Re: DT-400W filter mod results comments


Michael Slattery
 

Hi Steve :)

Thanks for expanding on your results with the mod. I agree that the
skirt selectivity is far from ideal. The published specification is
claimed to be better (35dB) than your measurement of 25dB. But I've
found that these figures can change with in-out load impedance
termination variations which are in any case difficult to set
precisely within the context of a ULR.

When I got my DT-400W I was extremely disappointed by the very poor
selectivity. The AM band was full of splatter. The mod was a quick fix
to calm the irritation I felt every time I used the radio.

I'm fully aware of the performance provided by the premium filters you
and others have been using for 9-kHz split DX chasing. I've used them
myself in my AOR AR7030 for many years but not yet in a ULR. So as
Gary commented (up the thread) I too have been spoiled by experiencing
the superior characteristics of this type of filter.

73

--
Michael Slattery
Sheffield, UK

On Thu, 2 Apr 2009 20:04:20 -0700, you wrote:

Hi Michael,
Thanks for your comments. Yes, I used the LTM450HT filter in the DT-400W
mod, purchased from the eBay UK seller Mainline. I bought 5 of them and
tested several on the bench before installing one. All had the same poor
skirt selectivity. Initial skirt depth was about 40 db, not bad at all
considering the tiny size of the filter, but it quickly rose to 25 dB about
5 kHz either side of the passband. That's not good enough to give decent
adjacent channel selectivity in strong RF environments. The Murata K 455 kHz
filter that I've installed in Eton E100's and CCrane SWP's has 75-80 dB
skirt depth and it doesn't rise past the passband. It's physically
approximately 4 times longer than the LTM filters, and is an example of an
excellent ceramic filter, and its performance is reflected by its
approximate $50 price.
I'm happy the LTM450HT filter mod worked well for your particular RF
environment. It didn't produce the desired results for me. Apparently our
North American urban RF environments are more challenging than one might
think. :) I don't have that problem in the rural area where I live, with no
strong stations nearby; I send radios to Gary DeBock to test. He's able to
properly evaluate how radios perform in his urban RF environment.
73,
Steve
----- Original Message -----
From: "Michael Slattery" <knallebo@...>
To: <ultralightdx@...>
Sent: Thursday, April 02, 2009 6:47 PM
Subject: Re: [ultralightdx] Re: SRF-M37V and SRF-M37W-Performance
differences?


Whether the 450 kHz filter modification would be useless or not probably
depends on the type of DX you pursue.
Before carrying out a mod you have to determine what you expect.

As the author of the DT-210L & DT-400W filter mod articles, I find the
comment that the mod is 'not worth all the effort' frankly incredible!

The mods were not offered as a panacea for turning these radios into
9/10kHz-split DXers machines. That would be impossible with the type
of filters (6kHz @6dB BW) used. Rather, the articles were offered in
the spirit of improving the usability of these radios in the domestic
context. The give-away line in my article was 'the intrinsic quality
of the audio remains.' By implication therefore, the mod filter is not
especially selective merely an improvement on the original.

The moral of the story is to make sure any ULR you intend to mod for
9/10kHz-split DXing has a 455kHz IF system and the ability to tune in
at least 1kHz steps or better. 450kHz IF radios are a waste of time
since narrow filters with a good shape factor are simply unavailable.

Whilst agreeing fully with the remark by Steve about skirt selectivity
and filter size, I have to ask:

Was the LTM450HT filter used? Nothing else will bring much improvement
except for a LTM450IT or even narrower filter, if you could find one.
The 'T' suffix in the filter model number is important since these
have superior skirt selectivity over those without the 'T' suffix.

Living here in the UK I frequently wonder what North American DXers
mean by a 'high RF environment'. Amongst others I live within 15 miles
of one 150kW & two 400kW MF transmitters & less than 1 mile of two 1kW
MF transmitters.

The DT-210L & DT-400W filter mods have transformed the reception
capabilities of these radios close to these powerful signals. Before
the mod, five (9kHz) channels either side of the frequency of one of
these strong locals were rendered useless for weak signal reception by
the original filter. After the mod I am able to hear weak signals on
the immediately adjacent channels (±9kHz) with only a small level of
adjacent channel interference.

Perhaps your RF environment(s) are not really that challenging? Maybe
that is the reason you are not reaping the reward from the mod?

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