On Mon, 10 Sep 2012 10:35:34 +0000, ULLS
wrote:

First off, I am attempting to auto switch/scan between two frequencies
on AM. I have FM and internet streams sorted. The tuner is a Cambridge
Audio hifi tuner with an RS232 interface for scanning. I don't recall
the exact model number.

If it's not too much trouble, could you make an effort to determine
the exact model number? I couldn't find a model on their site that
has an RS-232 port for frequency control. Once the model number is
determined, we can then look at the rear panel to see what you're
working with.

While you're at it, it might be useful if you describe the path that
the antenna coax is expected to take on its way to the roof. Is it
metal conduit? Plastic conduit? Random run? Diameter? Numbers?

The tuner can't be relocated as the server is
controlling the frequency scan.

I beg to differ. RS-232 will go about 150ft at 9600 baud without
difficulties.

Obviously I didn't attach it to the FM coax input! I do have a small
amount of common sense.

Assumption, the mother of all screwups.
That which is most obviously correct, beyond any need of checking, is
usually the problem.

The coax cable was already installed, I assume it's standard CT100
75Ohm.

Assumption, the mother of all screwups again. Look at the coax and
get the numbers. Also, what kind of connector.

the loop was just one of those cheap plastic things they bundle with
tuners.

Yep, that's what I figured. They're resonant in the AM band and do
not work with extension wires on the leads. If you're ambitious, you
could tear the plastic loop antenna open, replace the fixed tuning
capacitor with a variable, and experiment with different parallel
transmission line lengths. However, I don't think it will work very
well.

somebody mentioned impedance matching, Thanks! I'm going to look into
this if I can't already find a ready made 50-75Ohm antenna.

Hopefully, you mean 50 to 75 ohms, not 750 ohms. To get the antenna
in this range, you need a 1/2 wave dipole. At 1MHz, that's 468ft
long. You can shrink the antenna with loading coils, but the usable
bandwidth will decrease to the point of uselessness. An active
antenna might work.

However, I suspect all this design effort is a waste of time. The
receiver should be able to hear stations with a minimal antenna. All
you have to do is figure out where someone else went wrong.

somebody else mentioned overloading with an active antenna....this is
central London so there are all kinds of local AM stations....what kind
of damage can be done??

No damage. The effect is called "blocking" where the front end
transistors or FET's rectify the incoming signal and change the bias
point to where the device no longer amplifies. Unless you have an AM
broadcast station literally next door, it's not going to happen. Your
antenna is too small and the field strengths involved are insufficient
to cause problems.

again thanks for your input, you have given me something to work with.

Exact model number please.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On Mon, 10 Sep 2012 08:48:38 -0700, Jeff Liebermann
wrote:

somebody mentioned impedance matching, Thanks! I'm going to look into
this if I can't already find a ready made 50-75Ohm antenna.

Hopefully, you mean 50 to 75 ohms, not 750 ohms.

Oops... When I put on my reading glasses, it became 75Ohms.
My apologies.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558