G8MNY > TECHNI 22.11.21 09:10l 160 Lines 7443 Bytes #999 (0) @ WW
BID : 53757_GB7CIP
Subj: HF Rx Suckout Trap
Sent: 211122/0759Z @:GB7CIP.#32.GBR.EURO #:53757 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Aug 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
When doing demo stations with more than 1 HF Tx on different bands (or with a
nearby ham), QRM between stations is quite normal, even with the high Q tuned
circuit of an ATU.
Here is one method to remove most of the problem from the aerial lead. (from a
system built up years ago for 2 FT101s by G0SYR & revisited for more modern
STATION A STATION B
³ ÚÄÄÄ¿ ÚÄÄÄÄÄÄÄ¿RF ÚÄÄÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄÄÄ¿ RFÚÄÄÄÄÄÄÄ¿ ÚÄÄÄ¿ ³
ÀÄ´ATUÃÄ´Rx TRAPÃÄÄÄ´TRANCEIVER³ ³TRANCEIVERÃÄÄÄ´Rx TRAPÃÄ´ATUÃÄÙ
ÀÄÄÄÙ ³ B FreqÃÄÄÄ´ ³ ³ ÃÄÄÄ´ A Freq³ ÀÄÄÄÙ
It uses switchable "Teed in series tuned traps" (notch) that are only in
circuit on Rx. If the transceivers have a transverting loop through ports, then
the circuit is quite simple, otherwise a relay operated by the PTT is needed as
This device also stops the Rx safety lamps (if fitted) or Rx front ends from
It is only connected in Rx mode, so there is no high voltages to deal with, but
good quality components & layout in a sold metal box are needed to give the
good Q & deep reject notch on the 50ê line.
50ê coax coax coax
TXR o)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄ(o ANT Rx<ÄÄÄÄÄÂÄÄÄÄÄÄÄÄ>Relay
_³_ ³ _³_ Socket Port o In/Out Port
Relay\_ \_ SW
LED L1 (___ O R L1 (___
+12V>Ä´>ÃÄÄÂÄÄÄÄÄ¿ ( ³ Hi/Low ( ³ Hi/Low
D1 _³_ ÚÄÄÁÄÄ¿ L2 ( ø/ Band W I T H A L2 ( ø/ Band
/_\ ³relay³ (___³ Switch (___³ Switch
D3 D2³ ÀÄÄÂÄÄÙ ³_ Rx P O R T ³_
PTT>´<ÃÄÄÄÁÄÄÂÄÄÙ ³/| ³/|
o Out/In === C1 === C1
/ SW /³ Tune /³ Tune
C1 2x 350pF Air spaced (MW radio cap)
L1 1uH, 12 Turns 1mm Enamelled Copper Wire on 1cm former (oval egotube).
L2 10uH, 44 Turns of 0.7mm ECW on 1cm former (oval egotube).
Both coils are on the same ÚÄ±±±±±²²²²²²²²²²²Ä¿
former in line, wound in the ³ ±L1±±²²²²L2²²²²² ³
same direction. So more L in ÀÄ±±±±±²²²²²²²²²²²ÄÙ
series & less L when L2 shorted. / | \
If you are using a rig accessory socket for ptt relay power, be careful not to
short it, Andy G0FTD recommends a series LED as a fuse. If your relay is not
very RF isolating to the winding or you experience Tx mode lock up add a
capacitor (1nF) across the PTT line.
³ ³ Scale calibrating:-
³ 160/80m ___ ³ connect to Rx & mark
³ Band (o) 80Üßú_._úßÜ40 ³ the band ends etc.
³ 40m-10m 20Ü/ / n \ \ ³ by nulling signals.
³ 15Üúú / \ úúÛ160³
³ 10Ü³ ³ ( ) ³ ß ³ The band switch is
³ Out |_| ³ colour coded, & the
³Filter(o) ³ same colours mark
³ In Notch Tune ³ the ham bands on
³ ³ the dial.
Typical notch depth is 30-40dB, but tuning is critical for the peak. However
notching the QRM down by 20dB or so, generally removes all Rx overload QRM.
Notch Rx attenuation width, does not extend to the next ham band, so adjacent
bands like 5MHz & 10MHz can just be used while notching out the 7MHz band etc.
10dB´ \ /
20dB´ | |
1.8 3.5 5 7 10 14 18 21 28 MHz
Note that 2 stations operating locally, have QRM paths other than the aerial!
e.g. via mains/12V leads/Mic, poor coax, poor metal case screening etc.
The trap does NOT remove any of the other station's Tx harmonics, so you still
will have some spot frequencies to avoid. e.g. a 100W of CW on 3.510 MHz will
give a strong signal on 7.020 MHz & that can't be notched out! Also the trap
does NOT remove any of the other station's broad band Tx noise or spurious, so
you have to put up with that, but read on!
TX MIXING & ATU BAND FILTER ACTION
High levels of coupling between Tx aerials can cause QRM to other HF services
as the Tx PA can mix its own signal & with other strong ones!
100W_³ TxA ³_ 0dBc
10W_³ ³ TxB ³_ -10dBc
1W_³ ³ ³ ³_ -20dBc
100mW_³ A-B ³ ³ A+B ³_ -30dBc
10mW_³ ³ ³ ³ ³ ³_ -40dBc
1mW_³ A-2B ³ ³ ³ ³ A+2B ³_ -50dBc
100uW_³ ³ ³ ³ ³ ³ ³ ³_ -60dBc
So there is still a need to have the aerials separated enough, so that there is
next to no power seen by the PA. e.g. the SWR bridge ATU on station A (between
ATU & Tx) should not be indicating power from station B (SWR direction).
STATION A close aerial B
A 100W> \|/ <direct \|/
ÚÄÄÄÄÄÄÄ¿ ÚÄÄÄ¿ ÚÄÄÄ¿ ³ <pick up ³ e.g.
³ TX PA ÃÄÄ´SWRÃÄÄ´ATUÃÄÄÙ ÀÄ< 100W
ÀÄÄÄÄÄÄÄÙ ÀÄÄÄÙ ÀÄÄÄÙ <10W B?
<100mW B? <100mW B?
With a good enough Q between the aerial & the Tx stage in this example 100mW
(-20dB of aerial signal) gets to the PA & will produce unwanted products in the
PA (acting as a Mixer) say at 10mW each, these then have to go back through the
same tuned circuit to reach the aerial @ 100uW each. With valve PA equipment
the high Q of the PA tank circuit also helps a lot, but this is not present on
a modern solid state broadband HF PA.
This is where a manual "T" or "Pi" Hi Q ATUs come in to their own, by providing
more band isolation (20dB off resonance & 18dB/O cut off), compared to the
simpler 2 component "L" type ATU (Automatic types).
oÄ´ÃÄÂÄ´ÃÄo oÄÂÄÄLÄÄÂÄo oÄ´ÃÄÄÂÄÄo oÄÄLÄÄÂÄÄo
L === === L ===
oÄÄÄÄÁÄÄÄÄo oÄÁÄÄÄÄÄÁÄo oÄÄÄÄÄÁÄÄo oÄÄÄÄÄÁÄÄo
0dB´ /\ /\ /\._ _./\
10dB´ | \._ _./ | _./ ~ ~ \._
20dB´ _/ ~ ~ \_ -~ ~-
ÅÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄ
T ATU Pi ATU L ATU L ATU
Good LF cut Good HF cut some LF cut some HF cut
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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