[Lowfer] Antenna questions

[WE0H]

Dang Lyle, that was so good I had to paste it into a Word document. Thanks.

Mike>WE0H
http://www.we0h.us/lf
"WEOH" on 185.3026kc
QRSS 15 & -.-.  .--

-----Original Message-----
From: lowfer-admin@mailman.qth.net [mailto:lowfer-admin@mailman.qth.net]
On Behalf Of Lyle Koehler
Sent: Wednesday, July 02, 2003 3:32 PM
To: lowfer@mailman.qth.net
Subject: Re: [Lowfer] Antenna questions

Ed, here's another response...

As Peter mentioned, Mitch's site at
http://technology.fanshawec.on.ca/tele410/ has details on the "Octoloop" and
the Burhans preamp, plus other loop info. My web page at
http://www.computerpro.com/~lyle/listen/listn1.htm has a couple of articles
describing loops and preamps, with primary emphasis on receiving LowFER
signals. Specifically, you might want to look at
http://www.computerpro.com/~lyle/listen/listn1.htm and
http://www.qsl.net/k0lr/simple-loop/Simple-Loop.htm

Now let's go down your list of questions. Warning -- long answers, and more
opinions than hard facts :-)

1. In any loop-with-ribbon antenna mentioned, the plan is to use adjacent
wires to make it a series-connected set of wires. Is this a better idea-and
if not, what?

I would expect the series connection of adjacent wires to be best from the
standpoint of reducing distributed capacitance. By the way, you should be
able to use Reg Edwards' RJELOOP3 program to calculate the parameters of a
single-layer square loop made of ribbon cable. Reg's web site is at
http://www.btinternet.com/~g4fgq.regp/

2. Is it worthwhile to use most/all of the ribbon-as opposed to either a
single loop of 'ordinary' (1 conductor I mean) wire; or some construct
using a single loop or two of the ribbon cable.

A tuned multiturn loop has the advantage of providing some front-end
selectivity, which reduces problems with intermod from broadcast stations
and Loran. If you run a tuned loop into a preamp with a high-impedance FET
input, you don't have to worry about all that impedance matching stuff.
However, from the standpoint of efficiency, it's simply the total amount of
copper in the loop that matters -- 48 turns in series or 48 turns in
parallel will theoretically provide the same efficiency. A multiturn loop
for LF is fairly easy to tune over a frequency range of about 3 to 1. For
example, 150 kHz to 450 kHz  is fairly easy to reach with a 365 pF air
variable capacitor and a multiturn loop. But if you want DC to daylight, or
even 10 kHz to 500 kHz coverage, you'll need to go with a broadband and
untuned system.

Except for very low frequencies, 48 turns on a large tuned loop would be
excessive. At 185 kHz, you would probably hit self-resonance at somewhere
between 20 and 30 turns on an 8-foot square loop. Above self-resonance, the
loop becomes pretty useless. Again, RJELOOP3 will predict the self-resonant
frequency and the amount of external capacitance needed to tune the loop at
lower frequencies. I would not expect very good results if you try to make a
"multiband" tuned loop by using several taps on the ribbon cable, because
the unused turns are still hanging there on the higher frequency bands.

3. IF I do such a loop, should I make every effort to keep the flat-side
parallel to the plane of the 4X4s, or not worry about that?

I don't think there is any significant difference between keeping the flat
side parallel or perpendicular to the posts. Actually I'd probably put some
twists in the ribbon to reduce its tendency to flap in the wind.

4. IF I do such a loop, should I try to imitate some of the old BDCST loops
where a wire starts in the center and is spiraled inwards, keeping the flat
of the ribbon within the plane of the posts? I could use the baseball net
to do this, and it seems like it would be interesting. (???)

Other than getting a lot of turns into the available space, which isn't a
problem in your case, I don't see any advantage in that configuration.
Efficiency is proportional to the effective enclosed area, so the center
turns wouldn't be doing much for you.

5. And, if my GPD-111 has a 10,000 ohm input, what do you think would be
the most efficient way to feed something like this into the 'shack'?

Impedance matching to the transmission line isn't of great importance at
these frequencies, within reasonable limits. However, with 50 feet of 50 ohm
coax, there would be about 1500 pF of parallel capacitance. At 185 kHz, that
amount of capacitance has an impedance of less than 600 ohms. Some kind of
skinny twin-lead could be used to reduce the shunt capacitance, but I'd run
50 or 75 ohm coax into the shack and put a step-up transformer at the
receiver. Or, if receiver sensitivity is not an issue, just connect the 50
ohm coax straight to the 10k ohm input.

6. Finally, could someone suggest several 'relatively easy' listening
objectives, ranging from ELF to LF?

The US Navy's ELF transmissions are centered at about 76 Hz, and originate
from northern Wisconsin and upper Michigan. I haven't detected them here in
Minnesota, but have never tried very hard, either. From a few hundred Hz to
15 kHz, there isn't much except for sferics and various exotic "Natural
Radio" sounds, which are buried under 60 Hz noise and its harmonics anywhere
in the vicinity of a power line. The Omega navigation signals between 10 and
15 kHz were shut down several years ago. The Russian Alpha system may still
be operating in this frequency range, but it's a difficult catch in most of
the US. Between 18 and 25 kHz you may hear several very strong RTTY-type
signals from US Navy transmitters. As far as I know, they are all encrypted
and not all that much fun to listen to. Not like the good old days, when
they sent the daily news in clear text. Other than the occasional
commemorative transmission from SAQ on 17.2 kHz (which I copied once), the
only CW I've heard down there in recent years has been VTX3 from India.
That's not exactly an easy catch either. Above 25 kHz, you may hear some
fainter RTTY signals. One very easy objective is WWVB at 60 kHz. Sounds like
someone sending slow CW with a sloppy fist. The next big signal will be the
woodpecker-like Loran C pulses, centered at 100 kHz. Then there's RTTY from
CFH (Halifax, NS) at about 136 kHz if they happen to be on the air. And of
course, LF ham transmissions in the 135-137 kHz range and LowFER Part 15
operations between 160 and 190 kHz (mostly near the upper end of that
range). There are still thousands of non-directional beacons between 190 and
530 kHz in North America and other regions of the world. Great for SWLing
because they ID constantly! And more RTTY in the range from about 290 to 320
kHz from differential GPS beacons.

Maybe someone else can add some signals to the "shopping list".

73

Lyle, K0LR