David:
Squelch is achieved by the com[arison of two voltages. One is
the voltage of the 'noise' on the frequency. The other is the voltage of a
received signal. When the latter is heard, the receiver sets the squelch
datum to just below the received signal voltage level. Since the signal is
stronger, the noise is below datum and not heard. If the noise level is the
higher voltage, then one sets the squelch to a higher datum (and all signals
below that datum are not heard).
What I believe is happening is that because of a poor
antenna connection/design/siting, the signals received are of too low a
voltage datum to overcome the natural background noise level.
In amateur radio, this function is adjustable (on FM where it is
most valuable) by two settings - (a) positive voltage datum calling for
quiet in background, and (b) negative voltage setting lowest received signal
level. However in AM, the receiver probably has an 'automatic' comparator,
and one may only set the background level for cutoff.
That's why you get the signal if you back off the squelch which mixes noise
with the signal. (In FM, the loudest signal captures the receiver)and by
design automatyically silences the background).
This leads to the conclusion that you (and probly Harrison)
have a less-than-ideal connection somewhere which limits the strength of the
received signal, knocking it down into noise level.
I have led a "interference" committeee for ham radio and our
experience is that 99% of the trouble originates with coaxial fittings. Most
do not comprehend the need to affix these fittings EXACTLY as prescribed,
That is, the fittings when properly attached create a continuous 50-ohm
impedance from antenna to receiver detector. Use TV fittings (70ohm) or
pressfit or poor soldering, or stray starnds and one will experience an
impedance "bump" which greatly attenuates the signal, but not the noise. Old
fittings sometimes let in moisture and copper components corrode, bumping
impedance or cutting circuit entirely. Also communications (vs.
entertainment) "coax" may be low quality or poor coaxial screen coverage
(minimum 95% screen). Low quality coaxial connectors is next on the list
(buy only the best brand lines from proper sources) and antenna
design/siting comes next.
Depending on the design, the antenna should be sited at least
one wavelength from adjacent antennas/lines. At 125MHz, this is 2.4 metres,
although many designs can accept 1/2 wavelength separation.
If your antenna is a halfwave dipole (a popular model on the Europa - up the
rudder closeout usually), it needs greater separation on the sides, much
less on the ends - but then it's usually aligned vertically and there ain't
much room left above or below. If it is a quarterwave vertical (about two
feet high), this design needs a 'groundplane' or metal disc about the same
radius, properly separated and at right angles to the vertical element. You
will see these on aluminum aircraft as the skin provides the groundplane.
there are other designs of course.
My first instinct in both your cases is that a poor connection
presides somewhere in the anteena delivery to the receiver. try a different
receiver if you can borrow one, and place it in exactly the same fitting as
the original. If that is better, your receiver is at fault. Either put your
radio on someone else's fitting and compare results. One may need to try
several to achieve a better signal/noise ratio as poor antenna paths are
more prevalent than ever. If the result is better, then your installation is
at fault. Another second test is to rig up an alternative antenna to compare
with yours. Again, a better result points to installation faults. If
stymied, chase up an older ham from the local club - each one usually has an
old hand at antenna design/siting and might just rise to the occasion - it's
a quandry they enjoy confronting often. I'll scare out the local club if you
wish. No charge - that's what amateur means - non-commercial.
I may be 'way off base, but my experience tells me I'm not.
Best of luck (and don't listen to Harrison).
Happy Landings
>Ferg
----- Original Message -----
From: "Cripps, David" <david.cripps@spsystems.com>
Subject: Re: Bundling of coax cables
> Bob,
>
> I was interested to read your message below. I seem to have the same
problem with my Icom A200 - what radio do you have?
>
> In my case I seem to get an intermittent cut-out on the receive (as if the
squelch is cutting in and out) and I therefore only get a small amount of
the info relayed to me. The problem however seems to be worst with the 'big'
radio stations - eg in my area: Solent radar, Bournemouth App, Bristol App
and Cardiff App. I seem to have no problem in receiving clearly from the
smaller A/G's. The problem does go away if I pull out the squelch test
button but of course that brings with it a nasty amount of other noise.
>
> Any ideas?
>
> David
>
> -----Original Message-----
> Subject: Re: Bundling of coax cables
>
>
> Hi! Fred.
> When you are next in the UK let me know and you can attack my
> installation!!!!
> Yes, on Bill Bells' recommendation after walk around tests with hand held
RT
> set with the engine running I've a considerable improvement by painting
out
> the cowl with a nickle based paint but the problem still with my radio is
> that the bloody auto squelch control is being activated on the same
receive
> frequency as most air traffic control frequencies and takes the required
> messages out with the trash. Viz:- if I "pull to test" the auto squelch
> control is dumped and I can "read" receive messages from afar but after a
> while the other trash gets very tiresome and inclined to drive me
> skitsu!!!!(Not the best of attitudes to be in when flying?). I have had
the
> stupid thing back to "authorised installers" who claimed it is set "to the
> book" and would only slightly adjust it at my insistance so where to now?
> (there is no squelch regulate facility either you have it or you don't !)
> regards
> Bob Harrison G-PTAG
> PS Alfred Buess do you understand all this now ?!!!!!!
>
> -----Original Message-----
> From: forum-owner@europaclub.org.uk
> Subject: Re: Bundling of coax cables
>
>
> Hi, Bob --
>
> If yer referrin' to ignition noise, that's radio freq interference
> (RFI), which don't care about no steenkin' _electrical_ grounds! If
> we look at a marker beacon "sled" antenna, it's grounded one end, with
> the antenna tap clamped at a proper point along it's length.
> Aumiminum is nonmagnetic, but that's the magnetic part of EMI -
> aluminum foil is used as a coax shield for better RFI shielding verses
> EMI, where braid is better. If one metalizes the firewall only, to
> block ignition noise, it will be only partially effective, since the
> RFI merely bends around the "plate," which in free space can even
> reradiate the RF. If have read where it's thus effective to also
> metalize the inside of a fiberglass cowling with a conductive paint,
> enclosing all in a box.
>
> Separate grounds for electrical items isolate the ground from noise in
> a "main" ground cable that's carrying everybody else's contributed
> noise. The only items of concern I believe are mic and headphone
> audio due to their signal/noise ratio issues, where one might shield
> both the hot and ground wires. Either in separate shielded wires, or
> twisted 2-conductor pair inside of a shield, with shield grounded only
> one end in both cases (at box in panel).
>
> Another possible trouble spot are sensor wires for instrumentation -
> another candidate for shields. Yesterday I opened up the little sensor
> module for an automotive temp gauge, to modify it for the Rotax
> sensors. It's design to deal with stray pickup stinks (for A/C use),
> and 1 lousy millivolt is good for a 1/8" of needle error!
> [Experimenting with cool gauges - only 2" round, 270-degree linear
> needle sweep, orange backlighting through only the white parts of the
> dial].
>
> Best,
> Fred F.
>
> Bob Harrison wrote:
> >
> > Hi! Alfred
> > Not being a "pro" with this "wire bound water" called electricity !!!!!
I
> > wasn't going to reply
> > especially since you have had a good responce, however I'll repeat what
I
> > said to another enquiry recently on this forum. If I had my "time over
> > again" I would ensure that I have a completely separate system for
> > "grounding" every concievable item back to the engine or battery direct
> and
> > so ensure that the "negative" part of all the power supply circuits are
> > completely separated from the "grounding" circuit with their own circuit
> > systems.
> > Someone would have done me a favour to explain there are two types of
> return
> > ........ !!!!!!
> > Also remember that aluminium has no capacity to shield magnetism ,like
if
> > you put a sheet of metal across the rear of the bulkhead it needs to be
> thin
> > gauge stainless, or you'll need to buy 5 litres (minimum order) of
Nickle
> > Based paint at about 200 a go !!!!
>
>
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