FM antenna curiosity

[amdx]

Hi Mike,

Normally, a too strong signal is not a problem with FM as FM literally
locks onto the strongest signal and rejects the competitors. This is
not a characteristic of the RF wave, but rather the modulation
employed.

Collapsing your antenna is the same thing as moving it.

Now, as to your experience of receiving signals on the wrong
frequency, that is a classic situation of image rejection being poor
due to the lack of a tuned front-end (something that dissappeared with
the dinosaurs). If I were to guess on the basis of 40 year old
experience fixing these suckers, your off-frequency signals are
probably shifted by twice the IF frequency of your receiver. The
classical FM IF frequency of 10.7 MHz might apply, but time has
marched on and designers may select their own. This old standard
would argue that you shouldn't experience images except where they
would be out-of-band (the 88-107 band with this IF would force that).
A simple test is to tune into at least two of these off-frequency
stations, note what frequency they should be and subtract the
frequency where they appear. If you are having image issues (no, this
is not a self-help hint), the two or more stations should come up with
the same differences.

If you come up with the same number, AND you have trouble with
interference from adjacent stations (there are guard bands to prevent
this), THEN you have one crappy receiver.

73's
Richard Clark, KB7QHC

The receiver is an undercounter mounting Sony AM/FM, radio with CD.
Nothing great, but Sony ususally does a fair job.
Mike

 

[amdx]

I have an FM radio inside an aluminum boat. The radio worked ok with the
AC cord as the antenna but I got interference when I used my laptop.
I found the circuit that ran from the power transformer to the antenna input
on the
FM IC. I installed a connector that is used on car radios and wired the
center pin
to the foil that went to the FM IC (capacitor on pcb isolated) and the
shield side to
dc ground near the IC. I then plugged in a telescoping car antenna and it
worked
great on the bench .
So I installed the antenna on the outside of the boat and the radio inside,
now one station I listen to is weak, but if I unplug the antenna and let the
center pin touch ground of the connector on the radio it comes in great.
Just curious why it is working this way.
BTW, the mod did cure the computer hash.
Mike

 

[Baron]

amdx Inscribed thus:

I have an FM radio inside an aluminum boat. The radio worked ok with
the AC cord as the antenna but I got interference when I used my
laptop. I found the circuit that ran from the power transformer to the
antenna input on the
FM IC. I installed a connector that is used on car radios and wired
the center pin
to the foil that went to the FM IC (capacitor on pcb isolated) and the
shield side to
dc ground near the IC. I then plugged in a telescoping car antenna and
it worked
great on the bench .
So I installed the antenna on the outside of the boat and the radio
inside, now one station I listen to is weak, but if I unplug the
antenna and let the center pin touch ground of the connector on the
radio it comes in great. Just curious why it is working this way.
BTW, the mod did cure the computer hash.
Mike

Sounds as if the antenna is fed with co-ax and has too much capacitance
across the input. Put a trimmer capacitor in series with the centre
pin and see if that improves things. Try 2-20 or 5-50pf.

 

[amdx]

Hi Mike,

You moved the antenna.

Most loss of signal as you describe comes from not being a weak
signal, but the mixture of signals that combine negatively at some
spot due to multiple reflections. When you replaced the line cord as
antenna for this better implementation, you also found that "sour (not
sweet) spot." This can occur for any frequency with the equal
likelihood of reflections combining negatively. Move your antenna a
quarter wave and see what happens.

Your description of your having an aluminum boat almost guarantees a
multitude of RF-bright reflections. At short wavelengths, this also
guarantees many, many regions that will exhibit destructive (as well
as constructive) combinations of those reflections.

Put your antenna as far away from the superstructure or hull as
possible. This will reduce the reflection path differences.


FM has what is called a "full quieting" effect. It would suggest that
your first signal levels were just barely above the level of full
quiet (and perhaps not even that good).

73's
Richard Clark, KB7QHC

Richard, after I read your letter I did a little better checking and found
my signal is not weak it is to strong, I'm getting interference from other
frequencies.
Also I get stations on the wrong frequency.
I went out and collapsed the antenna to minimum about 1/3 of what it was
and
my problem station is perfect and the other station I listen to is still
good. The local
NPR station isn't good though. But I can download the podcast of Science
Friday
Thanks, Mike

 

[Joerg]

Richard, after I read your letter I did a little better checking and found
my signal is not weak it is to strong, I'm getting interference from other
frequencies.
Also I get stations on the wrong frequency.
I went out and collapsed the antenna to minimum about 1/3 of what it was
and
my problem station is perfect and the other station I listen to is still
good. The local
NPR station isn't good though. But I can download the podcast of Science
Friday
Thanks, Mike

The usual, lousy FM tuner. They don't make'em like they used to. It's
the same with television sets, one large signal and they fall off the
rocker.

If you have a radio with a signal strength meter you could notch out a
strong station. But that only works it it's just one and far away from
the NPR frequency. The only other options are to get a better radio, a
directional antenna, or just live with it and use the podcast.

 

[Joerg]

Hi Mike,

Normally, a too strong signal is not a problem with FM as FM literally
locks onto the strongest signal and rejects the competitors. This is
not a characteristic of the RF wave, but rather the modulation
employed.

Collapsing your antenna is the same thing as moving it.

Now, as to your experience of receiving signals on the wrong
frequency, that is a classic situation of image rejection being poor
due to the lack of a tuned front-end (something that dissappeared with
the dinosaurs). ...

One solution: Try to get an old Becker car radio. And I mean old, at
least 40 year, the first transistorized ones that could still be
switched to 6V. They used to be standard issue in Mercedes Benzes.

[...]

 

[amdx]

You really need a better quality radio and with radios and a lot of other
stuff older = better

I mean, considering what the boat must have cost ...

Well the boat is a pontoon boat (maybe $6,000) that I run my business
from, my wife or I are on the boat 70 hrs a week.
The radio was fine when I used the original AC cord antenna, except when I
was on my laptop, it caused hash
in the audio. That's why I isolated the antenna from the AC, that did
eliminate the computer hash.
I have put together a car radio and wall wart that I use with a pillow
speaker at night. Maybe later I'll put
together another one for the boat.
Mike

 

[Phil Allison]

"Richard Clark"

Now, as to your experience of receiving signals on the wrong
frequency, that is a classic situation of image rejection being poor
due to the lack of a tuned front-end (something that dissappeared with
the dinosaurs). If I were to guess on the basis of 40 year old
experience fixing these suckers, your off-frequency signals are
probably shifted by twice the IF frequency of your receiver. The
classical FM IF frequency of 10.7 MHz might apply, but time has
marched on and designers may select their own. This old standard
would argue that you shouldn't experience images except where they
would be out-of-band (the 88-107 band with this IF would force that).

** Hearing the same FM station at more than one spot is still possible even
with a 10.7 MHz IF frequency - if the signal is very strong. The reason
is harmonics of the incoming carrier generated in the RF stage interacting
with harmonics of the local oscillator in the mixer.

Eg:

A 100MHz FM carrier generates a harmonic at 200MHz in the receiver.

When the local oscillator is adjusted to 94.65 MHz, its second harmonic is
189.3 MHz.

The difference frequency is then 10.7 MHz - so goes through to the FM
detector.

In this situation, the FM deviation is doubled so the recovered audio will
be distorted on loud passages.



..... Phil

 

[Phil Allison]

"Richard Clark"
"Phil Allison"

in a very crappy receiver.

** Not by any method you alluded to - fuckwit.


It has already been said.

** But not in any detail - fuckwit.

No surprise a radio ham cunthead like YOU deleted all the facts.

Pure embarrassment to a know nothing turds like radio hams.




..... Phil

 

[amdx]

The usual, lousy FM tuner. They don't make'em like they used to. It's the
same with television sets, one large signal and they fall off the rocker.

If you have a radio with a signal strength meter you could notch out a
strong station. But that only works it it's just one and far away from the
NPR frequency. The only other options are to get a better radio, a
directional antenna, or just live with it and use the podcast.

This morning I got on the boat and the signal that was improved to good by
shortening the antenna is now bad. 94.5 has interference from 101.1.
Oh well!
Mike

 

[Joerg]

This morning I got on the boat and the signal that was improved to good by
shortening the antenna is now bad. 94.5 has interference from 101.1.
Oh well!

You really need a better quality radio and with radios and a lot of
other stuff older = better

I mean, considering what the boat must have cost ...

 

[Joerg]

The receiver is an undercounter mounting Sony AM/FM, radio with CD.
Nothing great, but Sony ususally does a fair job.
Mike

I have learned not to trust any radio that's newer than 30 years,
whether name-brand or not. And that's from experience. Unless it is from
companies like Icom.

 

[Joerg]

The usual solution on boats: Use a decent car radio.. Well shielded,
expects outdoor antenna. Runs on 12Volts... Can be had with CD player,
separate input for your Ipod etc.., good audio power to speakers
etc.... And designed to work inside a metal vehicle....

Best of all, it can be bolted down. That's really important on a boat.
However, many newer car radios (newer as in "last 20-30 years") don't
have very good tuners. Best to get one from the era of Ge-transistors,
those radios were usually good.

Better yet, get an Icom, Yeasu or whatever comms receiver. Most have a
WFM setting. Ok, no stereo sound but one can easily listen to NOAA
radio, ship-to-shore channels and so on.

 

[Phil Allison]

"Dave" <[email protected]>

all antennas are dipoles, you just can't always see the other half.
and Luxembourg has nothing to do with it, your silly frequency
doubling notions should be packaged up in art's box and never see the
light of day.


** ROTFL !!

Dave should be writing scripts for Mickey Mouse cartoons.

Cos he has the IQ of Daffy Duck.



.... Phil

 

[Paul Keinanen]

It sound like the "Luxemburg Effect". The signal was from the dipole
antenna.

The Luxemburg effect is usually contributed to the suspected Radio
Luxemburg intermodulation products caused by the _ionosphere_
nonlinearities.

Similar intermodulation effects can be obtained by the nonlinearities
caused by rusty bolts in a transmitter tower.

 

[Joerg]

Or a tuned trap to reduce the signal from that one station.

Well, that's what I suggested above, notch = trap

But it's tough and can be impossible if there are useful weaker stations
near the one you want to muffle. In Europe they had a pager service
right at the lower end of the FM band. Whichever committee signed off on
that one should be dunked into a moat for gross incompetence, until they
either learn or quit their career. Anyhow, the inevitable happened, and
despite being a school kid I predicted that: A barrage of complaints by
FM listeners. In Germany they pay a radio tax so that makes them sort of
constituents with rights. Long story short the governement had to
furnish rather expensive notch filters to anyone who complained.

 

[Kevin McMurtrie]

I have an FM radio inside an aluminum boat. The radio worked ok with the
AC cord as the antenna but I got interference when I used my laptop.
I found the circuit that ran from the power transformer to the antenna input
on the
FM IC. I installed a connector that is used on car radios and wired the
center pin
to the foil that went to the FM IC (capacitor on pcb isolated) and the
shield side to
dc ground near the IC. I then plugged in a telescoping car antenna and it
worked
great on the bench .
So I installed the antenna on the outside of the boat and the radio inside,
now one station I listen to is weak, but if I unplug the antenna and let the
center pin touch ground of the connector on the radio it comes in great.
Just curious why it is working this way.
BTW, the mod did cure the computer hash.
Mike

It's probably a severe impedance mismatch between your new antenna and
the old pickup coil. You might need to create a little step-up
transformer between the board and jack. I looked at a few FM chip specs
and they leave it up to the designer to figure out the right input
transformer. You'll probably need trial and error to figure it out.

 

[Jeff]

Yes. THY receiver is tuned to the transmitted frequency.
Luxembourg effect means that another receiver tuned to the doubled your
frequency hears you.

Perhaps you should read up on what the Luxembourg Effect actually was!!

It had nothing to do with antennas!!

It was Cross Modulation in the ionosphere between Radio Luxenbourg and
other radio stations, where by the modulation of Radio Luxenbourg was
heard superimposed onto the second station.

Jeff

 

[Joerg]

So you move the trap slightly to one side. A good trap can be narrow
enough to only affect one or two channels if it is built with the right
components, but it isn't cheap. ... ^^^^^^^^^^^^^^^^^^

Bingo!


... Glass piston capacitors and glass
inductors are temperature stable and have a very high 'Q'.

I just hope someone was read the riot act for making that frequency
allocation. I mean, that allocation was really borderline daft ;-)