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

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:

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 3^rd order IM was +37 dBM and the 2^nd order IM was +53 dBM.   By comparison the only active antenna that beats this is the Dressler ARA-60 which shows a 3^rd 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 3^rd 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
*****************************************8*************************************