Alberto di Bene wrote:
Hi Uwe,
probably you haven't found it because it is referred to as "Amrad
active antenna".
Here is the PDF file describing it :
http://www.arrl.org/tis/info/pdf/0109031.pdf
and here you can find some notes by Amrad on it :
http://www.pacificsites.com/~brooke/LF-Ant.shtml
It uses a rather unusual power FET as active device, the
Crystalonics CP666. A few have built it using a couple of U310 in
parallel, at the maximum drain current compatible
with the max dissipation, and with the supply voltage
reduced from 24 to 12V.
If the Roelof's mini whip antenna performs similarly,
then there isn't the need of searching for the (expensive)
CP666 FET.
73 Alberto I2PHD
As regards the AMRAD LF Antenna, here is Frank K0BRA's latest write up
on the subject, dated 29 January 2006.
73
André N4ICK
********************************************************************************************
The
AMRAD Active
Antenna After Five Years
Frank
Gentges K0BRA
Its been five years since the September 2001 QST
article on
the AMRAD active LF receiving antenna developed by several of us at
AMRAD. Since then we have heard from a number
of
people and the indications are that somewhere between 300-500 antennas
have
been constructed pretty much in accordance to that article. Since publishing it in QST, ARRL has posted
an Acrobat file of it on their web site at
http://www.arrl.org/tis/info/pdf/0109031.pdf
so you don't have to dig out your old QSTs. There is a web page on this antenna on the
AMRAD web site at
http://www.amrad.org/projects/lf/actant/index.htm
The web site has additional information to the
details in
the QST article. Especially, see the FAQ
on a number of issues and one correction to the schematic.
EXPANDED APPLICATIONS
A fellow in one of the big EMI/EMC labs uses this
antenna in
their lab for testing. Several local
organizations are using the antenna for non-amateur field work and seem
to like
it. If it is built with a one meter
whip, the sensitivity at the higher end of HF will be a bit below the
background noise. A ninety-six inch
whip will fix this but nearby AM broadcast stations may cause
intermodulation. It is best to just try it.
I built one up using a copper pipe in place of the
PVC
pipe. A copper pipe cap on one end has
a BNC connector. A small piece of brass
tubing of three inch length was
soldered into a hole in another copper pipe cap on the other end. A wire lead from the amplifier input goes
out the brass tubing. This has proved a
useful tool for sniffing local RF fields inside a chassis or around
equipment. Coupled with a selective
voltmeter a decent field plot was made
around an antenna counterpoise model to predict the overall effect of
the
counterpoise.
Dr. Dallas Lankford's original design essentially
had two of
these back to back to make up an active dipole. This
would have a nice null that could be used to null out
interfering stations.
Four of these could be arranged in a square to
build a small
Adcock direction finding array. A small
processor, a pair of digitally controlled attenuators and a pair of
wideband
combining transformers should be able to generate a rotatable pattern.
CRYSTALONICS TRANSISTOR
The largest number of questions over these five
years have
been on the use of the Crystalonics JFET.
This device is only generally available from them directly and
they were
kind enough to honor orders for one or just a few units from
individuals. I asked them very recently if they
were
still willing to make it available on this basis. Paul
Weinstein commented on how they had sold many to individuals
and to OEMs that had incorporated the device in their designs. He assured me that “it has been a good
arrangement and I will continue as long as you keep republishing or
refreshing
the article”. I know they have shipped
these transistors all over the world.
They are now making some of the Motorola parts that ON
Semiconductor quit making when they
took on the Motorola discrete component line and are a valuable niche
producer. See Crystalonics at:
http://www.crystalonics.com
and by mail at:
<>Crystalonics
2805 Veterans Highway-Unit #14
Ronkonkoma, New York 11779
We found the Crystalonics CP-666 was
extraordinarily linear
and gave us very good intermodulation and overload characteristics. This transistor presented a real challenge
to measure. We found the 3rd
order IM was +37 dBM and the 2nd order IM was +53 dBM. By comparison the only active antenna that
beats this is the Dressler ARA-60 which shows a 3rd IM point
of +50
dBM on their data sheet. If less IM
performance can be tolerated, it can be replaced with a
lesser U310 metal case or J310 plastic case
JFET if we lower the DC power to 12 volts and biased at about 30 ma of
drain current. With
these JFETs, the 3rd order
IM will be more like +10 dBM and the second order IM will be more like
+18
dBM. This option allows the deletion of
the heat sink which simplifies the construction.
The use of 12 volt power makes the unit a nice
LF/MF/HF
mobile receiving antenna running it directly on the vehicle power with
just an
isolation transformer. While running LF mobile all sorts of LF PLC
(Power Line
Carrier) control signals were heard near a large AC power transmission
line. The simpler antenna also
performed well on the Outer Banks in an LF mobile receiving setup. It did, however show some intermodulation
with the strong LORAN-C signals there which would interfere with the
weakest
signals. These LORAN-C signals were the original reason to develop a
better
antenna which moved the design to the 24 volt/CP-666 version where
these
artifacts were not seen. By getting rid
of these intermodulation products it demonstrates that the LORAN-C
system is in
fact clean and the artifacts are a result of imperfect active antennas,
receivers and nearby rusty bolt joints re-radiation.
OTHER PARTS
Originally, Signal Transformers assured us that
they would
honor orders for single units.
However, that changed and now
require a minimum order of $50. About
the same time an email from Jon Lerner
at Mitchell Electronics indicated they would like business from radio
amateurs
and were quite willing to sell single units.
A unit was ordered and tested.
the Mitchell design (P/N DP 241-4-24) is better for this use. You can reach them at:
http://www.yesmec.com,
914-699-3800
Mitchell
Electronics
85 West Grand Street
Mt. Vernon, NY 10552
It was also found that Amadon was not selling to
amateurs
any more. At Dayton Hamvention
Bytemark, who is an Amadon distributor, was displaying and indicated
they
gladly sell to amateurs. Bytemark has
been very helpful and can be reached at:
http://www.bytemark.com/
800 679-3184 (USA only) or
714-547-3276, by fax to 714-547-4433
[email protected]
Bytemark
1510 E Edinger Ave #B
Santa Ana, CA 92705
The PC boards are still available from FAR
circuits. They have a large number of different
boards
and you can find this board set under “RECEIVER PREAMPLIFIERS” on their web site at:
http://www.farcircuits.net/
(847) 836-9148 Voice/Fax
and mail at:
FAR Circuits
18N640 Field Court
Dundee, Illinois 60118
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