[Lowfer] antenna location (K2ORS remarks)

[jrusgrove at comcast.net]

Good discussion ... but I believe two separate topics somehow got mixed together - 1) noise 
mitigation involving the feedline and 2) divorcing the feedline between an elevated e probe and 
ground level.

Warren and I were discussing the VLF  part of the spectrum,  the 'mini whip' antenna in particular 
and how it is virtually impossible to divorce the length of feedline between an elevated e probe and 
ground level.

One can prove that the feedline between an elevated e probe and ground level is indeed part of the 
antenna. Pick a signal and measure received signal strength with the e probe mounted right down at 
ground level and then watch the signal rise dramatically as the e probe is elevated. This is why 
most folks mount the popular 'mini whip' up 20' or more. Try mounting a 'mini whip' down at ground 
level - I've done it and it's unusable.

On the subject of chokes  ... one could install a choke that *might* develop sufficient choking 
action in the hf range (where the impedance is 10's of thousands of ohms) in order to prevent the 
connecting feedline above ground level from acting as part of the antenna  ... but at VLF (where the 
impedance is megohms), the task becomes much more difficult.

Noise mitigation is a whole different discussion involving the feedline is a whole different 
discussion.

Jay



----- Original Message ----- 
From: "Steve Ratzlaff" <steveratz at wildblue.net>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Friday, December 10, 2010 8:27 PM
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: <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
>
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