RF interference issue again (esp. for E Drucker and Jim Weir and other RF wizards)

On 6-Nov-2003, (Snowbird) wrote:

I'm still trying to understand how a powered OFF radio with
the power OFF in the plane might be contributing to this
problem and hoping someone will explain this to me.



Sydney,

It IS possible, but it is also extremely unlikely to be the cause of your
problem.

Those that propose this theory are assuming that the RF signals being picked
up by your com antennas are strong enough BY THEMSELVES to provide power to
the receiver of a radio that is not otherwise powered. This seemingly
strange phenomenon is most often observed when a car (with a typical AM/FM
car radio that is turned off) is driven right up to an antenna farm.

In your case, I assume that you are flying either far enough horizontally
away from or high enough above the antennas for safety. Let's say a minimum
of 1/2 mile horizontally. Now lets assume that the strongest broadcast
signal is 100,000 watts "effective radiated power" and a frequency somewhere
around 100 MHz. At a distance of 1/2 statute mile the power from that
signal that would be picked up by an aircraft com antenna would be about 10
milliwatts. This is a spectacularly strong signal for purposes of
reception, and certainly more than enough to make it impossible for the
attached receiver to pick up any other signal, but nowhere near strong
enough to cause the receiver, if not otherwise powered, to generate and
re-radiate an intermodulation product.

So, if the signal WERE strong enough to provide enough power to cause an
otherwise unpowered receiver to generate and re-radiate an interfering
signal, the same powerful signal would pretty much wipe out operation of any
other nearby receiver that IS powered on.

By all accounts, what you have is plain old garden variety intermodulation
interference. The intermodulation products doing the interfering are being
generated in the same receiver that is being interfered with. In an earlier
post I offered a relatively simple way to prove this.

--
-Elliott Drucker

-In your case, I assume that you are flying either far enough horizontally
-away from or high enough above the antennas for safety. Let's say a minimum
-of 1/2 mile horizontally. Now lets assume that the strongest broadcast
-signal is 100,000 watts "effective radiated power" and a frequency somewhere
-around 100 MHz.

In a metropolitan area 100kW ERP would rank you in the lower third of broadcast
signals, but for grins and giggles, let's make that presumption.



At a distance of 1/2 statute mile the power from that
-signal that would be picked up by an aircraft com antenna would be about 10
-milliwatts.

More like 5 milliwatts, but let's not debate how many milliwatts can dance on
the head of a dipole.


This is a spectacularly strong signal for purposes of
-reception, and certainly more than enough to make it impossible for the
-attached receiver to pick up any other signal, but nowhere near strong
-enough to cause the receiver, if not otherwise powered, to generate and
-re-radiate an intermodulation product.

Oopsie. 10 milliwatts is about 0.7 volts RMS, or about a volt peak. If for
whatever reason that front end were wide open to the interfering signal, a volt
is sure as little green apples capable of turning on the B-E diode of the RF
amplifier. Now any lower power signal is perfectly capable of being mixed with
our newfound "LO" and being reradiated.


-
-So, if the signal WERE strong enough to provide enough power to cause an
-otherwise unpowered receiver to generate and re-radiate an interfering
-signal, the same powerful signal would pretty much wipe out operation of any
-other nearby receiver that IS powered on.

Ah, no. NOt if the other receiver had enough filtering in the front end to get
rid of it.

-
-By all accounts, what you have is plain old garden variety intermodulation
-interference. The intermodulation products doing the interfering are being
-generated in the same receiver that is being interfered with. In an earlier
-post I offered a relatively simple way to prove this.

I'm not debating that it is intermod. I don't know. I'm not there. But front
end reradiation is a phenomenon that should be investigated also. And, for that
matter, that is exactly why two brands of ELT have had factory recalls...the C-B
junction in the output transistor was a wonderful intermod generator.

Jim

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com