[ARC5] Loop antenna interface to BC-453B

[Bruce Long]

Hello all:
i have been lurking on this thread but I think I have something useful to communicate.

I assume the loop antenna that you want to connect to the receiver is electrically resonant.  If so there is a likely problem connecting it to the BC-453 directly.  I understand the front panel antenna connector is connected to the first RF stage input resonant circuit with an 8mmf capacitor and that the trimmer cap can be used to trim the "leftover" reactance.

If you couple one resonant LC circuit to a second LC resonant circuit (assuming you get the coupling coefficient exactly right)you will get a very nice flat topped bandpass filter response and excellent rejection of off frequency signals.  If you fail to achieve proper critical coupling you will have significant signal loss with an undercoupled response or even worst with overcoupling your nice flat top or pointy bassband will degenerate into a nasty over-coupled response with two signal peaks, neither occurring at the frequency of interest.


Problem is I think your chances of achieving said "Exactly Right" coupling coefficient are essentially zero without application of fairly significant filter design math and accurate characterization of both resonant antenna and RF resonant input circuit. And even if you do change the 8 mmf coupling capacitance to exactly the right value for critical coupling,  the right value is a function of frequency which means you will have the well behaved critical coupling coefficient at only one frequency.


I suggest instead you should add isolation between the resonant loop antenna and the resonant RF input circuit.  I FET source follower or bipolar emitter follower would be ideal  but the noise levels are so high in the lower MF range that it might be good enough to use nothing more than a series resistance or pi or tee resistive attenuator in the signal path between the resonant loop antenna and the BC453 antenna and ground posts.

As someone else mentioned it is highly desirable as well to have a transformer somewhere in the signal path to provide common mode noise rejection by eliminating any galvantic path from the receiver to the antenna.

So I suggest:
Parallel resonant loop antenna (matched to 50oHms if possible)
Isolation amplifier consisting of Fet source follower or bipolar emitter follower.  A balanced isolation amplifier with a pair of transistors would be ideal for improved linearity.
a RF transformer providing common mode noise rejection and impedance step up:

or

same as above with the isolation amplifier replaced by a resistive attenuator.

I suspect the atmospheric noise level will be high enough that the resistive attenuator will have little harmfull effect for any reasonably big loop antenna.

bruce kj3z







________________________________
 From: "kgordon2006 at frontier.com" <kgordon2006 at frontier.com>
To: D C _Mac_ Macdonald <k2gkk at hotmail.com> 
Cc: ARC5 at mailman.qth.net 
Sent: Wednesday, October 31, 2012 11:08 AM
Subject: Re: [ARC5] Receiver input impedance.
 
On 31 Oct 2012 at 11:54, D C _Mac_ Macdonald wrote:

> 
> Isn't all of this discussion a case of "straining at gnats?"

Well, I don't think so because, in my case at least, I want to interface my BC-453B to a 
balanced loop antenna. Impedance matching is a big deal here.

Ken W7EKB
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