AM Antenna for Zenith Tube Radio

On the back of my Zenith Tube Radio are G and F terminal connections. There is a blue wire that comes out from within the radio and hooks on to the F terminal. I realize that G is ground and F is the antenna hookup. I decided to disconnect this blue wire and hook up my own dipole antenna. I can tell my antenna is making an effect based on how I move it. I decided to hook my AM antenna to these terminals. It's just a basic 5 turn, rectangular, loop antenna. It's not making ANY effect at all. I'm trying to pick up better AM stations and am not having much success.
answers:
You neglected to mention a model number but this sounds typical of 50s-era AM-FM Zeniths.
The blue wire from under the chassis is a capacitively coupled connection to the AC line cord for FM only. The terminals allow you to use an FM antenna using twinlead....rabbit ears, outdoor aerial, etc.
There is no connection (typically) for an external AM antenna. If you'd like to add one a simple ~2-3 turn coupling link around the existing internal antenna (ferrite rod loopstick or back panel loop) will do the trick. Tie one end of the coupling link to chassis via capacitor (0.05 at 600 volts will do) and the other end to your wire antenna. It will not affect the FM connection.
I'm not sure what size your 5-turn loop is but if its not resonant it would probably work better unwound Being non-resonant its simply a random piece of wire compromised into a smaller space.
If you want a good external loop antenna there's plenty of discussion to be had on that. They can be link-coupled to radios which use an internal antenna by the method described.
Depends on your needs. An outdoor random-wire tends to have more pulling power for weak signals but the loop (resonant) will allow you to tweak the directivity to null out adjacent stations and/or local interference.
answers:
Thanks for help. The model is H 845. The AM antenna was included with my 1991 Onkyo Tuner/Receiver, when I purchased it. There is a back panel loop on the Zenith. If I have the blue cord connected to F, and I place my loop antenna across G and F, then am I already following your advice, with exception of not using the cap that you mentioned? That's probably the trick that I missed.
answers:
Oh, I think I misunderstood you. I will need to find the ferrite rod loop inside and find the terminal that does not go to chassis. I could attach one terminal of my am external loop antenna to. The other terminal would connect to a capacitor and from capacitor to ground. Essentially I would be having two am loop antennas in parallel?