to make a loop resonant
to cover the entire AM band
I recently constructed an AM loop antenna using a 29
inch square wooden frame with 8 turns of coated 22 guage wire
spaced one-quarter inch apart and a variable capacitor I salvaged
from a table model radio I acquired at a local thirft store.
It works great. Only problem is the antenna is resonant from
approx. 1100 kHz to 2000 kHz. What can I do to lower the resonant
freqencies of the antenna to cover the entire AM broadcast band?
the loop antenna tune lower, you need either more inductance
or more capacitance.
will do the trick but with the size of your frame, I'd try only
two or three at first, more only if needed and then only one turn
at a time. My air-core loop is 1 metre on the side, having 9 turns,
the centre turn wrapped with a pickup turn.
old papers, I see the Sanserino loop was 10 turns, with 1/2" between
turns, with a tap at two turns to feed the receiver. I always
had a FET preamp connected instead of a low impedance tap on mine.
variable cap used in old radios was 365 pF per section but not
always. If you have a two gang cap and the plates of one section
are larger than the other, use the bigger section. Note that the
connections are between the frame and the tab of one section,
not between the two tabs of the two sections. Most caps would
have a built-in trimmer cap-a copper plate separated from the
frame by a piece of mica, and an adjustment screw. This can be
removed to get the maximum spread between min and max C.
are tuning from 1000 kHz to 2000 kHz, the proper proceedure would
be to add turns to bring the upper limit down closer to 1600.
Since you are getting a 2:1 tuning range, you won't get 540 to
1600 directly, about a 3:1 range. Now add a switch to connect
an extra fixed capacitor in parallel with the tuning cap, 270
or 300 pF should work well. You'll end up with two bands, perhaps
540-1000 with the switch on and 800-1700 with the switch off.
adjust the number of turns and the size of the extra cap to set
the freq coverage you need. In some cases, people had three band