[R-390] Image problem solved - Not.

[Jim M.]

I turned the Collins 390a on today with a quiet 2-4 Mhz band and tuned
around.  It is connected to an inverted vee outdoor antenna.  I also have a
second 390a (Stewart Warner) in the garage connected to a wire in the attic.
Here's the scoop:

The Collins 390a:

Wow, talk about images!  I never noticed before.  

Here is my nearby strong signal environment:  One broadcaster operates at
1240 (75 dB signal) and is now carrying Rush Limbaugh.  Another strong
signal at 920 is carrying Bill O'Reiley (82 dB signal). An oldies music
station is bombing in on 1560 (85+ dB signal). All within a few miles.

Switching up to the 2 Mhz band, on 2480 Mhz (twice 1240) I see a 32 dB image
which is a mix of oldies music (from 1560) and Bill O'Reilley (from 920 AM),
but I hear no Limbaugh (from 1240). Up the band, there is a 2800 Mhz image
that has a strength of 20 dB, and is a mix of oldies (from 1560) and
Limbaugh (from 1240), but not Bill O.  I mention the broadcast content only
because it was easy to recognize by ear where they were coming from.

Next band up, on 3120 Mhz I hear a clean image (30 dB) of the oldies station
(from 1560) all by itself.  I hear nothing at higher frequencies.

Now for the Stewart Warner in the garage:

No images detected at all on the SW, even though the original signal meter
strengths of the stations is about the same as for the Collins.
(unfortunately there was a 30 dB line noise from attic wiring, but I heard
no image carriers under the noise with BFO turned on).

So.... This Collins 390a (with Collins RF deck) is hopping with images from
AM broadcasters, I just never bothered to notice them before.  But
apparently not the Stewart Warner.  When I get the chance, I am going to
connect the SW to the outdoor antenna for another comparison.  It is unknown
what the differences between the two RF decks could be, however I have
noticed that the Collins does tend to be a much "hotter" receiver than the
SW, good AGC action... but tending to pop up the noise floor when no signals
are present.  Maybe too hot.

Jim M.


-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Flowertime01 at wmconnect.com
Sent: Friday, October 26, 2007 10:19 AM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Image problem solved - Not.

Roy,

Good points.

Most of the stuff I hear then has to be mixed products.
Because the side bands are not doubled and sounding like garble.

I have a bunch of mixed signals hear from a local powerful AM station.
I have always liked in urban areas and have just accepted the birdies.

Roger.
> 
> 
> 
> 
> 
> At 10:15 PM 10/25/2007, Flowertime01 at wmconnect.com wrote:
> >Tony,
> >
> >If you have one and only one station giving you a second harmonic you
have 
> no
> >problem. Its easy to get enough second harmonic from an AM station.
> 
> Just a fine point that may help:  A harmonic from an AM station does not 
> sound like the fundmental signal. All the signal is doubled (or trippled) 
> including the modulation sidebands. So it won't sound right.
> 
> A mixing product will sound right or at least more intelligible.  Some 
> other signal combines with the AM station one to produce a signal shifted 
> in frequency, just like in the receiver as signals are mixed (usually down

> in frequency) to the lower IF frequency.
> 
> I think that in this situation, a lot can be learned with a signal 
> generator fed into the antenna jack.
> 
> Roy
> 
> 
> 

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