[Fwd: LF: Re: E-field antennas]

[Andre Kesteloot]

Derek Atter wrote:

> 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
> datter@btinternet.com
>
> ----- Original Message -----
> From: "Alan Melia" <Alan.Melia@btinternet.com>
> To: "LF-Group" <rsgb_lf_group@blacksheep.org>
> 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
> > alan.melia@btinternet.com