Does this group have a FAQ? If so, is it posted here on occasion or is there
a URL that I can access to read it?

As for my questions:

First of all, here's the technical info. I'm using a Grundig S350 with a 75'
longwire antennae oriented (out of sheer luck) east to west, (does that
matter?) and I'm in Seattle, WA, USA.

I'm receiving WWVH at 910 KHz below their listed frequency of 10000 KHz.
After a bit of research on the net, I've discovered that this is a "harmonic
image" that isn't being filtered out by my single I.F. design radio. What
does that mean in plain English?

Is this 910 KHz harmonic only going to show up -below- the actual broadcast
frequency, or will it sometimes appear 910 KHz above as well? (I imagine
that depends on what causes the harmonic, which obviously I don't
understand. Musically, harmonics appear at equal "distances" above and below
the node. Are these harmonics similar to radio harmonics?) And why is this
"image" so strong? My reception at 9090 KHz seems to be as good as that at
10000 KHz. (Which admittedly isn't great.)

Are there other harmonics of any given frequency that I haven't stumbled
across? For example, if I'm receiving WWVH at 910 KHz below their
broadcasting frequency, will I ever get them at say, 1820 KHz (910 X 2)
below?

How common are these images? I like to sit and spin the dial, looking for no
station in particular. When I find one, I look in my "Passport to World Band
Radio" book to see who it is. Am I going to have to check the displayed
frequency, -plus- the one 910 KHz above what my radio display says in order
to tell who it is I'm listening to, and what frequency they're -really-
broadcasting on? That's a hobby-killer right there.

Are certain bands more susceptible to these images than others? Why is it
that I get an image from a station broadcasting on 10000 KHz, but when I
checked for one for a station broadcasting on 10855 KHz I didn't get one?
Again, I'll probably have the answer to that one when I find out what causes
the images in the first place. g

I realize I've asked quite a few questions...if anyone knows of a decent
site that will help clear all of this up for me and wants to save themselves
some typing, I'd appreciate the URL. I don't mind doing my own homework, but
I've looked, and I haven't had much luck.

Another thing...has anyone ever come across a site that has audio files of
what heterodynes, birdies, etc. sound like? I hear long whistles that change
in pitch as I tweak the dial slightly above and below a given frequency, but
I never hear any chirping sounds. For that matter, are heterodynes and
birdies the same thing? I recall reading at one site that they were. Or was
it one of Yoder's books? sigh Too much input in too short a time...there's
a lot to learn!

P.S. Please don't tell me to go out an buy a more expensive radio, as I have
two kids and a boat. (That should sufficiently explain my financial
situation.) Also, please make your responses to the group. To avoid the
plague of spam, I'm using an address that dumps any email it receives right
into a virtual round-file. Responses to my personal mail box won't make it,
and thanks in advance for any help you folks give me!

These come in handy on some Chinese radios with official coverage
gaps.

On Thu, 08 Jul 2004 08:37:13 GMT, "Honus"
wrote:

Not to mention listening to cellphones on blocked scanners.

2 X IF = 90 mHz.

Honus wrote:
I'm receiving WWVH at 910 KHz below their listed frequency of 10000 KHz.
After a bit of research on the net, I've discovered that this is a "harmonic
image" that isn't being filtered out by my single I.F. design radio. What
does that mean in plain English?

Actually, it's just an "image". A "harmonic" is something else.

This will get a bit long but will hopefully be clear...

In the early days of radio, when you tuned your radio to 10000KHz, there
were several amplifier stages, all operating at 10000KHz, to increase
the strength of the WWVH signal until it was loud enough to drive the
speaker.

If you decided you'd rather listen to Radio Japan on 9525KHz, you had to
retune *all* of those stages from 10000 to 9525. At the time, each
amplifier stage required a different knob; you might be adjusting four
or five controls every time you change stations. Also, high-frequency
amplifier circuits above 2000KHz or so were a lot less efficient and
stable than low-frequency circuits.

So Edwin Armstrong invented the "superhetrodyne circuit". In this
circuit, when you tune your radio to 10000KHz, you tune a "local
oscillator" to 10000+455=10455KHz.

The output of this local oscillator is mixed with the signal coming from
the antenna. You get four outputs from this mixing process:

- 10000KHz, from the antenna
- 10455KHz, from the local oscillator
- 10455+10000=20455KHz, the sum of the two
- 10455-10000=455KHz, the difference of the two

These four outputs are then passed to an "intermediate frequency" (IF)
amplifier, tuned to 455KHz. The signal is amplified here, then
"demodulated" to audio and sent to the speaker. Selective circuits in
this IF amplifier reject anything that isn't 455KHz; the 10000, 10455,
and 20455KHz signals are removed, and all you hear is WWVH.

Next, you decide to change frequency to 9525 for Radio Japan. You
retune the local oscillator from 10455 to 9525+455=9980KHz. (again
assuming my math is right!) Now, your four outputs from the mixing process:

- 9525, from the antenna
- 9980, from the local oscillator
- 9980+9525=19505, the sum
- 9980-9525=455, the difference

You don't have to tune the IF amplifier: it's already tuned to 455KHz,
and now it passes the Radio Japan signal instead of WWVH. Whatever
station you tune, its frequency is "converted" to 455KHz, and amplified
in the IF amplifier, which is always tuned to 455.

Now, let's say you decide to listen to Radio Slobovia on 9090KHz.
(910KHz below WWVH-10000) You tune the local oscillator to
9090+455=9545KHz. And you get your four outputs:

- 9090, from the antenna
- 9545, from the local oscillator
- 9545+9090=18635, the sum
- 9545-9090=455, the difference

And you hear Radio Slobovia. But... there's nothing to stop WWVH-10000
from getting to the mixer. So you also get:

- 10000, from WWVH on the antenna
- 9545, from the local oscillator
- 10000+9545=19545, the sum
- 10000-9545=455, the difference

You have two different 455KHz signals! One comes from 9545 mixing with
10000, the other from 9545 mixing with 9090. You'll hear WWVH on 9090.

In practice, a receiver should have a "preselector". When tuned to
9090, this circuit should prevent the WWVH signal on 10000 from getting
to the mixer. If it doesn't get to the mixer, it can't mix with 9545 to
make 455. But preselectors aren't perfect, and if WWVH is strong enough
enough will "leak through" to be easily heard.

The difference between 9090 and 10000 is relatively small, it's hard to
make a preselector that can knock the unwanted signal down far enough.
Higher-quality receivers use an intermediate frequency much higher than
455KHz. For example, if the IF was 9000KHz (a common figure) then when
tuned to Radio Slobovia on 9090, the unwanted response would be on
27090KHz. It's much easier for the preselector to tell the difference
between 9090 and 27090 than it is between 9090 and 10000!

Is this 910 KHz harmonic only going to show up -below- the actual broadcast
frequency, or will it sometimes appear 910 KHz above as well? (I imagine

Only below.

As you might guess, it's possible (and common) to design the radio to
have the local oscillator 455KHz *higher* than the desired signal, in
which case the image would always be *above* the actual frequency.

Are these harmonics similar to radio harmonics?)

Yes, but what you're hearing isn't a harmonic. (there *are* harmonics
in radio. For example, you might hear Radio Slobovia on 18180KHz when
they're actually broadcasting on 9090. This kind of harmonic is usually
- but not always - the result of a problem at the transmitter, rather
than receiver design.)

And why is this
"image" so strong? My reception at 9090 KHz seems to be as good as that at
10000 KHz. (Which admittedly isn't great.)

Your radio's preselector isn't very good. Indeed, it's possible it
doesn't even have one. (this tends to be the first thing designers
leave out when they're trying to cut costs...)

Are there other harmonics of any given frequency that I haven't stumbled
across? For example, if I'm receiving WWVH at 910 KHz below their
broadcasting frequency, will I ever get them at say, 1820 KHz (910 X 2)
below?

Your particular example won't happen. However there are other spurious
receptions possible. For example, the local oscillator in your radio
will have harmonics. When the oscillator is tuned to 9545 to listen to
9090, it will also have some output on 9545*2=19090. And if there's a
strong signal on 18635, you might get:

- 18635 from the antenna
- 19090, the second harmonic of the oscillator
- 18635+19090=37725
- 19090-18635=455

and you'll hear the 18635 station too.

How common are these images?

If you hear it on one frequency you'll probably hear it elsewhere. It's
inherent in the design of the radio.

If there is a preselector, it's probably more effective on lower
frequencies. You may be less likely to hear these images on the 49m
(6000KHz) band and the 540-1700KHz AM broadcast band.

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

ken wrote:

Congratulations! The Grundig S-350 is one of the best radios with
which to study superhetrodyne theory. Not only because of its rich
offering of images, mixer products and harmonics, but its digital
readout lets you accurately read frequencies.


You'd make a *very* good promotional content writer.




The posting could be construed as "I just ran over your dog...but it's
collar is sure nice!" g



mike

As the other folks have said, you are receiving an image response when
you are tuned 910kHz below the actual frequency. This radio is using
high-side injection, which means that the LO is operating 455kHz above
your received frequency. If you look at the response of the mixer stage
of your receiver with a spectrum analyzer, you will see three dominant
signals; these will be the upper sideband signal (the image), a
suppressed LO signal, and the lower sideband signal (the desired
signal). Since the LO is operating 455kHz ABOVE the desired signal, and
the upper sideband response is 455kHz above the LO signal, it becomes
clear about that 910kHz image response.
There are a couple of reasons that double conversion is used; these are
equally important.
The first reason is that of image rejection. If a 1st I.F. of 45MHz
were to be used, the image response would be 90MHz above your desired
frequency. A low-pass filter with a 33MHz cutoff frequency ahead of the
front end would reject that image, because the response would be so far
down on the filter slope that you wouldn't know it was there, unless you
injected a very strong signal into the front end of your receiver.
The second reason to use a double conversion system is that of gain
distribution...........you generally don't want to have more than 70dB
of gain at any one frequency in a receiving system. By distributing the
gain between two different frequencies, an extra measure of stability
can be obtained. Now, about that 910kHz image............with a double
conversion system, you could still have that image if there isn't enough
selectivity at that 1st I.F. Nowadays this is a non-issue, because with
the use of monolithic crystal filters at the 1st I.F. the delta 910kHz
image response is pretty significant. If you look at crystal filter
specs for roofing filters that are intended to be used in a double
conversion system with a 455kHz 2nd I.F. the manufacturers will usually
give a suppression spec at that 910kHz offset. 50 to 60dB is a
reasonable spec, although the 8-pole crystal filters can have an 80dB spec.
Some of the more expensive receivers, such as those made by companies
such as Racal, Harris, Norlin, Watkins-Johnson, Rockwell-Collins, Cubic,
and others will use these multi-pole filters at the 1st I.F.
About that 9990kHz response of the 10MHz signal..........since you are
using a long wire antenna, fundamental overload could be the problem,
I.F. filter feedaround could be another, or as Mr Doug Smith stated, it
could be an image response. You would be amazed at how a receiver falls
apart (figuratively speaking), when you inject a very high level signal
into the front end.
I hope this helps!

Pete

Honus wrote:
What I meant was that 10000 KHz had an image 910 KHz below it, but a station
that I received at 10855 KHz didn't produce an image 910 KHz below that
frequency. I was wondering why that was; why some frequencies produced
images, but others didn't. Sorry if I wasn't clear.

Hmmm.

I think I explained why frequencies that are very different (like
2300KHz vs. 10000KHz) might not produce images. Can't explain why 10000
vs. 10855 would make a difference. Unless the 10000KHz station is MUCH
stronger. (which is very possible) It could be the image of the 10855
station exists, but is too weak to hear. If the preselector is present,
then the image will be weaker than the real station.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

"Honus" ) writes:

What I meant was that 10000 KHz had an image 910 KHz below it, but a station
that I received at 10855 KHz didn't produce an image 910 KHz below that
frequency. I was wondering why that was; why some frequencies produced
images, but others didn't. Sorry if I wasn't clear.

Because there is some sort of filtering at the front end.

Thus the image frequency will always be attenuated to some extent. If the
incoming signal is weak enough, then the attenuation will be sufficient
to knock out the signal at the image frequency.

The problem of image rejection increases as the signal frequency increases.
At 1MHz, the image 910KHz away is quite a large percentage of the signal
frequency, and even fairly simple front end filtering can knock out the image
so that only very strong signals will ever appear. At 10MHz, the 910KHz
is a much smaller percentage of the signal frequency, so you either need
better front end filtering, or start to think of moving the IF frequency.
As the signal frequency approaches 30MHz, image rejection will become awful
with such simple receivers, because the 910MHz is an even smaller
percentage of signal frequency.

I've seen reviews of cheap receivers from the late sixties, with 455KHz IFs,
and comments like "image rejection was almost non-existent on the top band
[20 to 30MHz]".

But note that the issue is not just the low IF frequency. It's also a matter
of the front end filtering. A really cheap receiver would have minimal
filtering at the front end, so it would only reject the weaker signals as
the signal frequency went up. But the better receivers would have better
filtering, and often could get away with a 455KHz IF; those would go to
two stages of RF amplification before the mixer, with the extra tuned circuits
to go with them. It was the extra tuned circuits that helped the image
rejection on the higher bands.

Michael

Noel ) writes:
On Thu, 08 Jul 2004 11:25:32 GMT, "Frank Dresser"
wrote:

With a single IF receiver, there will be only one image. They can be either
above or below the received frequency, depending on the design of the radio.

With a single IF, what you say is true. But with a dual conversion
surely part of the design is that the image is almost inevitably out
of the coverage range of the receiver?



No, because there is nothing about double conversion that requires the
first IF to be above the signal frequency.

Early double conversion receivers would have their IF in the 2MHz or so
range. In some cases, that extra conversion would only come into play on
the highest band, 20 to 30MHz, where it was especially needed. The filtering
at that first IF was minimal, but was sufficient.

And there was a whole other design of double conversion receivers that were
common at one time. These were in effect a single conversion receiver
that covered a fixed range of 500KHz or so. The exact turning range varied
with receiver, but it was usually in the low MHz range. In order to
get other bands, a crystal controlled converter was placed ahead of this
receiver, and you'd need another crystal for each band you wanted to tune.
In some cases, the tuneable receiver covered a band that the receiver did
tune, and the converter was disabled on that band. But more commonly, it
was double conversion on each band.

When crystal filters in the HF range came along, there was a wave of ham
transceivers (and likely shortwave receivers) that had a first IF of 9MHz, but
converted to 455KHz after that, because the designer wanted the selectivity
at that lower frequency, or because they wanted to add some feature, passband
tuning or variable bandwidth selectivity, that used the extra conversion.

It's only in the past twenty to thirty or so years that upconversion to a
frequency above 30MHz became common, especially for hobby receivers.

Michael

"Honus" wrote in message
...


What I meant was that 10000 KHz had an image 910 KHz below it, but a
station
that I received at 10855 KHz didn't produce an image 910 KHz below that
frequency. I was wondering why that was; why some frequencies produced
images, but others didn't. Sorry if I wasn't clear.





My first wild guess is 10855 wasn't the actual frequency of the
transmission, but it was the image of 11765. 11765 is in a standard
shortwave broadcast band. My older copy of Passport says that the BBC used
that frequency, but I don't know if they're still using it.


Here's some numbers:

Desired frequency -- 10000 kHz
Image frequency -- 9090 kHz

Desired frequency -- 11765 kHz
Image frequency -- 10855 kHz

Frank Dresser