Surprised by the Heathkit CW filter

Discussion in 'Straight Keys - CW Enthusiasts' started by AA4OO, Feb 12, 2018.

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  1. AA4OO

    AA4OO XML Subscriber QRZ Page

    I just finished getting a Heathkit HW-101 back on the air. It came with a CW crystal filter...


    On the filter it says 400Hz @6DB. I didn't expect much of a slope for rejecting nearby stations but after using it for a couple of days I'm surprised at how well it's rejecting nearby stations. Don't most modern CW filters have more than a 6DB slope, or am I missing something?

    Other than the HW-101 not having a RIT, it's turning out to be an enjoyable CW rig.

    Richard, AA4OO
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    The 6dB points on that filter don't define the slope or shape factor, the ratio of 6dB bandwidth to 60dB bandwidth is what defines how steep the slope is. Based on what's printed on that filter, it has a shape factor of 5 (2000 Hz 60dB BW / 400 Hz 6dB BW). That's not exactly cutting edge but it's not half bad.

    Basically it means if you center a CW signal near the center of the 400 Hz filter passband and have another CW signal 200 Hz above or below your center frequency you'll get 6dB of attenuation on that nearby signal. But if the offending nearby signal is plus or minus 1000 Hz from your operating frequency you'll get around 60dB of attenuation of that QRM. You may or may not see that much attenuation due to blowby or signal leakage based on things like filter mounting, shielding of stages before and after the filter and normal variability of the filters themselves but you should still expect a lot of attenuation of signals even three or four hundred Hz away from your operating frequency.

    FWIW, here's a plot of a fairly expensive 7 pole Inrad CW filter for Collins gear showing the 6dB and 60dB bandwidth points (standard reference points for calculating shape factor) and the resulting shape factor of approximately 2.6:
    They market this as a 500 Hz CW filter with the -6dB points measuring at almost 600 Hz. To get 60dB of attenuation you'd want the off frequency signal to be around -800 Hz or plus 1000 Hz. Viewed from that perspective your Heath filter specs are pretty good and compare favorably.
    Last edited: Feb 13, 2018
    W7UUU, N2EY, K3XR and 3 others like this.
  3. KL7KN

    KL7KN Ham Member QRZ Page

    There are a number of mods to add the RIT function to your -101. Sometimes that is the beauty of the other HEath - gear, someone has a proven mod.

    I purchased a HW-7 with known problems, because most problem with this older gear have long been sorted and documented.... Just plain fun to play with as well.

    Have some fun!
    AA4OO and WD4IGX like this.
  4. KE4OH

    KE4OH Ham Member QRZ Page

    Yes! The Heathkit CW filter is very good. It's also really helpful to put you right on a received signal at the correct offset.

    As KL7LN mentioned, there are several different mods out on the interwebs to add RIT to the HW-100/101. There is one that was published in 73 magazine that doesn't require opening up the VFO. However, I'll tell you that I don't miss RIT when I use the CW filter.

    I also have an SB-101 with the SB-640 external LMO. It also has the Heathkit CW filter. So, even though the SB-640 gives me full RIT capability, I find that I almost never use it on CW except for working DX stations split. The filter does what I need without RIT.
  5. AA4OO

    AA4OO XML Subscriber QRZ Page

    Excellent info guys. Thank you.
  6. N8AFT

    N8AFT Subscriber QRZ Page

    Your rig sounds Great OM!
    AA4OO likes this.
  7. N0JI

    N0JI Ham Member QRZ Page

    Love the Hot Water on CW. It was my first rig as a novice, still have it. Not a bad radio at all. However, I do miss the Russian Woodpecker every time I fire it up!
    WA4CZD likes this.
  8. N2EY

    N2EY Ham Member QRZ Page

    What @K7TRF wrote is 100% spot-on!

    (btw, that HW-101 pic looks super-clean)

    There are actually at least three different Heath CW filters. The earliest one was used in the SB-300, and has a 60 dB bandwidth of 2.5 kHz. (not sure of the part number. You have the improved one, 404-284-1, with 60 dB bandwidth of 2.0 kHz. (The third is used in the SB-104 and is not directly compatible).

    There are both silver and black versions of the 404-284-1. AFAIK they are identical performers.

    btw - one of the ways Heath saved money on the HW-100 and HW-101 was to use a less-expensive SSB filter.

    The SB-101, and probably all the other SB-series rigs, used the 404-283 SSB filter, which has 6 dB bandwidth of 2.1 kHz and 60 dB bandwidth of 5 kHz.

    But the HW-100 and HW-101 use the less-expensive 404-328 SSB filter, which has 6 dB bandwidth of 2.1 kHz and 60 dB bandwidth of 7 kHz.

    How much difference this makes in real-world operation I will leave to others. But I suspect that the 404-283 filter would work fine in an HW-101.


    40 years ago - 1978 - I was at the Rochester NY Hamfest, which was the biggest hamfest I've ever been to. I came across a seller there who had a collection of Heathkit parts for various things - including a box of crystal filters. They were all identical SSB filters - the 404-283 - except for one, at the very bottom of the box, which was a 404-284-1 CW filter. $5 each - so I got a CW and SSB filter.

    Around that CW filter I built a CW transceiver.....but that's another story.

    73 de Jim N2EY
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    FYI, I found this listing of HK filters (link). It sez your pn 404-284-1 filter is 400 Hz wide @ -6dB and 2000 Hz wide @ -60 dB. That's Shape Factor = 2000/400 = 5:1. I hate to say this but, BEFORE filter technology improved, that was awful. Still, it it works for you then, OK. :)
  10. N2EY

    N2EY Ham Member QRZ Page

    Awful? I disagree!

    Look at any of the OEM CW filters from the 1960s/70s and you won't see much better. (Remember we're talking CW filters, not SSB filters).

    73 de Jim, N2EY

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