[R-390] Broadcast radio at 2 meg

[Flowertime01 at wmconnect.com]

Murph,

Not just close AM will get you. WBZ Boston was all over the HF bands at Ft 
Devens Ayer Mass some real miles from the Towers near Boston.

Every thing in LA County was all over my R390 in west end of LA County in 
California.

Every thing in San Diego was all over my R390 in Spring Valley San Diego 
County.

So many local stations with real power to mix it up.

On a real second AM broad cast harmonic we would expect the voice modulation 
to be spread twice as wide as on the fundamental. So its distorted. If the 
signal is nice and clean you think a mixer formed the move.

If you have only a couple AM stations mixing over its likely over load.

If you have the whole broadcast band mixing over its likely a problem.

Think about what you are getting.

One station on it second harmonic is the station doubling.

We have lots of cases where stations have harmonics. Every loose joint in the 
AM station antenna tower wants to mix the carrier and signal with modulation 
to a second harmonic at the station tower. 

Several stations spaced some frequency above their original are mixing with a 
fixed signal from some where else. Do the math to find that frequency. It 
could be external like a 1 MHz time base on the bench. Not your bench, but any 
one down the street.

Crystals will have spurs that will mix for you.

Get the signal generator out and plugged into the antenna input. Get the 
level down low. turn every thing but the generator and receiver off and start 
listening. Run the generator over to the frequencies of the ofending stations. 
Listen to the band where you have the second signal. See how much signal 
generator output is needed to get you a signal on that second dial location. If the 
fundimental frequency shows up on the second dial location with no chage in 
generator output you likely have a crystal with a spurous output. It will be one 
of the crystals that is active in the mix for the dial setting you are on. 

You need to decide if the problem is internal to the receiver or external. 
You may need to run some attenuation or band pass filters on your receiver.

I had to put my R390/A into a full metal jacket (small cabinet) and ground 
that jacket with a real braid and real ground rod to keep AM 600 out of the 
receiver in LA. I have real twin AX between the antenna input and a feed through 
coming out of the jacket. Then a length of twin Ax into another metal box 
(antenna tuner) where the antenna goes single ended to my wire.

Roger.
    


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