Hello Bob,

Here's a link that will give you an idea of what is
involved. The active antenna is just the circuit at the
upper left, and this one works all the way to 40 MHz!

http://www.qsl.net/on7yd/la8ak.htm

Remote operated active antenna for 10kHz-40MHz
If you do a google search on "active antenna" you will find
a great deal of information that may be of help.

Good luck!

Chuck




Bob Medico wrote:
Hi,

Your post interested me because I need a coupler for a SITEX unit. I am
not an electronics expert but have successfully fabricated circuits from
schematics in the past. The long and short of is that I would like to
attempt to build my own coupler. Do you or anyone else here have access
to SITEX service manuals that may show the circuit used. I would think
that such low frequency stuff would not be too touchy to fabricate if
one had the schematic and parts list. I have a SITEX LORAN 797.

TIA

Bob

chuck wrote:

Morrow is one of the companies that use "active" antennas to
overcome the fatal signal losses resulting from enormously
high impedance (~ 1 Megohm, albeit mostly reactive) of
extremely short antennas fed into 50-ohm receiver inputs.

Your circuit board probably contains an FET follower to
convert the high impedance of the antenna to a low impedance
of 50 ohms. To do this with a coil would probably render the
system so lossy as to not work at all.

You should be able to vary the antenna length modest amounts
(I'm thinking +/- 20%)with no noticeable difference. Just
how far you can go will be a question you can answer easily
by trying different lengths of substitute antenna.

On the other hand, if it was working fine with no antenna at
all, (was the coax connected to the circuit board?) do you
really need to go to this trouble?

Good luck, and let us know how you make out!

Chuck

RB wrote:

Does the length of a LORAN metal whip matter much? I recall a time when I
found I didn't even have an antenna on my LORAN (years ago); just a coil of
coax under the console, and it worked fine. So, can I stick just about any
whip on the coupler and get good results? Or, is there some reasonant
length that is needed?

I'm putting a LORAN in my 18' cc, and want to use a short metal whip on the
coupler, if that'll work OK.

"Dennis Pogson" wrote in
:

That's about the same frequency as my sextant works on.


You should see the TRANSMITTER and it's huge antenna at this frequency.
I've visited many LORAN "C" installations across the South and calibrated
their test equipment in a mobile cal lab. Try reading a scope in between
megawatt power pulses when you're lab is located BETWEEN the huge capacitor
hat 800' above you and the massive ground system buried right under you.
Everything in miles has LORAN pulses on it....

If you ever get the chance to go by any LORAN station, do so. Knock on the
door and tell the Coasties thanks for providing you with nav at sea.
They'll be glad to show you their beast.

The antenna, by the way is 800' high with a 36 spoke capacitor hat about
500' diameter made of bridge cables at the top to tune it at such a low
frequency. The transmitters are all solid state. The output power
actually comes from multiple drawers of Silicon Controlled Rectifiers,
thousands of amps, pulsing from cesium-beam frequency standards as the
pulse timing of LORAN is very critical to its positional accuracy.

It's all obsolete, now that GPS works so good. Politics keeps it on the
air....

LORAN "A", the one from WW2, was on 1.9-2.0 Mhz above the standard
broadcast band. Its antennas were much smaller because of the higher
frequency. It was useless at night or in a storm because of the "skip",
same physics that lets you listen to an AM radio station 800 miles away all
night. Charleston had one on the very northern tip of Folly Beach run by a
few sailors who lived across the street from the transmitter. Fine duty
back in the analog days...(c;