Are you trying to build a better avalanche receiver or what? The
trancievers available today seem to be pretty good.

Paul, KD7HB

I am, but I'm trying to build a longer range receiving antenna for
specialized purposes, possibly for use in a heli...

AI4QJ wrote:

...
I assume you are missing the word "is" in the second clause but I need to
further correct your statement by changing the word "proposition" to
"observation". Go ahead and impeach my and others' "observations". Better to
dismiss observations that may be difficult to explain as not having been
actually observed. We should keep our observations to ourselves.



I like you already ... I mean that in the "right way" too LOL

Regards,
JS

AI4QJ wrote:

...
How violently must I agree with Roy before you believe me?



Imagine yourself in a room filled with deaf, dumb and blind
people--you'll know what to do ...

Regards,
JS

What specific kind of signals are you trying to pick up, and what is
it that will prevent you from hearing them? That is, if the signals
are buried in atmospheric noise, a more "sensitive" antenna that also
picks up more noise as well as more signal isn't going to help the
signal-to-noise ratio. For that, you may need to use some additional
knowledge about the signal that you can use to differentiate it from
the noise. At 457kHz, atmospheric noise is very high amplitude, and
it doesn't take much of an antenna plus receiver to get all the signal
that will do you any good.

On the other hand, if the thing that keeps you from hearing the
desired signal is an interfering signal, the null of a loop (or
ferrite rod) antenna can be used to get rid of that signal that comes
from one direction (which lets you listen more easily to the desired
signal, provided your desired signal isn't coming from the same
direction).

Cheers,
Tom


The signal itself is coming from an avalanche transceiver ( a pulse at
457khz every second or so) which has a relatively small antenna and
the signal attenuates quickly , most receivers only have about a 25m
straight line range and I'm trying to improve on that. I thought the
small range was due to the weakness of the signal, I'm a bit clueless
as to how much of the problem would be from radiant noise, although
these posts are starting to give me some insight. Thanks for the help
thus far.

-Chris

John Smith wrote:

[his normal chit]

Oh yeah, and by the way, 10-4 'gud buddy! ROFLOL

Just remember, 26-27mhz is radio too! :-)

Regards,
JS

AI4QJ wrote:

I think you were right, it is a front end overload issue. But I think adding
an attenuator that simply reduces all input signal strength in a linear
manner does not act the same as de-tuning. Comments from others on their
experiences would be interesting. It is easy enough to try out out; just
find a strong 75m station and using a tuner, tune out the noise and see if
you can still receive (and more pleasureably). Is this the same as
attenuating all input signals 20dB? I lack a technical explanation and we
haven't agreed that the effect is real so I will drop out of this discussion
unless someone else wants to pick it up.

Let's take a look at what mismatching does.

At the frequency you're listening to (and for practical purposes some
bandwidth on either side), mismatching does the same as an attenuator --
it simply reduces the strength of the incoming signals and noise by the
same factor. (It has to reduce both by the same factor because neither
the antenna nor the tuner has any way of distinguishing which is which.)

At other frequencies the story is a bit different. Assuming you don't
want to listen to other frequencies at the same time, everything
off-frequency can be categorized as noise. Adjusting your tuner will
cause the match to improve at some frequencies, increasing the noise at
those frequencies which reaches your receiver. It will also degrade the
match at other frequencies, decreasing the noise from those frequencies
which reaches your receiver. A good quality receiver will reject the
noise at all frequencies except the bandwidth you're intentionally
listening to, so increasing or decreasing the noise at those frequencies
shouldn't make any difference, and again the result won't be any
different from an attenuator. Only if off-frequency noise is strong
enough to cause your receiver to distort will the mismatching have any
effect that might be different from an attenuator. If that's the case,
different mis-tunings will have different effects depending the
frequencies of the overloading signals and which frequencies are favored
or not favored by the mis-tuning -- mis-tuning could make matters better
or worse.

There's an added wild card here if the receiver is being overloaded and
creating intermod. Two off-frequency signals ("signals" in the broad
sense, here, which includes both noise and intentionally transmitted
signals) can combine to produce an in-band intermod product which you'll
hear and regard as noise (this also in the broad sense, meaning anything
you don't want to hear). This intermod product isn't on frequency until
after the receiver front end, because that's where it's created. Messing
with the tuning could possibly attenuate one or both the offending
signals more than an attenuator would, so you'd see more of an
improvement in S/N ratio than with an attenuator. I think this would
only be an occasional lucky happenstance, though, and not something
you'd consistently see. But a plain attenuator can provide quite a
dramatic improvement when intermod is present. Adding 10 dB of
attenuation ahead of the receiver will reduce your desired signals and
in-band noise by 10 dB. But it'll reduce second order intermod products
by 20 dB, third-order products by 30 dB, etc. So an attenuator (or
mis-tuning or a smaller antenna) can provide a real S/N ratio
improvement when some of the noise is intermod being created by your
receiver. A preselector can also be helpful in this situation. Again,
though, you won't see this with a quality receiver unless you have some
remarkably huge off-frequency signals. I wouldn't be too surprised to
see it on 40 meters in Europe, for example.

Roy Lewallen, W7EL

cmor wrote:

The signal itself is coming from an avalanche transceiver ( a pulse at
457khz every second or so) which has a relatively small antenna and
the signal attenuates quickly , most receivers only have about a 25m
straight line range and I'm trying to improve on that. I thought the
small range was due to the weakness of the signal, I'm a bit clueless
as to how much of the problem would be from radiant noise, although
these posts are starting to give me some insight. Thanks for the help
thus far.

-Chris

As Tom says, once your antenna is good enough to make atmospheric noise
louder than receiver noise, you can't do anything to further improve the
S/N ratio unless you can null out some noise from a single direction (or
opposite directions). And it doesn't take much of an antenna or receiver
to do that at this frequency.

The only way to improve range, then, is to make the transmitted signal
stronger with respect to the atmospheric noise. To do that, you'll need
to increase the strength of the transmitted signal by increasing
transmit power or improving the efficiency of the transmitter antenna.

Roy Lewallen, W7EL

cmor wrote:

...
Thanks,
Chris

Oh yeah, somethin' Roy said, making the antenna "directional."

Lucky you, that ~1ft. long ferrite rod is a dream for it, use aluminum
foil shielding!

Just place some foil on the side(s) of the rod you have no interest in
getting a signal from.

If you doubt the effectiveness of this, wrap a 1ft. "circle" of foil
around the darn ferrite antenna (DON'T short the ends of the foil--so as
to complete a "shorted turn." And, you'll get a lot of silence with
this shield ... you might even find it works rather well. :-)

These guys, most likely, consider you a "slow cousin." Heck, I'll give
'ya the benefit of the doubt--you'll figure it all out--and, probably,
and most likely, quickly!

I like your questions, makes me remember when I played around with
similar stuff ... good luck!

Regards,
JS

Great thought on making it directional, hopefully even the slow cousin
can figure out how to wrap foil.

As for the receiver would a simple MK484 based one suffice? Or would a
Superheterodyne provide better performance?

Lastly is their a cheap commercially available receiver I could plug
my antenna into to compare the performance to my home built one?

Thanks,
Chris

cmor wrote:
Great thought on making it directional, hopefully even the slow cousin
can figure out how to wrap foil.

As for the receiver would a simple MK484 based one suffice? Or would a
Superheterodyne provide better performance?

Lastly is their a cheap commercially available receiver I could plug
my antenna into to compare the performance to my home built one?

Thanks,
Chris

The MK484 should be a good device for your project, almost everything
you need on a single chip. I have used the ZN414, these devices were
fun to play with ...

Ham swaps are a good place to look for "cheap" equipment ...

You should be able to set up the MK484 as "hot" as you would like ...
why bother with looking for other equip? Chuck a low noise MMIC in
front and off 'ya go ... as others point out, noise will be your only
limiting factor.

Would be interested in how your final project turns out ... keep me
posted if possible ...

Regards,
JS