[ARC5] Loop antenna interface to BC-453B

[J. Forster]

I don't think the loop is electrically resonant at or below the frequency
of interest.  This is not a single frequency receiver.

Some loops are/were. The loops used for 60 kHz of WWVB and 100 kHz LORAN-C
were.

In fact, the loops used for the MN-26 and RA-10 used a very special cable
composed of roughly 5/8" flex conduit with loose wires, rather than
twinax. AFAIK, that cable only came in one length, about 6 feet. I suspect
that the cable was designed to reduce the capacitances and keep the
self-resonant frequency high

-John

===========


> 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
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>