Active Antennas
 

Hi, would anybody know (or like to know?) the "secrets" hiding in the pvc
tube antennas like Dressler ARA-500, 900, 1500, 2000 LF-Engineering H800,
etc. What kind of element is used as antenna, also the config of the preamp.
and the semis used? Their overall efficiency? I know my questions are many
fold but still I'd like to know. Thanks to All. Just Curious.

Active Antennas
 

Just Curious wrote:
Hi, would anybody know (or like to know?) the "secrets" hiding in the pvc
tube antennas like Dressler ARA-500, 900, 1500, 2000 LF-Engineering H800,
etc. What kind of element is used as antenna, also the config of the preamp.
and the semis used? Their overall efficiency? I know my questions are many
fold but still I'd like to know. Thanks to All. Just Curious.


No secrets, just a short and probably simple antenna and low noise
amplifier. The antenna is likely to be a simple monopole which would be
no longer than about wavelength at the highest frequency to ensure low
angle omnidirectional response. There's no practical way I know of to
define the efficiency of an active antenna. Other than amplifier noise
figure, what would separate a good active antenna from a bad one would
be the dynamic range of the amplifier -- poorly designed ones would be
prone to overload and spurious signal generation from strong signals.

Roy Lewallen, W7EL

Active Antennas
 

Correction:

Roy Lewallen wrote:

No secrets, just a short and probably simple antenna and low noise
amplifier. The antenna is likely to be a simple monopole which would be
no longer than about wavelength at the highest frequency. . .

That should be "no longer than about a half wavelength at the highest
frequency. . ."

Roy Lewallen, W7EL

Active Antennas
 

On May 5, 10:17*pm, Roy Lewallen wrote:
Correction:

Roy Lewallen wrote:

No secrets, just a short and probably simple antenna and low noise
amplifier. The antenna is likely to be a simple monopole which would be
no longer than about wavelength at the highest frequency. . .

That should be "no longer than about a half wavelength at the highest
frequency. . ."

Roy Lewallen, W7EL

From memory the ARA-500 (50-500MHz) used a fat printed circuit
biconical style dipole with a broadband balun feeding something like a
BFT66 bipolar transistor RF amplifier with ferrite transformer derived
negative feedback (Similar to a broadband pre-amplifier design
originally published in the German 'VHF Communications' magazine
sometime in the 1970's). The later versions used FETs or broadband
amplifier MIMIC's of various types and a strange helically wound
antenna element constructed from foil tape.

As long as you can achieve reasonable impedance matching to the active
device across the required frequency range, and an amplifier noise
figure which is better than the atmospheric noise level which can be
received on the physically small antenna element, then it should work
as well as (or in some cases better than) a full sized antenna.
However this is a tall order, as strong signal handling is the main
issue with low noise amplifiers, especially when used in a broadband
configuration in the presence of strong local signals (usually
broadcast stations).

A good design of balanced HF active antenna (100KHz to 50MHz) can be
found on the link at the bottom of this web page.

http://sites.google.com/site/datongarchive/ad270-ad370

UKM