Topband: Re: Sept/Oct NCJ article on DX Prowess (LONG)

[Bill Tippett <btippett@alum.mit.edu>]

        Attached is a response from Tadek SP7HT to my
recent letter.  Tadek is not on this reflector so I'm forwarding
his response.  As you can see, Tadek targeted his article
primarily to the DX community and not contesting.

                                73,  Bill

W4ZV wrote:
>Hi Tadeusz,

I just got my new NCJ and scanned your article. I was surprised by the 
following statement (next to last paragraph on page 4):

"These statements mean for me that somebody unfamiliar with all measurement 
set-up and procedures nuance cannot use data derived "directly" from Swept 
BDR or IMD DR3 oscilloscope graphs close to the listening frequency instead 
of point results measured manually. Therefore, I have consequently used in 
my Table only manually measured test point data at 5 kHz and 20 kHz spacing.?

I had believed that ALL discrete points plotted on the ARRL graphs came 
from manual measurements, so I asked Mike KC1SX who did ARRL's measurements.
I see Mike copied you on his reply which did confirm that ALL discrete 
measurement points in the ETR swept graphs are done manually. Thus the IMD 
and BDR data down to 1kHz spacing (always the innermost point in all ARRL 
plots since 1997) are perfectly valid for comparison (although sometimes 
difficult to interpolate...I notice Mike has included the 1 & 2 kHz 
measurements in the tables for the IC-7800 ETR).

SP7HT Response:
>May be, that was caused by language problem.

English is not even my second but a third language. During my professional 
career in satellite microwave telecommunication I?ve made hundreds of SWEEP 
Amplitude / Frequency Response, Group Delay Response (and so on) graphs. 
They ALL have been made automatically in decided frequency range. The 
result was always a SOLID line.

In case of point measurements the result was a Table or graph with results 
indicated. Sometimes, an approximation was made between points. But there 
have to have be used a DOTTED or DASHED line (never solid line) between 
measured points.

Therefore my interpretation of SOLID line used in ?Swept BDR? and ?Swept 
IMD DR3? graphs was as made during automatic CONTINUOS SWEEP in programmed 
frequency range.

I think that terminology of ?Swept BDR? and ?Swept IMD DR3? is misleading 
for me.

And the question: why, if they are made ?point manually?, are they named 
?Swept??
I propose rather ?close and wide range BDR? and ?close and wide range IMD 
DR3? instead of.

Now I know that all are manually made point data.

W4ZV wrote:
In general, I agree with much of what you said, but have some significant 
differences in the following areas:

1. 5 kHz spacing (BDR and IMD) is far too wide for realistic situations in 
my opinion. I actually favour ARRL's 1 kHz measurements as being more 
realistic, especially for contesters. When do you last recall a contest 
with signals spaced only 5 kHz? In my opinion, 500 Hz or even closer is 
much more common, especially in the major contests.

SP7HT Response:
>As in stated above (in my article title and many times in the article 
text I?ve consequently pointed out that ?it is valid for DX oriented 
Hams?.) My article was directed towards DX Community and NOT towards 
Contester Community.

W4ZV wrote:
>2. By only looking at 5 kHz spacing, several of the receivers you 
reviewed appear to be much better than at much closer spacing. A good 
example is the Elecraft K2. I have a K2 and like it very much for its 
intended use, but the receiver basically falls apart at close signal 
spacing. BDR drops to ~116dB and IMD drops to ~67dB for 1 kHz spacing. 
Compare this to Orion with BDR of ~119dB and IMD DR3 of ~84dB at 1kHz 
spacing (ARRL data using the 1000 Hz roofing filter).

SP7HT Response:
>I fully agree with you. Have you experimented with 2 diodes across Xtal 
Filter in K2 receiver chain? Such modification has been announced by 
Elecraft some time ago.

W4ZV wrote:
>3. Given "proper design" (similar to your "not good" comment on page 11 
about IMD DR2 and IP2), I would rank IMD DR3 as the most important 
parameter for the following reasons:

a. Unless you have an extremely close neighbour, or unless your BDR is 
terrible, BDR is not likely to be as significant a problem as IMD DR3. The 
reason is very simply that BDR performance is normally much higher than IMD 
DR3, so the latter becomes the most critical since you will reach the IMD 
DR3 limit sooner. W8JI cites an example on 160 below where he sees the 
maximum dynamic range needed being around 95dB. As long as BDR is above 
this number, or unless you have a very close neighbour operating at kW 
power levels, any BDR above 95 dB should be sufficient.
<http://www.w8ji.com/receiving.htm>http://www.w8ji.com/receiving.htm

SP7HT Response:
>I think, this is Contester point of view, when many strong signals are 
inside main selectivity Crystal Filter pass-band.

But, in typical DX-Pedition pileup, they are spread out from 5 kHz to 15 
kHz from the listening channel. Such case ? in my opinion - the BDR is the 
main factor because:
a)                    even single, but strong enough signal, can cause 
blocking,
b)                    for IMD DR3 problems two strong signals, with strict 
frequency relation shall appear at the same time (less probability to 
occur). I repeat: this is DX?Pedition Hunters point of view.

W4ZV wrote:
>b. Phase noise also becomes critical at very close signal spacing. An 
example of poor phase noise performance is the IC-7800. At close spacing of 
1 and 2kHz, you can see in ARRL and RSGB results that Phase Noise overrides 
the receiver's IMD and BDR performance (see asterisks by measurements on 
pages 17 & 18 of ARRL's ETR). As KC1SX describes on page 34 of his August 
2004 QST article, Phase Noise becomes "effective BDR on a noise limited 
measurement". I fully agree with ARRL's definition since it makes no 
practical difference to the user whether the source of the noise is due to 
the internal BDR or Phase Noise performance. Thus, assuming the receiver's 
Phase Noise is always below IMD and BDR, it is not so important. But of 
course if Phase Noise is poor, then it becomes more important than either 
BDR or IMD DR3 since it will over-ride those effects at close spacing.

c. Assuming Phase Noise is below both BDR and IMD DR3 effects, and assuming 
BDR is "adequate" for your operating conditions, then I feel very close 
spaced IMD DR3 (1kHz or even less) becomes the critical parameter. This is 
primarily because it is not uncommon to have many strong signals spaced at 
500Hz (or even less) in a contest capable of creating IMD DR3 products. 
This was my primary motivation in working with Inrad to develop the 600Hz 
#762 filter for Orion. At this BW, even 500Hz spaced signals will fall 
outside the +/- 320Hz 6dB BW of the #762, and will be attenuated by >20dB 
(and hence IMD DR3 products should be attenuated by >60dB).
<http://users.vnet.net/btippett/inrad_.htm>http://users.vnet.net/btippett/inrad_.htm

SP7HT Response:
>Yes, I am watching ?Orion Digest? mailing with most interesting comments 
/ news and I?ve noticed your modification several weeks ago.

W4ZV wrote:
>I would rank very close-spaced IMD DR3 as my #1 criteria, as both W8JI 
and Sherwood Engineering do. Granted, this assumes that both BDR and Phase 
Noise are "adequate" as described above. W8JI has some very practical 
advice below which relates to what levels of MDS, BDR and IMD DR3 are "good 
enough".
<http://www.w8ji.com/receiver_tests.htm>http://www.w8ji.com/receiver_tests.htm

SP7HT Response:
>You are (as W8JI is) a 160 meters operator and you reflect in your 
opinion realities of that band.

I am not 160 meters operator: I live in block of flats in big town. The 
local industrial noise level is so high that, even using ANC-4, it is hard 
to receive DX even on 80 meters. DX operation on 160 is not possible yet. 
Therefore I am planning to build FLAG or Pennant antenna on the roof. I do 
not know if it will improve the situation.

I do not have any experience on 160. I can only receive your opinion as a 
new piece of knowledge about top band demands for effective DX-ing. Thanks 
for that.

I?ll look Websites you have advised me.

Many thanks for all.
Tadek, SP7HT




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