I’m slowly getting into CW at the moment and have started with some SOTA CW chases. I’m using an ICOM 718, which is not equipped with CW filters. When the activator is “alone” on a band, I manage fine. However, when the activator is transmitting on a crowded band like 40m, it becomes very annoying to listen to, especially with 2 or 3 loud stations nearby. It’s overwhelming, and I end up turning the radio off. This got me thinking about getting a CW filter.
Option 1
Buy a second-hand official 250 or 500 Hz filter, such as the FL52a or FL53a. These are hard to find and expensive—around €200 from the US. For a €400 radio, it feels a bit excessive.
Option 2
Build my own audio filter to place between the radio and the headset. The Variable Bandwidth Filter Module SSB/CW from Sotabeams is only €45 and quite appealing. Another option is a classic passive LC filter (example here), which could be very cheap if I can build the inductor properly.
Option 3
Sell or retire the ICOM 718 and buy an amplifier for my TX500 portable radio. The TX500 has a great receiver, lots of filters, and would address the issue. Financially, it could be a neutral operation, but I’d lose a radio in the process.
I don’t know the difference in performance between embedded IF filters and “post-radio” audio filters. I’m really not an involved chaser; I just want a solution to limit the QRM. It doesn’t have to be top-notch.
Are they a lot better than audio filters ? From what I can see, my only option for them is eBay US , at around 150€ + probably 50€ shipping costs. Not cheap.
An ICOM 7300 would be great but it means selling the 718 and adding an extra 600€ to get it second hand. Truly excessive for the few minutes I spend on the radio every week / month.
I think I’d start with this option. It will almost certainly help, and may solve your problem completely. You will also have fun making it, and the enjoyment of using something you have made.
If it is not good enough, you will have learnt something and lost nothing
As a person just getting started with Morse this might be biting off more than you can chew, but a zero-cost Option 4 would be to train yourself to distinguish signals of different pitches coming through the wider filter.
There is old, free Windows software called Morse Runner that could help with this. And I recently learned of a newer free web-based practice tool called Morse Walker https://morsewalker.com/
With Morse Walker you can use the “Basic Contest” mode and under the Responding Station settings, adjust the number of simultaneous calling stations as desired.
I’ve used a Sotabeams DSP audio CW filter with my FT817 for activations and found it very effective. I didn’t like all the leads (which was part of the motivation for homebrewing my rigs) but this doesn’t matter for a shack setup. One issue to be aware of is the AGC will act on any signal in the SSB passband even if blocked by the CW filter so you may have to turn off the AGC.
I would understand you, if you were in a contest like WPX or CQWW or if you were the operator of a DXpedition.
My recommendation is “Avoid using filters” unless it is practically impossible.
Why don’t you try playing with the RIT, CLAR or the encoder?
Even though I bought the Icom IC-705 last year, I never used the filters and I assure you that in Japan the 7mhz band is pandemonium all day long.
I’m not saying don’t have a filter, just avoid using it. This will help you be more attentive and have a sharper ear.
You are probably right but I have very sensitive ears and even with a top quality headset, the sound not so loud, my head is about to explode after a few minutes. I’m using ear protection even when I hoover the floor, just to say …
I tried but no success. It’s just moving the bandpass up and down and when I get rid of a qrm signal I just gather a new one on the other side. Maybe I’m doing wrong but holly banana it’s a killer
Je veux savoir si ça vaut le coup de payer un filtre quartz relativement cher (la moitié du prix de la radio !) ou si un filtre audio dsp fait sensiblement le même travail pour une fraction du prix.
@F4LEK
If you decide to buy and you are faced with a choice of CW Filters if I couldn’t have both and it was for SOTA /P operation I would always get the 500 Hz one. I find the background noise listening to a 500 Hz filtered background is far easier on the ears than the bandwidth noise produced by a 250 Hz filter. I am prepared to tolerate the wider skirt from the 500 Hz on reception, as my ears provide me with sufficient filtering to pick out the signal of interest.
I’m not fond of audio filtering for CW. I’ve tried several and I don’t think its particulary effective as earlier in the thread as @MM0FMF also noted…
Finally the right answer!!! Training your ears is the best solution!!! Below 500 Hz, the PileUp is stressful. Above 500 Hz, the ears start to work. As Phil @G4OBK says, the best solution!!
I built and tested a switched capacitor audio filter (SCAF), it was from a QRP club in the US and found it to be very good. The two controls vary centre frequency and bandwidth. Notably even at narrow bandwidth there was no ringing, a common problem with IF crystal filters. The filters that don’t ring are the Collins mechanical filter, i have that in an ic703 (the now-defunct dual filter mod by W4RT - the filters are no longer manufactured by Collins as DSP has taken over their role).
I have used the kenwood 270 hz filter, the Icom fl53, the dsp filters in other rigs (IC7000 and IC7610 and several audio filters over the years and while the AGC pumping effect caused by the bandwidth in the IF still being 2.4 kHz or similar, the impact is less than i thought it would be. I haven’t operated in a contest in Europe or the US so perhaps the giant signals in the IF bandwidth do make a huge difference. However i am pretty sure that some audio filtering is much better than none.
Which leaves us to consider the DSP filtering in the filters sold by SOTA beams. Have never seen or heard them but I dare say they would be worth trying.
I think the drawback to making LC filters using inductors etc is the bulk of the filters, also making them sufficiently equal in inductance to satisfy the needs of the design. One inductor is not enough, you need a number of them in a decent filter. Op-amps based on RC filters are ok, MFJ used to have them in their catalog. I experimented with one of those on my ft817 listening to a contest and the mechanical filter beat it easily. The stop band suppression is what makes the mech filter so good. No good cutting a s9+40 signal by only 40 db, it’s still s9 so if it’s too close to the desired signal and the comparison between signal strengths too great, it is very difficult to copy the weaker one.
If you don’t mind waiting till you see a ft817 or any other rig with a Collins filter going for disposal, you would be well served by a surplus Collins mech filter at either 500 or 250 hz. Be aware they were made for many different IF frequencies, not just 455 as used for ham rigs.
If close-in cw contesting was easy, more would be doing it, so the QRM would increase.
If you can find a used crystal filter 500 or 250/270 cycle for the ICOM that really is the best way to go. You can’t imagine how much difference it makes unless you experience it for yourself. Audio filters do not really cure the QRM problem because the strong adjacent signals still cover up the weak signal you’re trying to hear before the signal ever gets to the audio stage of the receiver. Now if you have a nice narrow filter in the IF, and then add an audio filter, you have something that’s even better!
I use a 150 cycle IF filter (yes, 150 cycle) and I can pull those really weak SOTA signals out of the muck, even if there is a bit of nearby QRM.
Something else to be aware of is that when you narrow down the IF passband you cut down the noise coming through. This makes the signals stand out more, improves the signal to noise ratio.
If you already have one rig with a good CW filter, you probably already know some of this just from taking a listen and comparing
the difference from one receiver to the other.
73,
John, K6YK, CW op for 67 years
