If there is one transmitter per band and no duplex, I think it would work
fine. (If you can get old timers to use knobless radios.) External filters
would easily correct any problems.
The issue is duplex on one band at high local signal levels, where an
external filter would be much too complicated.
----- Original Message -----
From: "bruce whitney" <zuceman@yahoo.com>
To: <topband@contesting.com>; "Tom W8JI" <w8ji@w8ji.com>
Sent: Monday, October 19, 2015 11:59 AM
Subject: Re: Topband: ADC Overload from MW transmitters
This has been an interesting discussion.
I heard a rumor that a very prominent and successfully competent multi-multi
contester in IL was going to an all SDR multiple computer control set-up.
Which would seem to be problematic in light of this discussion?
Anyone else hear this?
Bruce W8RA
--------------------------------------------
On Mon, 10/19/15, Tom W8JI <w8ji@w8ji.com> wrote:
Subject: Re: Topband: ADC Overload from MW transmitters
To: topband@contesting.com
Date: Monday, October 19, 2015, 10:58 AM
I think the problem here is some
people read this as a SDR radios never
overload, or are superior in every case.
Apparently one person thought they were junk because
multiple modest
strength signals would add up to overload them, and that
triggered the
response that was misinterpreted to mean they never overload
under any
condition or were always superior to roofing filtered
systems common in
standard receivers.
In the case I had here, a *single* transmitter totally wiped
the SDR out.
The overload was nothing like the desense or noise in a
traditional
receiver. It just was totally useless. It was useless at any
signal spacing,
because it had no front end selectivity at all that would
reduce levels.
For my application, it was useless. It was far worse than a
K3, which a few
kHz spacing would duplex on most antenna combinations. When
the K3 (or
FT1000MP MKV's) did overload, the overload was a
desense or composite noise
type sound. It would take out noise floor signals worse, be
progressively
less problem for stronger signals, and never be bothered
with any antenna
combinations with strong signals. When the SDR overloaded,
it was just
totally gone for everything, and wider frequency spacing
with the local TX
made absolutely no difference like it does with a normal
receiver. I assume
this was from overflowing the ADC, but it was a very
dramatic sounding
overload.
That, coupled with the fact it did not have a traditional
knob and panel
system and had some transmitter spurs, made it useless here.
But that was
this setup and this application, where a local 1500 watt
transmitter within
a few thousand feet of the RX antennas was being used while
receiving. This
was a single transmitter multi-op, where one TX signal was
allowed on the
air at a time but two or more operators were making
contacts.
I still never find any SDR I listened to, even that one
without a
transmitter running, better than analog detection for my
ears on
"in-the-noise" signals.
73 Tom
----- Original Message -----
From: "Steve Ireland" <vk6vz@arach.net.au>
To: <topband@contesting.com>
Sent: Monday, October 19, 2015 8:07 AM
Subject: Re: Topband: ADC Overload from MW transmitters
> Hi Jim
>
> Clearly in a large US city, there is going to be a
whole larger degree of
> difficulty than here.
>
> Perth is still pretty much a small city in world terms,
with a population
> of about 2 million. In addition to the ABC
transmitters, we have about
> half a dozen other transmitters, but only two of these
have signals of any
> size – 6PR (10kW) and 6IX (2kW), with the former of
these putting in the
> largest signal to me, with its transmitter/antenna on
the banks of the
> Swan River estuary about 15km away.
>
> When I used my HPSDR, originally I had no filtering in
front of the ADC
> and had some overload problems on 160m from the local
BC stations.
> However, a simple Chebyshev HPF got rid of this. Later
when I added the
> Alex bandpass filters, which are part of the HPSDR
design, there was no
> longer any need for the HPF.
>
> The main point, as Phil says in his post, is that the
amount of protection
> an ADC is going to need will vary widely, depending on
factors such as
> local AM BC transmitters and how strong they are. In my
case, all I had to
> do was to use the general coverage facility of the SDR
to look at the
> medium wave here, see which of the signals were largest
and look for a
> suitable HPF design accordingly .
>
> Vy 73
>
> Steve, VK6VZ
>
>
>
>> That's typical of medium-size cities in the US for
high power
>> broadcasters, but major cities typically have twice
as many. Both large
>> and medium-size cities, as well as smaller ones,
typically have 6-10
>> stations in the 5kW range, and more in the 1kW
range. Chicago is typical
>> of a large city (like New York, Boston, Los
Angeles, San Francisco) -- it
>> has 50kW on 670 kHz, 720 kHz, 780 kHz, 890 kHz, and
1,000 kHz. There's
>> also a daytime only station with 50kW on 1160 kHz.
Cincinnati is typical
>> of smaller cities like Indianapolis, Detroit,
Minneapolis, Cleveland, St
>> Louis, and New Orleans, with 2-3 50kW stations and
many smaller ones.
>> Cincinnati 50kW stations are on 700 kHz and 1530
kHz.
>
> I grew up in a small town in WV, with three 5 kW
stations within two miles
> on 800 kHz, 930 kHz, and 1470 kHz.
>
> Bottom line -- there's a lot more broadcasting in the
US than in most
> countries.
>
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