LF: screened room and the probe/mini

[Laurence KL7UK <hellozerohellozero@hotmail.com>]

"Now place the receiver and the rest of the local world with all its noise and QRM generators in a screened room and make the feed coaxial through a bulkhead connector into teh screened room and see what happens.   All the QRM is contained within the screened room and cannot pass outside.    The antenna is now a strange sort-of dipole with one end in free space, and the other connected to a solid mass made from the outside of the screened room.   It can't pick up any QRM from inside the screened room, and all it will now receive are signals generating an E-field between the probe and the solid mass.  Perfect!

 

The shack and the rest of the world is not in a screened room, so there lies the problem. "

 

 

Andy notes -

 

This is a very good prinicple and has served me very well in hotels, high rises etc that tended to have a metallic rebar/frame content.... we dont have trees in a lot of the locations I find myself in**

 

I found that often, by selecting a metal window frame, outside door or similar bonded to the structure appx 30-40dB common mode noise coming from the hotel room up the cable is eliminated. keep the strap from the coax short  (small reactance needed) though if your listening on HF. Mine are max 0.3m long...

 

Selecting the actual structure to %^&**# (nearly said it) is an art form and its amazing the number of windows or doors are defacto half or fully touching the rebar/building frame. This as Andy says gives a defacto (sometimes leaky) Faraday cage and if you can select the probe in a favorable position outside and  up over the edge of a building youll probably get fair results. Ive used this solution in Singapore, China, Indonesia, Malaysia, Chile etc and got what I would call good results. Not ruffling my feathers but received Eu on 500 and 137 on a PA0RDT from most of those locations on a quiet/favorable day, and I would lay a hefty wager for these locations a 20/30m long wire suitably matched would have been worse....just intercepting too much rubbish...

 

Im guessing some of the UK brick houses built in the 40/50 and 60's with the older window/outside frames will oblidge...

 

Now a lot of Buildings in say Alaska/Oklahoma  are only wooden framed and thus are fairly transparent to RF at LF/MF and HF - (use a portable radio on SW with its whip and try to see what you can hear inside a metal framed/bonded building,:-) )) - For instance in Alaska I have all sort of problems from common mode and direct radiation noise - I found that a single or more earth rod where the coax comes in helps a tadge, but with common mode choke using peaky turns at the required freq helps a little more, sometimes, isolation transformers more as long as you can keep inthe intercap down and remind yourself they have to be phased properly on a toriod. But the best Ive found was to extend my ^%$# mat <nearly said it again>  (mulitple wires radiating away) from the shack window out to where the probe is in the air as far as possible - and to keep these wires way way away from the electrical protection earth rods from the incoming supply which have their own noise and currents....

 

I like the idea of remote power, optic cables, with small counterpoises or balanced array, but for simplicity an PA0RDT/AMRAD or modified themes are hard to beat for those poor souls like me that have to select a small quietish spot, dont have trees or real estate, or a nearly handy bonded and rebar/metal framed building...

 

If I build a new house Im going to get them to pull and staple 12 gauge solid cable every metre up and over vertically the whole length of the building and bond this to the @#%$# <nearly said it > rebar....

 

Laurence KL7UK et al

 

 

 

**Acacia trees for some reason are my tree of choice for hoisting up - Palm trees for some reason arent so good. Im testing Pecan trees at this time :-))
 

---

From: g3kevmal@talktalk.net
To: rsgb_lf_group@blacksheep.org
Date: Wed, 10 Aug 2011 17:08:37 +0100
Subject: Re: LF: Re: HB9ASB...

 

----- Original Message -----

From: Andy Talbot

To: rsgb_lf_group@blacksheep.org

Sent: Wednesday, August 10, 2011 4:30 PM

Subject: Re: LF: Re: HB9ASB...

I'm surprised so many seem to misunderstand how high impedance E-field probes work.  There is too much hearsay and false comments being peddled.   Stop thinking of the feeder as 'part' of the antenna as if it were all one entity and consider each stage.... And forget that thing called Ground completely.

 

It starts with a high input-impedance amplifier which must have two connections - ignore any common reference for now, it just has two input pins.  A voltage is imposed across these from any antenna with two ports,  like a short dipole.   This input voltage is buffered, amplified,  and sent to the two output pins from which it travels down a feeder - balanced, coax, or whatever -  and into your receiver.   So far we haven't made any connection between input and output, and they could (and ideally would) be independent and isolated from eachother

 

However, they're not.   One input connection is usually common with one output - usually the  0V DC supply pin and the reference  (and please note, I am not referring to this as ground; ground is taboo, a dirty word,  and will not be mentioned)    Which means the other hot-side input pin now has a voltage imposed on it with respect to the reference.  This will probably come from a probe antenna which is coupling to the E-field of a radiated signal.   Now, bear in mind any antenna must have two output ports, so where is the other side?   As the reference input pin is connected to the output reference pin, any other connection to this point will form the other 'half' of the high impedance short dipole antenna.

 

Now, the feeder dropping down from the amplifier / probe assembly (or across, or up and over, or buried in the soil [see, still didn't say the taboo word] ) is connected to the input reference pin by virtue of the amplifiers internally linking them, and must therefore form the other half of the dipole.   This is bad.   We have a hi-Z dipole, with one short element in the air where it should be, and the other element being one conductor of a probably long length of feeder with its end connected to we know-not-what.

 

So what do we now have?  A long assymetric dipole plus amplifier assembly, stretching from some arbitrary shack connection up into free space.   The voltage at the not-middle of this is the stuff that is amplified and fed to the receiver.    So, as the bulk of the dipole is close to the shack expect the majority of signal received to be locally generated noise.  

 

That is the case for a completely unscreened and un-earthed (still didn't use the word :-)  system.  Picking up any locally generated E-fields at high levels onto the dipole. 

 

Now place the receiver and the rest of the local world with all its noise and QRM generators in a screened room and make the feed coaxial through a bulkhead connector into teh screened room and see what happens.   All the QRM is contained within the screened room and cannot pass outside.    The antenna is now a strange sort-of dipole with one end in free space, and the other connected to a solid mass made from the outside of the screened room.   It can't pick up any QRM from inside the screened room, and all it will now receive are signals generating an E-field between the probe and the solid mass.  Perfect!

 

The shack and the rest of the world is not in a screened room, so there lies the problem.   However much you don't want it to be, with this setup the feeder will always form the majority of the antenna because there is nothing else there that can be the other half of the dipole.   And if the bottom end of the dipole intrudes into the rest of the world, it will pick up stuff it shouldn't

 

Andy

 

So the FEEDER does form the majority part of this type of antenna, simply one side of a very poor asymetrical dipole.

It would appear this is not the antenna for a noisy urban environment

The long element of the dipole is picking up every possible noise and amplifying it and feeding all this noise plus signal to the RX, the minute Probe element is insignificant. 

This has been my observation all along.

 

G3KEV