Tunable carrier (birdie) 20445khz

[Edward R Cole]

Just for fun I tested the response of my K3 to external signals with
ANT1 directly terminated with a 50-ohm term (no coax).  The radio has
2m transverter connections, 6m preamp in/out, and both IF out
connections, 10-MHz Ref cable, keys and Inhibit control lines
connected to ACC per usual set up.

Using my XG3 is a signal source on 14.020, I get an S3 signal in CW
with 0 dBm level, but nothing is heard at -33 dBm.  I think this
indicates pretty good shielding.  I would doubt that external birdies
would be heard with ANT terminated.  They would have to be very
strong, radiating from something in the shack, to be heard.

Therefore, making internal birdie survey should work well if you
terminate the antenna (use no coax).
PS:  test with ANT1 open with no cable connected is the same.  I did
note than if I placed the XG3 antenna over the speaker grill of the
K3 I could get a weak signal at -33 dBm.

Note:  My XG3 is installed in a diecast Hammond box with BNC
connector so radiation is only via the BNC connector to a 2m rubber
duckie.  Using this enclosure with a 20-dB fixed and 30-dB step
attenuator I can reduce the XG3 signal to -157 dBm for testing my eme
system at the preamp input.


73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-100w, 1296-60w, 3400-?
DUBUS Magazine USA Rep [hidden email]
======================================

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[Edward R Cole]

Don,

For those who chase weak signals, any spurious signal generated
inside the receiving system on the receiving spectrum is trouble.  I
operate with a 144-MHz receiver with -156 dBm sensitivity and add
21.3 dBi of antenna gain to that, so I hear VERY WELL.

I find most of the problem signals come from external to my station
and are directly picked up by the antenna system.  Nothing to do
about that unless one moves to the far side of the moon.  But any
internal spurious or mixing product signal is a problem when you are
working with such extreme weak signals.

One asset of the preamp gain is sky noise is amplified by 25-dB which
can cover up the weakest internal "birdies".  At 144-MHz sky noise is
approx. 200K min.  but at 1296-MHz it is 10K, or less, so birdies can
be more troublesome.  28.000-28.100 MHz is the sub-band that needs to
be birdie free for VHFers, etc. as most transverters output there for
the weak-signal frequencies.

If you are one of the growing users of WSPR on HF, those frequencies
are sensitive to interference from "birdies", as detection is -29 dB
below noise in 2.5 KHz bw.  HF sky noise really sets a limit on
receiver sensitivity, though.

73, Ed - KL7UW

------------------------------

Message: 24
Date: Sat, 20 Aug 2011 23:28:15 -0400
From: Don Wilhelm <[hidden email]>
Subject: Re: [Elecraft] Tunable carrier (birdie) 20445khz
Cc: 'Elecraft' <[hidden email]>
Message-ID: <[hidden email]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Ron and I will have to just disagree with the definition of a "birdie" -
and it is only a matter of definition.

My definition is the result of multiples of the various signals and
oscillator frequencies present in the receiver, and since they are
always multiples greater than 1 of any signal (or oscillator), will
always result in a fast tuning response.

OTOH, there are unavoidable mixing products in any down-conversion
receiver that will tune as a normal signal.  The goal of the designer is
to choose the IF frequencies to keep those spurious responses out of the
bands of interest to the target users - in this case, the ham bands.

So, if your definition of "birdies" agrees with Ron's, so be it - I will
continue to refer to extraneous direct mixing products (those responses
that do not produce fast tuning signals) as spurious responses.  It is
just a matter of definition.

BTW, this is one of the advantages of up-conversion - those direct
responses are so far away from the desired signal that they do not
become troublesome, but up-conversion designs have their own share of
troublesome problems.

73,
Don W3FPR




73, Ed - KL7UW, WD2XSH/45
======================================
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 50-1.1kw?, 144-1.4kw, 432-100w, 1296-60w, 3400-?
DUBUS Magazine USA Rep [hidden email]
======================================

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