[Lowfer] E probes .v. loops

Steve Dove lowfer@mailman.qth.net
Thu, 11 Sep 2003 22:00:53 -0000


Hi Peter,

We're actually saying pretty much the same thing.

>?? I don't understand this. Surely if the probe is properly decoupled,
>say, by a ground plane or other means, the feed line remains just that.
>

Quite right.  Except that it is unusual for an e-probe's body to be mounted directly on 
something adequately groundlike, or to be attached to an adequate ground 
plane/screen;  'adequate' in either case meaning presenting a *much* lower impedance 
than that presented by the feeder outer  -  difficult.  Proper decoupling is for most e-probe 
installations the exception rather than the rule.  All the feeder outer and anything 
connected to it until it connects to 'real' ground (or an awfully *big* plane) is part of the 
antenna.

In the worst case installation (sadly most) that means that the feeder run back to the 
shack plus much else, with all the horrors that implies, is active.

>The important consideration when using a probe is to isolate it away
>from all signal hoarding structures, and if around trees, it should be
>above their effective canopy. This is according to accepted chat here on
>the list.

Also agreed, hi-Z antenna and 'lossy green capacitors' and all that, but in corollary with 
the above, a lot of the improved performance from getting it higher is due to the additional 
feeder (read longer antenna) required to reach it. 

With such an effective and fat conductor available in the form of the feedline screen, 
there is little point in running a separate ground wire to the e-probe.  The feedline should 
be run directly to the nearest available real and quiet RF ground instead, and attached 
thereto.  An isolation transformer, or at least a common mode 'choke' of as high an 
impedance as attainable, to decouple the feedline back to the shack should serve to 
keep antenna the antenna, feedline the feedline (cognizant that 'antenna' still does 
include the bit of feeder going from ground up to the probe). 

Don't get me wrong  -  I like e-probes (countless Altoids tins and fish-food containers to 
prove it)  and they work well when good installation is possible.  When it isn't, they're deaf 
and noisy.

        Cheers,

                Steve