[Lowfer] Alien e-probes

[Bill Ashlock]

Hi Steve,

I agree with your analysis of the first (tall) hotel. The 100ft by 100ft 
profile is small compared to a wavelength and the true ground plane is 
actually the ground; not any part of the hotel. There would be a large 
increase in signal compared to the signal measured at ground level, however, 
due to the 1000ft effective height of the antenna. Based on my experiments 
this increase is approximately equal to the length of the antenna (assume 1 
meter) divided into the total height above ground (~312 meters). I believe 
this effect is due to probing the changing E-field at two locations 312 
meters apart. The instantaneous voltage induced is proportional to this 
distance as long as it is much less than a wavelength. I therefore conclude 
that VO1NA's signal at ground level would be approx 100uv/321 = .32uv and 
assuming a 1 meter antenna .32uv/m.

The second hotel has such a large profile that its structure would act as 
the ground plane for the signal. I therefore believe that the .32uv/m 
E-field would produce a .32uv signal on the top floor.

Let me know what you think.

Bill A


>From: Steve Dove <dsp@hifidelity.com>
>Reply-To: lowfer@mailman.qth.net
>To: lowfer@mailman.qth.net
>Subject: [Lowfer] Alien e-probes
>Date: Sat, 13 Sep 2003 01:23:28 -0000
>
>Hi Bill,
>
>Well, I was beginning to think I quite fancied Laurence's gig what with all 
>the flash hotels
>and such, until the copper sheathing showed up.  Anyway, despite knowing 
>better than
>to rise to the bait . . . .  Coming up with hard numbers would involve 
>making up some
>arbitrary assumptions about the whip, input impedance of the sense 
>amplifier etc.
>
>The short answer is not much difference between any of the three 
>conditions.
>
>A longer answer, third case first:  The mondo-hotel of the third condition 
>is big enough to
>start to acting like a ground plane, so the only conditions affecting 
>sensitivity are the
>basic ones of whip reactance vs. amplifier input impedance;  no return line 
>impedances to
>take into account.
>
>The first two cases are a straightforward choice between the return current 
>finding
>ground via a skinny conductor (coax outer) or a fat one (building steel).  
>The antenna
>sensitivity is largely defined by the very high reactance of the 
>electrically very short whip
>vs. the amplifier input impedance;  the ground return in this circumstance 
>cannot help but
>be comparitively lower impedance, and won't affect this much.  That said, 
>1000 feet is a
>noteworthy proportion of the wavelength (1/7-ish), so whichever return is 
>going to show
>some non-negligible impedance, with the skinnier wire likely a bit more 
>reactive than the
>fatter one, so there may be a (probably very) minor sensitivity variation 
>between them and
>a small affect on gain overall, though probably just fractional-dB-ish.   
>(At higher
>frequencies, where the whip is far longer with respect to wavelength so 
>lower in
>impedance, and with the feedline often measurable in wavelengths, this 
>'feeder/antenna'
>gain effect is quite demonstrable, as described the other night.)
>
>So all three would behave similarly, like antennas 1000' up, and  I want 
>one.
>
>You know, I might just toddle off and model this, unless some other sicko 
>already has.
>
>         Cheers,
>
>                 Steve
>
>
>
>
>_______________________________________________
>From the Lowfer mailing list
>Send messages to: Lowfer@mailman.qth.net
>To sub/unsub visit: http://mailman.qth.net/mailman/listinfo/lowfer

_________________________________________________________________
Compare Cable, DSL or Satellite plans: As low as $29.95.  
https://broadband.msn.com