AMRAD LF Active Whip Antenna

© Brooke Clarke, N6GCE

AMRAD Active LF Antenna
AMRAD active antenna with 1 meter whip

Background
Noise
Blown Transformers
Photo Descriptions


Background

As part of their effort in support of a U.S. <-> Europe contact in the 136 kHz amateur band AMRAD has published the design for an active whip antenna covering 10 kHz to 30 MHz in the September 2001  issue of QST magazine.

The design took into account that a strong front end is needed to minimize intermodulation distortion and some attention was paid to isolating noise from the AC mains, both by allowing for the use of battery power and by choosing a power transformer for the PS box with minimal primary to secondary capacitance.

Noise

It turns out that for use below about 200 kHz the power supply suffers from excessive power line noise.  By adding a couple of 0.1 caps, one across the secondary and another next to the printed circuit board and adding a couple of 100 uH chokes the noise was drastically reduced.
When I remember to get a couple of 1 mH or larger chokes with low DC resistance I will try to improve the noise floor some more.
Prior to this mod WWVB at 60 kHz was buried in the noise(GIF), this is in comparison to the McKay Dymec DA-100 active whip noise floor(GIF).
After the modification WWVB is 20 to 50 dB above the noise floor(GIF) depending on the time of day, but there is still room for improvement in the noise floor.

May 2002 - replacing the two 100 uH chokes with 1 mH chokes cut the frequency where the noise floor rises about in half.

Blown Transformers

The first time the transformer was blown (coax hit by lawnmower) the fuse looked too strong so was replaced with a 1/4 amp fast blow.

April 2009 - This time I looked into the transformer specs and talked with the maker.  They say the limit on current is way below 1/4 amp, i.e. should be 0.08 Amps.  To get this you need to use a self resetting fuse.  So the AMRAD design did not properly specify the protection of the transformer.
I'm adding a MOV across the AC line input, a Resettable fuse for the primary and the secondary, and a TVS across the coax connector.  Maybe the transformer will last longer this time.

Photo Descriptions

(top of page) Antenna mounted at ground level using a wood stake and plastic tie wraps so as to not add any extra capacitance to ground.  The whip is a Radio Shack 1 meter 21-952A Mobile CB Antenna Stainless Steel Replacement Whip with a male 1/4-20 thread base.  I added a 4' ground stake right at the antenna base while trying to improve the noise floor, but it did not make a noticeable improvement.  Although the antenna is mounted near the ground it is on the edge of a slope and is also relatively in the clear above, tree wise.

AMRAD Power Supply
            ModificationPower Supply Modification

added caps and inductors are marked.  They can be installed without removing the PCB.  Note that the secondary output is shown at the back side of the transformer (the primary is down near the PCB).

Spectrum Plots made with the AMRAD antenna:
0 to 200 Khz overlaid with the DA-100  <19 Jly 2007 on line>
200 to 400 kHz - airport NDBs             <19 Jly 2007 on line>


Sources


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