Many times, during rtty contests, I have a big problem with qrm. A lot of strong signals in a small bandpass, so, if you reduce the RF Rx gain or connect the 6-12 dB attenuator you shall loose the weak signals. In fact, the receiver (the preamplifier, mixer, etc) have to deal with a lot of unwanted (by the operator) signals, so the transceiver performance will be degradaded. Usualy, the modern transceiver have a quality FEF (Front End Filter) after the 1'st mixer. It's wonderfull, but still need to reduce the RF Rx gaind during some periods of the contests.
So, last cqwwrttycontest (2020) I tried an old solution, a FEF connected direct between antenna and transceiver, with 5 comutabloe monocrystals (7041, 7045, 7055, 7070 and 7082 kHz.).
1'st, some history....
Few explanations: circuits A & B give us the posibility to have a small variable bandwidth; circuit C is a simple one, with the minimum bandwidth obtainable from a specific crystal. A short theory lesson for the "C" configuration we can find at page 106 of The Radio Amateurs Handbook - 14th edition.
And now, others people experience:
(http://eddystoneusergroup.org.uk/Restoration%20projects/Tech%20Short%205%20-%20Receiver%20Selectivity%20and%20Crystal%20Filters.pdf)
and M0AYF QRSS band pass filter
And now, my experience:
Note 1.
Even you know the Fs of a crystal can be modified with a C in series, you shall find that the RL, or selectivity is depending how far you are from Fs. So, for a crystal with Fs= 7045 khz, the optimal aria for this kind of filter is very close from Fs, only 1 khz up and down (even you can move Fs with 5-6 khz up, with Cv=5-10 pF). The explanation is Q factor wich is degradeding when you modify Fs.
Note 2.
Ct have to be a good trimer, air or mica, with small capacity. Dont forget that you have to adjust somewhere around 3-5 pF, exact the value of crystal Cp.
Note 3.
Find a good variable rezistor, with a very low rezidual inductance.
General note
Take care of practical setup: minimum wires, short wires, screening. If you can not adjust Ct for a good diagram, it seems you have parasitic capacitance in circuit, wich are greater than your Ct. Also, dont forget you need another module, the relais module. You need to disconect the FEF during Tx, to bypass the FEF and you are not using him, etc. You need minimum 3 good relais: the antenna connector, the transceiver connector and the relais for commanding those 2 relais. The circuit depends what transceiver you are using, etc.
How to adjust the FEF (Front End Filter)
My first prototype: only 2 crystals, and a toroidal transformer on the output, to transceiver.
The adjustment is done with a RigExpert analyzer (AA-230 Zoom): 1st step adjusting the SWR for the lowest value (in the bandpass), and after that checking the whole diagram for RL (returning loss). If the value is more than 30 db, is OK (dont forget, 1 S point = 6dB, so 30 dB = 5 S points attenuation for unwanted signals).
Actual prototype has 5 crystals, a plastic box with aluminium tape for screening and a variable rezistor replacing the second toroidal transformer. It was not my best day when I put everything inside.... but I promise (to me) to build a more adequate aluminium box for cqwpxrtty contest.
Dont forget to adjust also the rezistor for a minimum SWR. Depending your crystals and setup you can obtain RL up to 50 dB.
... hmmm... I am litle shame of my presentation, I dont know if you understand what is in last 2 pictures, but the device is working.
How to use ? Very simple. Connect the device inline, put on BYPASS, and when have the biggest QRM around a frequency where you have crystals inside, connect the unit and try the Cv for an optimum Rx. Also, dont forget the RX RF gain, because nothing is an ideal solution.
So, thank you for your attention, and do not hesitate to ask if you need more info.
Good luck in contest (and QRM),
YO4DFT - Cristian
cristian.moldovanu@gmail.com
P.S.
2 crops from my log, when I used FEF to deal with QRM.
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QSO: 7071 RY 2020-09-26 1530 YO4DFT 599 20 DX RV3ZN 599 16 DX
QSO: 7070 RY 2020-09-26 1533 YO4DFT 599 20 DX LY3CY 599 15 DX
QSO: 7069 RY 2020-09-26 1537 YO4DFT 599 20 DX JH4UTP 599 25 DX
QSO: 7069 RY 2020-09-26 1538 YO4DFT 599 20 DX S57X 599 15 DX
QSO: 7069 RY 2020-09-26 1540 YO4DFT 599 20 DX SP2UU 599 15 DX
QSO: 7069 RY 2020-09-26 1541 YO4DFT 599 20 DX YL2BR 599 15 DX
QSO: 7069 RY 2020-09-26 1541 YO4DFT 599 20 DX UY7MM 599 16 DX
QSO: 7069 RY 2020-09-26 1542 YO4DFT 599 20 DX DA2X 599 14 DX
QSO: 7069 RY 2020-09-26 1543 YO4DFT 599 20 DX LY9Y 599 15 DX
QSO: 7069 RY 2020-09-26 1543 YO4DFT 599 20 DX RA3TT 599 16 DX
QSO: 7069 RY 2020-09-26 1543 YO4DFT 599 20 DX YL3CU 599 15 DX
QSO: 7069 RY 2020-09-26 1544 YO4DFT 599 20 DX SM5MX 599 14 DX
QSO: 7069 RY 2020-09-26 1545 YO4DFT 599 20 DX OK5NW 599 15 DX
QSO: 7069 RY 2020-09-26 1548 YO4DFT 599 20 DX IT9RBW 599 15 DX
QSO: 7068 RY 2020-09-26 1549 YO4DFT 599 20 DX IZ3BUR 599 15 DX
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QSO: 7040 RY 2020-09-26 2144 YO4DFT 599 20 DX DH8BQA 599 14 DX
QSO: 7040 RY 2020-09-26 2145 YO4DFT 599 20 DX LA2XNA 599 14 DX
QSO: 7040 RY 2020-09-26 2147 YO4DFT 599 20 DX OH1SIC 599 15 DX
QSO: 7040 RY 2020-09-26 2148 YO4DFT 599 20 DX PA3BFH 599 14 DX
QSO: 7040 RY 2020-09-26 2148 YO4DFT 599 20 DX PB7Z 599 14 DX
QSO: 7040 RY 2020-09-26 2149 YO4DFT 599 20 DX DF2RG 599 14 DX
QSO: 7040 RY 2020-09-26 2150 YO4DFT 599 20 DX IU2LTO 599 15 DX
QSO: 7040 RY 2020-09-26 2151 YO4DFT 599 20 DX DJ1OJ 599 14 DX
QSO: 7040 RY 2020-09-26 2151 YO4DFT 599 20 DX UV5EDW 599 16 DX
QSO: 7040 RY 2020-09-26 2152 YO4DFT 599 20 DX IU4HRJ 599 15 DX
QSO: 7040 RY 2020-09-26 2153 YO4DFT 599 20 DX DL3SYA 599 14 DX
QSO: 7040 RY 2020-09-26 2155 YO4DFT 599 20 DX DL5ARM 599 14 DX
QSO: 7040 RY 2020-09-26 2155 YO4DFT 599 20 DX OZ1OXQ 599 14 DX
QSO: 7040 RY 2020-09-26 2156 YO4DFT 599 20 DX DM1FS 599 14 DX
QSO: 7040 RY 2020-09-26 2156 YO4DFT 599 20 DX R6CC 599 16 DX
QSO: 7040 RY 2020-09-26 2157 YO4DFT 599 20 DX DL3KUD 599 14 DX
QSO: 7040 RY 2020-09-26 2157 YO4DFT 599 20 DX DL4ME 599 14 DX
QSO: 7040 RY 2020-09-26 2158 YO4DFT 599 20 DX PA1JM 599 14 DX
QSO: 7040 RY 2020-09-26 2200 YO4DFT 599 20 DX DL0BI 599 14 DX
QSO: 7040 RY 2020-09-26 2201 YO4DFT 599 20 DX IU2CIQ 599 15 DX
QSO: 7040 RY 2020-09-26 2204 YO4DFT 599 20 DX IK1BPL 599 15 DX
QSO: 7040 RY 2020-09-26 2206 YO4DFT 599 20 DX SQ3MZ 599 15 DX
QSO: 7040 RY 2020-09-26 2206 YO4DFT 599 20 DX DQ9Y 599 14 DX
QSO: 7040 RY 2020-09-26 2208 YO4DFT 599 20 DX DL6UAA 599 14 DX
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