Radio (Side-Tone)

[asaybe]

In reference to radio communcations, does anyone knows what is the "side tone" in a VHF Radio???  I' am battling noise coming into my intercom especially at High RPMs. and it gradually stops when the RPMs are very low. I have an Apollo SL 40 transceiver. It has noise filters installed, but it makes no difference.  

Thanks,
Afif

[Waiter]

"SideTone" refers to "Hearing your own voice" when your talking on the radio.

There are two different ways interference can come in, and this applies to electronic ignition or magnetos.

1) Radiation - In this case its actually coming in via the receiver RF path. This is most likely caused by a bad spark plug, wire, or distributor cap. ALSO - Install a "P" lead filter if you havent already.

2) Power Supply - The noise pulses are getting into the radio via the power source. Actually, I find that intercoms are more suseptable to this than radios. However, the same fixes apply. Check for bad plugs, wires and distributor caps. ALsao make sure the "P" lead filter is installed. If these are OK, then install a DC Line choke in the power line that supplies the radio and intercom.

Radio Shack used to sell these, However, I think this item is the same
http://www.tessco.com/products/displaySkus.do?groupId=619&subgroupId=65
Its a Speco Technologies HLF10


Good Luck

Waiter

[asaybe]

Thank you for the sidetone clarification, Waiter....

I will inspect the the electrical system.  Which particular "P" lead filter are you recomending???

I have to agree that most of the interference comes from the INTERCOM (which is incorporated into the radio)rather than the radio and I do have LSI ignition system.
 
Please get back to me with "P" lead filter...

Best Regards,
Afif

[Waiter]

Take a look at ACS, Chief, Wicks, etc. The P lead filter runs about $40. Looks like a 1 inch diameter tube with wires coming out one end.

ALSO;

noise could be entering through your microphone lines, make sure you are using good quality shielded twisted pair for the wires for any microphone and audiono input lines. I would also recommend tieing any unused audio inputs to ground through a 560ohm resistor. This guarentees that they are not the source of your noise.

Waiter

[Drew]

Waiter,
Any chance there is something wrong with the alternator?  May need an alternator filter.  You can isolate this as a problem real quick by turning off the alternator to see if you have a problem.

Regardless, sounds like you recommend to choke the power line prior to entering the noise effected device rather than filtering at the alternator (if that is the problem).

Good stuff!

[Waiter]

Drew, excellent point.  Alternators will generally whine anyway, but if one of the bridge diodes is open, this will defiantly aggravate the noise.

If your able to do like Drew recommend, i.e. isolate or disconnect the alternator, this would tell you if its an alternator problem.  ALSO, If you can't isolate the alternator, try loading it up, i.e. turn on everything electrical, lights radios, etc.  If the whine gets louder under load, then its definitely alternator.

An Alternator filter will help, but generally the filter capacity of the over the counter "Alternator Noise Filters" is nowhere near the filtering capacity of the smaller "individual filters".  

You may not be able to get rid of the whine with the alternator filter, but keeping it out of the intercom and radio power supplies with the small individual filter should solve the problem.

Normally, in an automobile, the alternator and the battery are only a couple feet apart. Te battery itself acts as a giant capacitor to filter the noise. In our application, these are separated by 15 feet, so the alternator noise can generate a very small voltage in that 15 ft of wire, this is the the actual "source" of the noise you hear.

I'd do the shotgun approach. P lead filter, Inductive filter the radio and intercom power, and make sure the mike and audio lines are routed and terminated correctly.

Waiter

[asaybe]

Thanks to both of you. Very good insight. I am not experiencing the "whine" as we speak, but yes noise, like "open squelch". This really difficults the hearing sometimes. I know the alternator also has the filter, because I've visually seen it.  Do these filters "fail" often??? I haven't been able to locate a specific "P" lead filter...other name for it???

AS

[Waiter]

Try Magneto Filter. Heres a link to Chief Aircraft. You should also check ACS, Wicks, etc.


http://6url.com/0CXZ


The P lead could be causing enough to trigger the squelch, Don't forget to check plugs and wires also.

Waiter

[asaybe]

Thank you sir......Will do.

Best regards,
AS