[Elecraft] K3: SSB - WPX - Filters - Controls

[Ed Gray W0SD]

Why a 1.8 filter?

Well put Ed-W0YK. I am no electrical engineer, ie have no electronics 
background, but I do my best to understand.   In addition to what 
Ed-W0YK mentioned there are at least a few more situations besides 
contests for a 1.8 filter on receive for SSB. I have been on the bands 
everyday working a lot of DX and I note the RTTY and CW and SSB DX 
stations are more and more going to split frequency.  Many are just 
saying UP and many are saying up 1 and many SSB stations are just saying 
up.  I have no problem with this as it is best if the DX station does 
not use anymore bandwidth than possible operating split.  I can see why 
they go split as "simplex" can be a real mess.  The point is calling 
stations are really getting close to the listening(DX)frequency.  As we 
move into a season of more sporadic E(can really be strong) on the 
higher bands calling stations this close are going to be a problem as 
they will be much louder than they are now.  With hopefully more 
sunspots down the road F2 signals more local will be very loud on 17M 
and up.  Even now on 20 meters and down they already often are loud so 
we need all the IMD protection we can get.  The l.8 filter roofing 
filter with DSP can make a difference.

On CW, RTTY and SSB I think a number of people lose track how close they 
are getting to the DX station and I often have stations on CW and RTTY 
calling with in 300-500 hz and on SSB with in 1000 hz.  What makes this 
especially bad is they can call that close for a long time as they don't 
get worked by the DX station.  Having operated a number of DX-Peditions 
I always made sure not to work anyone real close to my frequency and I 
note others do the same to prevent station getting almost on top of the 
DX stations frequency. Sometimes the calling stations figure it out that 
they are to close and sometimes the "policeman" get them moved(not easy 
as they often are not listening to their transmit frequency) but then 
you have the policeman QRM.

Another thing that happens quite commonly is two DX stations with the 
one having a listening frequency range that includes or gets extremely 
close to the other DX stations frequency.

A second reason is a DX station operating split often accidentally gets 
"TRASHED" by a very close station(usually does not know the DX station 
is there) 1 or 2 Khz away or right on top of them and then there are the 
"Policeman" trying to get them to move. It can be difficult to copy a DX 
station even with reasonable strength when this happens.  Obviously if 
they are right on top there is nothing we can do with the rcv but often 
they are not right on the frequency and the rcv comes into play.

A third possible reason that I am interested in finding out about under 
actual operation is combating QRN on SSB with 1.8 and the proper AGC vs 
a wider filter and the proper AGC. I think this somewhat depends on the 
hearing from person to person but there is a lot of difference between 
2.7 to 1.8 in filter width.  This would mainly apply to 80 and 60 meters 
I would think as there is little SSB on 160M on the low end but could 
come into play higher in the band.  I know from a lot of moonbounce 
experience a narrower filter often helped as long as it was not to narrow.

A fourth reason which has often been mentioned is if you have hams that 
live close to you! Admittedly they are not likely to operate that close 
to you normally that the difference in filters would make any difference 
but you both might want to chase the same DX station at the same time as 
sometimes the window to a rare one is just 10 or 15 minutes a day and 
the DX station is just listening in a narrow range up a little.  In this 
situation the other station can be pinning your meter so you need all 
the help you can get!

Fifth even if you don't work DX or Contest but have any schedules or 
just get on and call CQ and ragchew so often even after asking several 
times if the frequency is in use some one can get very close to you and 
also band conditions change.  Also if you like nets it is not at all 
unusual to have a very strong station interfering with NCS and other 
stations on frequency.  Maybe it is because I live in the middle of the 
country but QRM very close in frequency is a constant issue for me; 
especially on SSB no matter what type of operation I am doing.

Obviously as W0YK said it depends on what type of operating you do. If 
you only do cw, digital and rarely if ever get on SSB then the 1.8 
filter is not needed.  If you get on SSB much at all to me it seems to 
make economic sense to eventually get a 1.8 or 2.1 filter for receive 
given we are not talking about a lot of money difference, maybe 4% -7% 
of the total price.  IMHO on SSB receive the major advantage a K3 has 
over any other radio on the market would be the narrower roofing filter 
than the competition.  The only other advantages I can see on SSB over 
others might be a more effective NR/NB but of course that only comes 
into play if you have noise.  There may be an advantage in lower DSP 
noise, AGC hangup, etc. based on what Bob Sherwood found.  Not having my 
K3's yet(I ordered two as you just can not argue with the spec's and 
experience of knowledgeable operators)I am not sure about these last two 
but comments from very knowledgeable posts here indicate there may be 
some advantages now and possibly more down the road with firmware 
updates in these two areas over any other radios on SSB.  The advantages 
on CW, RTTY, etc. are obvious.  The good news you can add a roofing 
filter at any time.

Since it has been consistently pointed out the DSP does a good job past 
the -30 db points of the roofing filter being used I made the following 
chart from various sources on the Elecraft sites.  Looking at the -30 db 
points we see the following figures for filter width:

1.8 Filter 8 pole  2500 hz
2.1 Filter 8 pole  3200 hz  (700 hz more than 1.8 filter)
2.7 Filter 5 pole  5600 hz  (3100hz more than 1.8 filter)
2.8 Filter 8 pole  3400 hz  (900 hz more than 1.8 filter)

Looking at the -6db points we see the following figures for filter width:
1.8 Filter 8 pole   1913 hz
2.1 Filter 8 pole   2175 hz  (262 hz more than 1.8 filter)
2.7 Filter 5 pole   2910 hz  (997 hz more than 1.8 filter)
2.8 Filter 8 pole   2888 hz  (975 hz more than 1.8 filter)

The following information came from Eric WA6HHQ of Elecraft 13-09-07 
post, 8 pole plots posted by Elecraft, 5 pole info from OH9HB plot for 
200 & 500 hz. filters.  There could be a slight error in my ability to 
read the points on the plots but they are close.  Hopefully I did not 
make any typo's or incorrect subtractions!

Filter Pole     BW(-6db)  -3db** -30db   -60db*** Shape factor
200      5         224      200    500     860       4.0
250      8         370             430     785       2.1
400      8         435             640     935       2.1
500      5         565      465   1100    1680       3.1
1000     8        1063            1400    1650       1.6
1800     8        1913            2500    2863       1.5
2100     8        2175            3200    3213       1.5
2700     5        2910     2710   5600    8439*      2.9
2800     8        2888            3400    4488       1.6


* calculated
** source Eric WA6HHQ 13-09-07 post
***Really not a significant factor with DSP and the K3 design

In closing I defer to the electronic experts on the significance of 
these figures in day to day ham radio operation.  I understand the point 
that bad signals with real output inside the filter can be the limiting 
factor if the difference in filter width is not large but to me 997 hz 
at -3 db and 3100 hz at -30db between the 1.8 and 2.7 filter is quite 
large even for SSB.  We can not control how other set their transmitter 
but at least when someone is "clean" it seems to me that the 1.8 or 2.1 
filter will give a noticeable advantage even with the DSP since these 
two filters are narrower at -6db all the way to -30 db.  Given the shape 
factor of 2.9 for the 2.7 filter it widens out quite rapidly from -6 db 
to -30db points where the other two have a shape factor of 1.5 and do 
not widen out nearly as fast.  Even stations that are not clean the 1.8 
filter should permit us to hear closer to them than the 2.7 filter.

Therefore with the 1.8 filter we get 997 hz more protection from strong 
signals at -6 db and 3100 hz more protection at -30 db over the 2.7 five 
pole "STOCK" filter.  For the 2.8 eight pole filter we get 975 hz more 
protection at -6 db and 900 hz at -30db from strong signals.

73, Ed W0SD