FM antenna curiosity

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

amdx Inscribed thus:

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.

--
Best Regards:
                     Baron.

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

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 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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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

"Richard Clark"

** 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

in a very crappy receiver. It has already been said.

Not the usual quality one expects from Sony, but Mike's testimony suggests otherwise hence we cannot discount a comb generator having been dropped into the LO chip's place. Pity that, it must have been a bad year.

73's Richard Clark, KB7QHC

"Phil Allison" wrote news: snipped-for-privacy@mid.individual.net...

It sound like the "Luxemburg Effect". The signal was from the dipole antenna. Are now the FM stations which use the dipoles?

S*

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....

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

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 ...

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

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

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

al.net...

le

=A0 The

is

e FM

ill

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.

"Dave"

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