[OKDXA] Yo... Mr. N5UW:

[Nelson Derks]

I recall you were asking about 160 Meter RX antennas a few weeks back...

I took my own advise and whipped together a receiving loop for the AM
broadcast band this week and gave it two thumbs up last night... Despite
being indoors and no more than three feet off the ground, it's delivering
good signal levels with much less noise than the random dipole I had been
using. Yes, the dipole is about two S-Units 'hotter' than the loop, but the
lack of noise more than compensates for the lower signal levels. If I had
the loop installed at the same height outdoors, I suspect the difference in
signal strength would be negligible while the noise reduction would remain.
As it is I can peak the loop on band noise with all attenuators turned on,
and that tells me I'm hearing all the way down to the practical noise floor.
Adding a pre-amp won't improve the S/N ratio and while tuning around 160
last night I heard plenty of activity with some very clean copy despite the
loop being out of resonance on that band. I still need to dial it in by
removing a turn or two...

So... What did I build? Being reluctant to waste materials, I bought a 10'
section of 1/2" thick-wall (600 PSI) schedule 40 PVC pipe and one 'X' plus
four 'Tee' couplers. Using a tubing cutter, I chopped this in to four 2'
sections and eight 3" sections. The four longer sections are glued to the
'X' with the 3" sections used to extend the T's mounted perpendicular on the
ends. This gives a 7" spread across the perimeter with approximately 12' of
wire in each turn and no wasted PVC. (BTW: I used CPVC glue as it has a
slower action and gives a bit more adjustment time before it sets) I also
gave the frame a shot of black enamel spray paint so it looks pro-fessional.
(Hi!)

The outside T's were notched with a hacksaw blade every 1/2" with a small
hole drilled through each T where the center lines cross. One T has a second
hole approximately 3/8" below the first hole and away from the outside edge.
These holes are used to thread a one-turn coupling loop nested inside the
outer turns. I used a junk spool of 20 gauge enameled wire and ended up with
13 turns on the outside loop. A medium-value (70 pf?) padder cap was
attached to the ends of the outside loop while the one-turn inside loop was
soldered directly to some scrap RG-62. If I didn't have the enameled wire
laying around I'd probably have opted for 22 ga telephone hook-up wire or
similar. The wire size isn't critical, and there's nothing magic about the
1/2" spacing. Use what you have and choose sturdier materials if it's going
outdoors. Mine has less than $10 worth of materials involved and it's moving
to the attic once it's dialed in...

I can tune the loop for a clean noise peak up to 1400 kHz or so. Beyond that
the padder cap opens up so much that it's out of range. I need to remove a
turn or two from the outside loop. The concept at play here is a series L /
C circuit being tuned to resonance. The larger the L, the smaller the C and
vice-versa (in very broad terms). When the C gets too small it will limit
the current in the loop and efficiency drops rapidly. The goal is to adjust
the number of turns in the coil until you're using maybe 30-40 pf of
capacitance at the high end of the band. On 160 the band is narrow enough
that you could probably get away with a fixed silver-mica in the 20 pf range
paralleled with another 140 pf of variable capacitance, and this can be in
the form of varactor diodes if you want to avoid motorizing a conventional
tuning capacitor. A clever guy can figure a way of using a DC block on the
coax so the tuning voltage runs up the coax, although this will reduce the
signal levels a bit. Since the AM broadcast band covers several octaves (540
to 1700 kHz) I'll need more tuning range in the capacitor, but last night I
found the Q is low enough that I'm hearing perfectly readable signals well
off the resonance peak despite being S-3 or lower. I have a hunch I'm not
going to worry too much about the extreme low end of the band. If I were
interested in only the CW or Phone segment of 160, I suspect a fixed value
cap would be close enough, although I'd probably weatherproof a trimmer cap
just so I could dial it in exactly where I wanted it.

In closing, it takes a little time to get used to seeing the S-Meter ride
lower than what you're accustomed to and I'm learning to trust my ears...
Around noontime yesterday KRLD in Dallas was an S-5 and barely readable in
the noise on my Windom, but 100% copy with moderate noise at an S-2 on the
tuned loop. Could have been a quirk in the wave angle, but whenever an
indoor antenna near ground level delivers better copy than The Wonder Wire,
it tells me I'm on to something... By 6pm KRLD was pushing 10 over and
sounding like a local on the loop. Also, I found the loop is very useable
for general SWL listening and has no problems with WWV at 10 and 15 MHz. The
Wonder Wire will beat the heck out of it on HF, but the loop can be built to
fit the back of a closet door and that makes it fully 'rotateable' for GP RX
use... All you need is 90 degrees of swing.

- AC5UP