On 7.3.13 4:30 , rickman wrote:
On 3/7/2013 2:15 AM, Tauno Voipio wrote:
On 6.3.13 9:00 , rickman wrote:

Since the loop is inductive, your first priority is to resonate it
with a
capacitor at the desired frequency. This will require a very precise
value, and even for a single frequency, may require a variable
capacitor
to account for manufacturing tolerances. In the AM BCB, a Q of 10 gets
you 50-160kHz bandwidth, so you only get a few channels for any given
tuning position. And if the Q is higher, you get even fewer.

Yes, that is loop antenna 101 I think. It was when I added a coupling
transformer with 100:1 turns ratio that I was told I needed to consider
the parasitics. I have found it is not useful to go much above 25 or
33:1 on the turns ratio. I am receiving a single frequency, 60 kHz.
There is no need for a wide bandwidth. Ultimately, I prefer a Q of
100 for the higher gain. If it gets too high, the off tuning by
variations (drift) in the parasitic capacitance affects the antenna gain
appreciably.

Please note that high Q will destroy the modulation sidebands on
the signal you're listening to.

I appreciate the advice from everyone, but much of it is not in the
proper context and way off target. "High" Q is how high? Where are the
modulation sidebands? My point is that I have already considered this.
The modulation sidebands of this signal are on the order of low 10's
of Hz. This signal is modulated at a 1 bit per second rate. I will be
demodulating a 30 Hz sample rate. So a bandwidth of 100 Hz is plenty
which corresponds to a Q of around 500.

I said I was looking for a Q over 100, maybe I should have said a Q of a
bit over 100. By the time it gets to 300 it is to peaky to hold a tune
setting. That is the problem I am concerned with.


In aviation, there are non-directional beacons which are transmitting
in a frequency around 300 kHz (1 km wavelength). The antennas cannot
obviously be of efficient length (250 m / 800 ft), so they are short
(20 m / 70 ft) force-tuned to the transmitting frequency. This creates
so high Q that the identification modulation sidebands for the customary
1050 Hz audio do not fit in, and the ID is modulated using 400 Hz audio.

Ok, but that is nothing like my application, receiving WWVB.


I'd still be wary of high Q. The antenna is, by definition, in close
interaction with its surroundings, and a high-Q thing is quickly
detuned.

At those low frequencies, the atmospheric and other outside noise is
far larger than the internal noise of an amplifier, so in my opinion,
the way to go is a loop tuned to 60 kHz with as low Q as easily comes
without extra attenuation and a good pre-amplifier. The preamp can
then contain a tuned interstage tank for interference suppression.

--

Tauno Voipio