[600MRG] Active Antenna Recommendation

[Brian Pease]

My comments assume a E-field probe antenna (which I will call a VPA for 
Voltage Probe Antenna) is mounted on a non-conductive pole away from the 
shack and fed with coax that has its shield well grounded at the base of 
the pole.
For question (1), the decoupling may be needed only if one has problems 
with mad made noise, which is typically worse at low frequencies.  One 
way to get a feel for whether a common mode choke might help is to first 
note how far the desired signals are above the mad made noise, then 
completely isolate the receiver from ground by operating it from a 
battery, and resting on a wood table.  It the noise drops, then a common 
mode choke with a high impedance at that frequency range might help.
For question (2), I will muddy the waters some more.
A VPA will in fact work if placed with its lower end (where the preamp 
connects) just above a large ground plane (or at ground level) , with 
the coax shield connected to the plane or to the ground.  This will 
provide a flat response at least up through the HF band, but the output 
level will be _very_ low.  If it is atmospherically noise limited in 
your receiver in the band of interest, then you are good to go.
I have actually modeled a VPA at different heights above ground and at 
different frequencies using NEC4.  I assumed a 1 meter vertical "probe" 
with the high impedance preamp connected between the bottom of the probe 
and the top of a grounded metal mast, which simulates the coax shield of 
the feedline.  At low frequencies, the VPA output with a 10 meter high 
mast is roughly 6X (16dB) greater than the output of the VPA on a 1 
meter high mast.
To try to remove the 1 meter probe as a factor, I changed it to a 1 
meter long horizontal "T", then repeated the simulation.  This time, the 
10m mast gave 8.6X (19dB) more VPA output than the 1m mast.
As a rule of thumb, VPA output seems roughly proportional (at low 
frequency) to its mounting height above ground, except that that a 
vertical probe will have some output at zero height.  When the mounting 
height approaches 1/4 wave resonance at HF, this rule no longer applies 
and gain will vary wildly with frequency.
Also, a VPA mounted (and grounded) at the center of a very large metal 
roof would have a gain closer to its height above the roof than to the 
ground far below.
VPA preamps typically have little or no voltage gain (such as AMRADs), 
but have a gigantic power gain due to their nearly infinite input 
impedance and 50-75 Ohm output.
For question (3), a cautious approach would be to slowly increase 
transmit power (if possible) while monitoring signal output on the VPA, 
possibly with an RF voltmeter or scope.  When the VPA starts to 
saturate, you might be getting near the limit.  That said, some designs 
can handle way more than that.  This is a good argument for mounting the 
VPA as far from the TX ant as possible, and as close to the ground as it 
can be and still work.  it does need to be in the open.

On 6/19/2015 9:32 AM, Frank Lotito wrote:
> For the past few days there has been some activity regarding a question posted on active antennas.  From a theory point, what I know about antennas would easily balance on the "tip of a sharp needle."  I have a few questions that you may be able to help me with:
>
>
> (1) For E-Field probe antennas (and H-Field loop antennas) how does one determine that there is a need for common mode decoupling?  For any particular setup-up I assume the need for decoupling is very frequency dependent.
>
>
> (2) Reference the AMRAD article in QST by K0BRA for an active LF E-Probe antenna design: The AMRAD author(s) suggest that a wire screen be placed just below the probe's feed point to "stabilize the E-filed." (See the Siting and Installation section of this article.)  In the recent 600mrg post on this topic, a reader replied that the mast forms part of the total antenna and the drain follower (cathode follower, or what ever) is tapped off-center and towards one end of the total antenna (probe plus mast.)  I am confused - what am I missing?
>
>
> (3) Any guidance on how one would determine if an E-Field antenna (and for that matter, a tuned H-Field loop) is dangerously coupled to the transmit antenna and some form of "protection" is required to keep from blasting the active antenna to kingdom come?  My stuff is home brewed. Blow it up - tough. For those with commercial active antennas, which I believe are reasonably water-proofed, you will be out a few bucks.
>
>
> 73 Frank K3DZ / WH2XHA
>
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