[Lowfer] antenna location (K2ORS remarks)

[Steve Ratzlaff]

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: <k2ors at verizon.net>
To: <lowfer at mailman.qth.net>
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