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low pass filters

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JHRichards

low pass filters

Good day - What is the consensus concerning low-pass filters to
minimize RFI both coming in and going out of the shack? Is there a
type or size, brand or model that comes highly recommended? Is it
prudent to employ one, regardless of whether or not there is a known,
identified problem , i.e., just in case? Does it assist reception as
well as transmission?

What is the real world skinny?

K8JHR
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Don Wilhelm

Re: low pass filters

A low-pass filter will not change RFI either coming into the shack or
going out. There are low-pass filters in all transceivers produced by
Elecraft (and most other manufacturers), and they are in both the
transmit and receive path.
The current FCC requirement is for the 2nd harmonic be suppressed by 43
dB over the fundamental.
Low pass filters come from a past era when the FCC requirements were not
as stringent and TVs responded to those higher order harmonics from
amateur transmitters. That is no longer the case.

Considering receive, we do have many sources of unintentional radiation
which is present in the HF spectrum, and other than locating and having
the offending device turned off, there is not much we can do about it.
Although if it is interfering with licensed operation, there is recourse
by cooperation with the FCC. The products giving problems range from
inexpensive switching power sources (read wall-warts) to control
circuits in many household appliances and other consumer devices, and
even some lighting products.

If you are operating in an environment like Field Day where there are
multiple transmitters operating in close proximity, bandpass filters and
single band antennas are the normal "cure" as well as physical
separation of the antennas.

73,
Don W3FPR

On 2/17/2019 5:59 PM, Richards wrote:
> Good day - What is the consensus concerning low-pass filters to
> minimize RFI both coming in and going out of the shack? Is there a
> type or size, brand or model that comes highly recommended? Is it
> prudent to employ one, regardless of whether or not there is a known,
> identified problem , i.e., just in case? Does it assist reception as
> well as transmission?
>
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Ken G Kopp

Re: low pass filters

OT: A caution related to external low-pass filters …

Many were made with cutoff frequencies -below- 6M and these
exhibited high reflected power above 10M.

Trivia, FWIW

73!

Ken - K0PP

On Sun, Feb 17, 2019 at 4:26 PM Don Wilhelm <[hidden email]> wrote:

> A low-pass filter will not change RFI either coming into the shack or
> going out. There are low-pass filters in all transceivers produced by
> Elecraft (and most other manufacturers), and they are in both the
> transmit and receive path.
> The current FCC requirement is for the 2nd harmonic be suppressed by 43
> dB over the fundamental.
> Low pass filters come from a past era when the FCC requirements were not
> as stringent and TVs responded to those higher order harmonics from
> amateur transmitters. That is no longer the case.
>
> Considering receive, we do have many sources of unintentional radiation
> which is present in the HF spectrum, and other than locating and having
> the offending device turned off, there is not much we can do about it.
> Although if it is interfering with licensed operation, there is recourse
> by cooperation with the FCC. The products giving problems range from
> inexpensive switching power sources (read wall-warts) to control
> circuits in many household appliances and other consumer devices, and
> even some lighting products.
>
> If you are operating in an environment like Field Day where there are
> multiple transmitters operating in close proximity, bandpass filters and
> single band antennas are the normal "cure" as well as physical
> separation of the antennas.
>
> 73,
> Don W3FPR
>
>
>
> On 2/17/2019 5:59 PM, Richards wrote:
> > Good day - What is the consensus concerning low-pass filters to
> > minimize RFI both coming in and going out of the shack? Is there a
> > type or size, brand or model that comes highly recommended? Is it
> > prudent to employ one, regardless of whether or not there is a known,
> > identified problem , i.e., just in case? Does it assist reception as
> > well as transmission?
> >
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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K9MA

Re: low pass filters

> On Sun, Feb 17, 2019 at 4:26 PM Don Wilhelm<[hidden email]> wrote:
>
>
> Low pass filters come from a past era when the FCC requirements were not
> as stringent and TVs responded to those higher order harmonics from
> amateur transmitters. That is no longer the case.

Here, Don is talking about external low pass filters intended to
suppress VHF harmonics. If you live in an area where TV signals are
weak, they may still be necessary. Of course, hardly anyone receives
their TV service that way any more, and most TV stations have moved to
the UHF bands, anyway.

The low pass filters in solid state radios and amplifiers are necessary
because their outputs are untuned, and the nonlinearity of transistors
generates lots of harmonics, including low order ones. In the days of
vacuum tubes, the pi network served as an effective filter. The popular
pi-L was even better.

It might be interesting, during a major contest, to listen on the second
harmonic, say around 14.120 when there's a lot of activity on 40. Many
are using solid state amplifiers now, and even 43 dB down may be
audible. I've never positively identified one, but a lot of stations
don't come back to me for other reasons, so it would be hard to tell.

73,

Scott K9MA

--
Scott K9MA

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W2xj

Re: low pass filters

OTA TV is growing rapidly which is why cable companies are hurting. I’ve been exclusively OTA for over 10 years. Also, with the repack a number of stations are being moved back to VHF.

Sent from my iPad

On Feb 18, 2019, at 11:41 PM, K9MA <[hidden email]> wrote:

>> On Sun, Feb 17, 2019 at 4:26 PM Don Wilhelm<[hidden email]> wrote:
>>
>>
>> Low pass filters come from a past era when the FCC requirements were not
>> as stringent and TVs responded to those higher order harmonics from
>> amateur transmitters. That is no longer the case.
>
> Here, Don is talking about external low pass filters intended to suppress VHF harmonics. If you live in an area where TV signals are weak, they may still be necessary. Of course, hardly anyone receives their TV service that way any more, and most TV stations have moved to the UHF bands, anyway.
>
> The low pass filters in solid state radios and amplifiers are necessary because their outputs are untuned, and the nonlinearity of transistors generates lots of harmonics, including low order ones. In the days of vacuum tubes, the pi network served as an effective filter. The popular pi-L was even better.
>
> It might be interesting, during a major contest, to listen on the second harmonic, say around 14.120 when there's a lot of activity on 40. Many are using solid state amplifiers now, and even 43 dB down may be audible. I've never positively identified one, but a lot of stations don't come back to me for other reasons, so it would be hard to tell.
>
>
> 73,
>
> Scott K9MA
>
> --
> Scott K9MA
>
> [hidden email]
>
> ______________________________________________________________
> Elecraft mailing list
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Phil Kane-2

Re: low pass filters

In reply to this post by K9MA

On 2/18/2019 8:41 PM, K9MA wrote:

> Here, Don is talking about external low pass filters intended to
> suppress VHF harmonics. If you live in an area where TV signals are
> weak, they may still be necessary. Of course, hardly anyone receives
> their TV service that way any more, and most TV stations have moved to
> the UHF bands, anyway.

Two assumptions that may not hold even in urban areas. Many cable /
satellite subscribers are "cutting the cord" and going to over-the-air
reception (I use that as a backup strategy when - not if - C*cast goes
down), and because of the FCC's ill-regarded "repacking" scheme where
UHF TV channels above 37 are being "re purposed" for the "broadband"
industries' voracious need for spectrum, some urban TV stations are
going back to their pre-UHF channels.

With that said - I still use the "good old" external low-pass filter on
my K2, rather than have it gather dust in the garage. Better be safe
than sorry.

73 de K2ASP - Phil Kane
Elecraft K2/100 s/n 5402

From a Clearing in the Silicon Forest
Beaverton (Washington County) Oregon
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K9MA

Re: low pass filters

Hang on to that low pass filter!

73,
Scott K9MA

On 2/19/2019 10:55, Phil Kane wrote:

> On 2/18/2019 8:41 PM, K9MA wrote:
>
>> Here, Don is talking about external low pass filters intended to
>> suppress VHF harmonics. If you live in an area where TV signals are
>> weak, they may still be necessary. Of course, hardly anyone receives
>> their TV service that way any more, and most TV stations have moved to
>> the UHF bands, anyway.
> Two assumptions that may not hold even in urban areas. Many cable /
> satellite subscribers are "cutting the cord" and going to over-the-air
> reception (I use that as a backup strategy when - not if - C*cast goes
> down), and because of the FCC's ill-regarded "repacking" scheme where
> UHF TV channels above 37 are being "re purposed" for the "broadband"
> industries' voracious need for spectrum, some urban TV stations are
> going back to their pre-UHF channels.
>
> With that said - I still use the "good old" external low-pass filter on
> my K2, rather than have it gather dust in the garage. Better be safe
> than sorry.
>
> 73 de K2ASP - Phil Kane
> Elecraft K2/100 s/n 5402
>
> From a Clearing in the Silicon Forest
> Beaverton (Washington County) Oregon
> ______________________________________________________________
> Elecraft mailing list
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> Post: mailto:[hidden email]
>
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> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

--
Scott K9MA

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