC300LP/QT-1", "Palstar AT1500CV
ATU", "Drake WH7 QRO HF SWR Bridge", "A Homebrew HF SWR Bridge",
"PEP Meter modification", "Meter Damping & Speed Up" & "QRP SWR Bridge".
Why don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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