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Old October 30th 08, 07:26 PM posted to
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Default DXing the AM BCB band in ECSS/SSB vs. AM

DXing the AM BCB band in ECSS/SSB vs. AM

I always DX my MW band in ECSS/SSB mode, but was vaguely aware that
some of
my MW buddies DX in the AM mode. From years ago I got used to the fact
SSB was better, so I was quite surprised when a chap I exchange mails
in Brisbane said I should have another try in AM mode. So I did and
amazed to find that in many borderline stations I could render it
audible in AM but not hear it properly in SSB. So now I will do more
DX in
AM and hope for better results on my excellent 7700.

When DXing weak AM MW stations in SSB I have found it best to use the
15 kHz roofing filter for the 3 kHz filter (before I was using the 3
kHz roofing filter) and then use the 3 kHz roofing filter with the 2.5
and 2 kHz filters. There is a slight but definite improvement using
the 15 Khz roofing filter if the station is not interfered with.

It is interesting to note though that this does not hold true for
reading weak hams on 14 megs. There definitely the 2 kHz filter with 3
kHz roofing filter gives the best results.
Interesting and useful to know when the chips are down.

I gave the 7700 a workout in AM listening daytime to the furthest and
faintest signal that I could.
Of the various combinations these were best in AM mode on an AM BCB
station 700 km's away:
6 filter and 6 roofing filter = best
3 filter and 3 roofing filter = next
9 filter and 15 roofing filter = last, but not by much

At the prompting of Richard in Brisbane I have been doing more MW
DXing in
AM mode instead of my usual ECSS/SSB mode. I have been amazed to find
the AM is better than SSB most of the time. I was unaware that the AM
on the
7700 in the MW band was so good, and had previously ignored it under
impression that SSB was much better. But not so - you live and learn
and old
dogs have to learn new tricks!

So I asked a friend in Vancouver BC to test the IC-7700 for AM
sensitivity on the MW band and he replied:

Hi John,

Managed to run a few tests on 1010 kHz, a nice frequency in the middle
the MW band. See:

The main objective was to demonstrate that the performance of the 7700
receiver is essentially undiminished below 1.6 MHz.

Cheers for now, 73,
__________________________________________________ ____

Richard Bogusz in Brisbane mailed me this:
AM.. I think many swl's have been under the spell of ssb/ecc
for years... probably for 2 reasons.. A/ they have listened to
utilities/hams in ssb using 2.1kc filters and have seen the benefits
narrow filtering and then thought that this will also always apply to
broad AM signal as well in a crowded band. B/ they have looked at
sensitivity figures for their rec's and seen that the ssb's fig's are
lower hence they believe will hear more.... It is true that there
situations that ssb is better than AM ... but hardly ever where
sensitivity and resolution is needed as far as I have seen.. This
not only to your 7700 but to older rigs unless they are of poor
design. The
Collins 390a has a very conventional envelope AM detector ... same as
99% of
radios out there .. you will hear AM sigs in AM mode that are
indecipherable in ssb mode... Note .. sync/dets are no more sensitive
envelope dets... but there are obvious fading/distortion benefits..


Assuming that selective fading is not an issue during listening,
demodulating the complete AM signal with an AM demod will yield 3 dB
baseband power than demodulating one sideband and "binning" the other
(neglecting the carrier, which does nothing more than supply an "LO"
demodulation of the sidebands.)

SSB sensitivity figures are always lower than those for AM, because
the IF
bandwidth in SSB (J3E) mode is 50% (or less) that in AM (A3E) mode. In
addition, the receiver supplies a carrier of fixed amplitude for
demodulation of the recovered sideband.

I have never encountered a situation where I could not recover an
intelligible baseband from an A3E signal, even though aggressive
conditioning was sometimes required (NR, NB, notch filter etc.)

Cheers for now, 73,

So what do you find when DXing the BCB AM band?
Is AM mode better or ECSS/SSB mode? (taking into account of course
that there are situations working the splits when you are forced to
use SSB)

John Plimmer, Montagu, Western Cape Province, South Africa
South 33 d 47 m 32 s, East 20 d 07 m 32 s
Icom IC-7700, Icom IC-756 PRO III with MW mods
ERGO software
Drake SW8. Sangean 803A
Sony 7600D, GE SRIII, Redsun RP2100
Antenna's RF Systems DX 1 Pro Mk II, Datong AD-270
Kiwa MW Loop.

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