[Lowfer] antenna location (K2ORS remarks)

[k2ors at verizon.net]

Bill,
   Good point about the input capacitance.

73 Warren K2ORS
          WD2XGJ
          WD2XSH/23
          WE2XEB/2
          WE2XGR/1


Dec 10, 2010 09:16:38 PM, lowfer at mailman.qth.net wrote:


Steve,

In order to get a good feel of what the input Z characteristics are like one has to understand that the input Z of the typical E-probe has a lot to do with the input capacity of the FET amplifier, which is usually about 10pf.  If for example you are operatining at 185KHz a 10pf capacity would be a reactance of approx 100K ohms, so you are correct in stating that the input Z is not 100s of megs but less than 100s of Kohms. Only at very low frequencies would the FET probe exhibit 100s of Megs (assuming an input resistor very high in resistance).

Bill



> From: steveratz at wildblue.net
> To: lowfer at mailman.qth.net
> Date: Fri, 10 Dec 2010 17:27:54 -0800
> Subject: Re: [Lowfer] antenna location  (K2ORS remarks)
> 
> I take strong exception to Warren's remarks. Common mode chokes can be very 
> effective in reducing or eliminating noise fed from the shack to the 
> antenna.
> 
> Here are some comments that Jack Smith K8ZOA of Clifton Labs also had to 
> say:
> 
> "Warren misunderstands the relative impedance points and how a common mode 
> choke works. While the whip amp impedance is high (and it's not hundreds of 
> megaohms, but more like a few hundred k-ohm to a few hundred ohms, depending 
> on frequency) but the coax shield against ground is around 300 or 400 ohms. 
> And the idea is to decouple the coax shield acting as a single wire 
> transmission line against ground. A few k-ohms is adequate.
> It's often better to think of an active E-field antenna as an off-center 
> dipole with the mast and coax out to the common mode choke as one side of 
> the dipole and the whip the other side."
> Jack
> 
> 73,
> Steve AA7U
> NE Oregon
> 
> ----- Original Message ----- 
> From: 
> To: 
> Sent: Friday, December 10, 2010 4:05 PM
> Subject: Re: [Lowfer] antenna location
> 
> 
> > Doug,
> >
> >  Jay and I were talking about locating  e-probes a few days ago - Jay 
> > pointed out something I hadn't thought about  - the impedance of an 
> > e-probe is so very high that it is impossible to isolate the feedline from 
> > the antenna.
> > Thus with any e-probe the coax is part of the antenna circuit, you just 
> > can't get any significant isolation by using ferrite chokes on the shield 
> > etc compared to the hundreds or thousands of megaohms input impedance of 
> > the e-probe a ferrite choke on the coax will present an insignificant 
> > impedance.
> >
> > A longer (or higher) run of coax will have a major effect on the 
> > performance of the antenna.
> >
> >
> >
> > 73 Warren K2ORS
> >          WD2XGJ
> >          WD2XSH/23
> >          WE2XEB/2
> >          WE2XGR/1
> >
> >
> > Dec 10, 2010 06:40:12 PM, lowfer at mailman.qth.net wrote:
> >
> > ===========================================
> >
> > Years ago, when I first started using an active e-probe antenna for
> > LF/VLF receiving, I placed it on a ten foot TV mast attached to a vent
> > pipe on my roof. The results were underwhelming, as noise (QRM) from
> > devices inside my house was quite strong. It wasn't long before I
> > moved my antenna to a spot in my yard about 50 feet from my house,
> > which produced much better results, and that has been my receive
> > antenna location for the various antennas that I have used over the
> > years. A couple months ago, we had a metal roof installed over the old
> > asphalt shingles. It just struck me today that the new metal roof
> > might provide an RF barrier for the QRM. I wouldn't mind having a
> > second receive antenna in place. Do you guys think I should try
> > putting one on my roof again, or should I just find another spot in my
> > yard?
> >
> > 73,
> > Doug KB4OER
> 
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