Hi all,
Re Alan Melia's posting on E-field antennas (Aug. 25th), I confirm his
comment that I did carry out a number of tests on two E-field active
antennas each having roughly a 3m whip, one an ex-Decca Navigator design
using op-amps, and the other an AMRAD high dynamic range design using a
Crystalonics high current FET and published in QST in September last year (I
think it was).Both of these designs feed DC up the coax to the active
element.
I compared these with a 50m long inverted 'L' antenna with a 15m vertical
section (resonated with a base loading coil) and also with a large
rectangular loop approx 8m per side in a very noisy urban environment. This
was at a time when I was experiencing interference on 136kHz radiated from a
hi-speed data cable which also coupled into local CATV distribution cables
(not connected to my QTH!).
The main findings were :
(1) Both E-field antenna designs performed in a similar fashion and when
mounted reasonably in the clear, ie more or less above roof-top height,
produced a signal to noise ratio similar to the inverted 'L' antenna.
Generally anything I could hear on the inverted 'L' , I could hear on the
active antenna but the active antenna appeared less susceptible to the cable
interference than was the wire antenna. Under conditions of radiated
interference which appeared to be primarily in the H-field from the local
data cable, the loop was virtually unusable which meant that I could not use
it for Loran cancellation!
(2) The slope of the variation in absolute signal strength at the output of
the E-field antenna with variation in height above ground was initially
fairly constant and surprisingly high at around 2 to 3dB per metre up to
about 12m. Above that the rate of change flattened off but the received
signal strength still continued to increase up to at least 20m which was as
high as I could measure.
The flattening off in signal strength increase at around 12m I assume is
as a result of the active antenna becoming less 'E-field shadowed' as it
rises above roof-top height.and local trees.
(3) It is strongly recommended that the coax feeder from the E-field antenna
should be allowed run more or less vertically downwards to ground level and
the outer of the coax then earthed at some point underneath the antenna. At
the shack end of the coax, noise currents on the outer of the coax should be
suppressed by passing the signal through an isolating 1:1 transformer (eg. a
bifilar winding of a few turns on a suitable small toroid). Without the
isolating transformer I found the performance of both the E-field antennas
very disappointing in a noisy environment. The transceiver in the shack
should be separately earthed
(4) The above results confirm those outlined AMRAD article in QST. I think
that Wallter G3JKV would also confirm that at his QTH, the results from an
active receiving antenna mounted at 20m+ above ground are similar to those
obtained from a large well-sited wire antenna.
(5) There is little advantage to be found in extending the whip much greater
than 1 metre or so - cross-mod then potentially becomes more of a problem. A
bandpass filter between the output of the active antenna and the RX input is
also strongly recommended to minimise the risk of intermod problems from
broadcast stations.
Regards to all, Derek Atter, G3GRO
[email protected]
----- Original Message -----
From: "Alan Melia" <[email protected]>
To: "LF-Group" <[email protected]>
Sent: Sunday, August 25, 2002 11:18 AM
Subject: LF: E-field antennas
Hi all, as I understand it the performance of an E-field antenna is more
related to its height of installation above ground that to any effect of
the
amplifier. As we all know an amplifier will amplify noise as well as the
signal, its real purpose is to extract a signal efficiently from the very
ofdd impedance of a short wire. It may well be that this type of aerial
is
better suited to the tight filtering necessary to keep the strong stations
in mainland Europe from generating intermodulation, and will also give
some
advantages to those without the 'real estate' or mast height to erect wire
aerials. To me this suggests that an E-field antenna at 20m height will
work
about as well as a 20m vertical wire......or have I missed something ?? I
suppose if you house is in a clearing amongst tall trees it could have a
distinct advantage over a wire.
I think Derek G3GRO has done some comparative tests?.
Cheers de Alan G3NYK
[email protected]
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