James/Dex et al - Good morning - I concur with the findings on the eprobe on the roof of the car. My mobile efforts tend to be was mounted on a trimagnet mag mount and a short stub mast of about 12 inches to which the probe is attached. The coax (about 12ft total length) routed directly into the car across the roof, but bonded to the rear passenger door frame where it enters. This bond must be short and low reactance to stop the in car noises getting up the coax to/close to the antennae. Makes big odds as you go HF.
Got to say even with this very small effective height it worked well enough on 137/500, but very dependant on the type of car, the bonding and currents flowing out/in/capacitance to real ground....
I have a feeling Ill be resurrecting the system in Oklahoma to pass the evenings away...
For the house PA0RDT - I shove a Hi Z at LF common mode choke on the coax outer (30T on J material or maybe it maybe a bit more lossy stuff ) just one meter back from the eprobe, same at the other end of the coax just before its bonded down to the door frame/balcony/rebar reinforcing. At the receive end its galvanically decoupled with a 4T:4T type 77 small toroid - good and flat till 15Mhz
Now, how long it the antennae? Bonding still reduces the noise so there must be small leakage from the coax past the upper choke - probably capacitive coupling...it doesnt take much.
Again good noise free bonding (if your lucky and get the right spot) can and has given up to 50dB of noise reduction without it. Sometime less of course. The BY3A apartment balcony was bonded back to the building rebar and gave around 40dB improvement signal/noise on 137, but only had a very small, maybe a dB increase in signal level.
Ive come to a conclusion to mount the probe away from the source of noises and "dumping" of the E field. In my case I get the probe as high and away so tend to have a 11m pole at around 45 degrees away from the building over the side - in some cases I prefer to do that (s/n wise) than mounting it 11m in the middle of an apartment roof. Ill always try to locate it on a corner and away from the building with as much clearance to the "real" ground for the greatest clear space angle. I cant help walking around towns now and view the skyline and each building and visualise how the LF E field signal is disrupted close in by these new high rises that now dominate some skylines.
Laurence KL 1 X
Enroute home.
> Date: Sat, 19 Dec 2009 08:49:52 -0500
> From: [email protected]
> To: [email protected]
> Subject: Re: LF: Noise cancelling by using optic transmission of RX signals
>
> James Moritz wrote:
>
> > I think the role of the ground with active whips is often neglected -
> > the output of the whip preamp is the voltage differential between the
> > whip element and the circuit ground, so the ground connection is just as
> > much part of the antenna as the whip element itself is. Clearly, an
> > electrically quiet ground is needed for the active whip, whether this is
> > provided by the coax feeder or a separate local ground connection.
>
> Years ago I built a simple MPF-102 active whip into a gutted VHF coil enclosure
> of a NMO mobile antenna. I quickly noticed the antenna performed much better
> when mounted on a vehicle roof than when mounted on a mast. Even when the whip
> was elevated up to 10 meters above the vehicle the roof mount received much better.
>
> As for the optical link, I attempted to link a remote VLF-3 by IR back to the
> shack. I never got that to work very well but I feel sure someone that knows
> what they are doing could perfect it.
>
> Dex
>
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