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G8MNY > TECHNI 09.01.22 10:04l 177 Lines 8335 Bytes #999 (0) @ WW
BID : 57850_GB7CIP
Read: GAST
Subj: HF ATU & SWR Bridge MFJ-904H
Path: DB0FFL<OE2XZR<OE6XPE<IW2OHX<IW0QNL<SR1BSZ<OK0NBR<OK2PEN<GB7CIP
Sent: 220109/0856Z @:GB7CIP.#32.GBR.EURO #:57850 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Dec 19)
(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 by 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
grease (not thin WD40), I was able to free it all up OK.
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
OTHER PROBLEMS FOUND
1/ There 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 minimum L positions (1 turn each) were too little inductance,
for 10m band! So I permanently added 2 turns of enamelled 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 much better 160m tuning
too. :-)
Both of these mods give the 12 steps more usable options from 160m-10m bands.
3/ Wiring of the 2 tuning caps was incorrect & UNSAFE! The shaft & mounting
nuts with insulation system, were wired at the VERY RF HOT (several kV) end of
the tuned circuit! I rewired mine (see below) for much lower RF voltage on the
shafts of just the Tx & aerial!
Bypass
Switch
³___³ Stator
Ü Ü_ _ _ _ _ Link from
\ n \ nÄÄÄÄn n===Coil
| =====²===== =====²=====
| =====²===== =====²===== The shafts should have been
| =====²=====\ =====²===== at Tx & Aerial voltage & never
| =====²===== | =====²===== the high voltage (kV) end of
| =====²===== | =====²===== the power tuned circuit!
\_=====²===== \_=====²===== Even though there are plastic
RotorÚÁÄÄÄIJÄÄÄÄÁ¿ ÚÁÄÄÄIJÄÄÄÄÁ¿ front shafts for the knob
tag ÀÄÄÄIJÂÄÄÄÄÙ ÀÄÄÄIJÂÄÄÄÄÙ brass sleeve & screws etc.
(moveable) ³²³\insulation/³²³
(position) ² kits ²
Tx Ant
FEATURES
150W max. now 160m to 10m (no sign of saturation @ 100W on 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 (linked from wire ant post)
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
ÀÄÄÄÄÄÄÄÄÄÁÄÄÄÄ>Earth
CIRCUIT
The SWR bridge is quite conventional.
TX Ferrite ring BYPASS SWITCH
(0)ÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>oÄÄÄÄÄÄÄo<ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄ(0) 259 ANT
1:1 Nul³/\ ===== o o ÀÄÄÄÄÄÄÄ(o WIRE
3-10p === ÚÄÄÄCCCCCÄÄÄ¿ ³_ ³_ |
/ÃÄÄ)ÄÄÄÄÄÙ ³ ³/³ ³/³ ====== |link for
n22 === ÃÄÄÄÄ68êÄÄÄÄ´ Tx === === Ant ÚÄCCCCCÄ(o 4:1
ÄÁÄ_³_ _³_ /³ /³ ³ ÄÁÄ BALUN
\_/ 4x10n \_/Ge ÀÄÄÄÂÄÄÄÙ Ferrite ÀÄÄÄÄÄÄÄ(o
ÃÄÄ´ÃÄÂÄ´ÃÄÄ´ A.T.U ³=======Inductor
100 ³ 100 ÀCCCCCCC¿
ÃÄÄ´ÃÄÅÄ´ÃÄÄ´ L /³\ ³
ÚÄÁÄ¿ ÚÄÁÄ¿ ÀÄÄÂÄÙ
100k 50k 100k 10k ÄÁÄ
PRE PRE PRE PRE
300wÀo oÙ30w 60wÀo oÙ6w
\ \ Meter ³
ÃÄÄ´ÃÄÂÄ´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
If doing this on someone's operating frequency, DO QSY 3kHz first! 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 & fiddly especially with no reduction verniers & the small knobs. The 2
adjustments are interdependent as their combined capacitance tunes the L for
the frequency used. On 50ê Tx & Ant load the Cs will be equal in value.
N.B. always use some Ant C & an Antenna connected, failure to have a load can
result in arcing, & possibly damage even at quite low powers!
LOW POWER is kindest tuning method for the ATU & also for minimum on band QRM.
A very briefly re-tweaking of Cs only, at full power may be an advantage as the
meters are more accurate (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 (cut 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 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) or add a series cap (200pF) or
L (20 turns).
The "T" type ATU is easy to use, although it is not so good at removing Tx
harmonics as the ã (pi) type. But it does an excellent job of protecting the Rx
from DC static, & strong signals on lower bands, that some poorly filtered SRD
Rx can suffer from! (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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