[HCDX] Re: coax antenna?

["Guy Atkins" <dx@xxxxxxxxxxxxx>]

Hi Patrick,

Two thoughts occur to me on this. The first is that your coax must have been
compromising the directionality of your NNW Beverage when you still had the
antenna wire hooked up. See John Bryant's article for info on this:
www.dxing.info/equipment/coax_leadin_bryant.doc  or
www.dxing.info/equipment/coax_leadin_bryant.pdf  I've used John's techniques
to improve the shielding of my own Beverage antennas.

There is a type of antenna using just coax on the ground called the snake
antenna, which you've probably heard of. It seems to work for some, and not
for others. The only reason a "snake" antenna might work is due to common
mode noise transferring from the shield to the inner conductor. Tom, W8JI,
says it best at his highly regarded web site: "A receiving example of an
antenna that works because of common-mode excitation is the "Snake" antenna.
This system accidentally (or intentionally) induces common-mode on a cable
shield in order to receive signals. The entire shield picks up signal, the
Snake is simply a reverse-fed random wire lying on the ground."  See
http://www.w8ji.com/common-mode_noise.htm  Tom also says: "If a feedline is
very long and lies directly on or is buried in the earth,  ground losses can
attenuate conducted noise or unwanted common-mode signals. Unfortunately, we
almost never know if the feedline shield is contributing noise, because it
is nearly impossible to measure the common-mode noise contribution of the
feedline!"

So, I think you have an accidental randomwire on the ground, and the signal
is leaking from shield to inner conductor. 200 feet isn't very long (for
directionality along the wire axis) in comparison to even a quarter
wavelength on the MW band, so it seems reasonable to me that you might have
various lobes from such a short length.

If you're going to use a randomwire on the ground, it would be better, I
think, to connect the shield directly to your receiver antenna input. Signal
strengths would be better, I'd guess, as you wouldn't be relying on leakage
between shield and conductor.

Guy Atkins
Puyallup, WA


> -----Original Message-----
> From: Patrick Martin [mailto:mwdxer@xxxxxxxxx]
> Sent: Monday, February 21, 2005 11:56 AM
> To: am@xxxxxxxxxxx; hard-core-dx@xxxxxxxxxxxxx
> Subject: [HCDX] coax antenna?
>
>
>
>
> I have discovered an interesting "antenna" I have purely by accident. I
> think I mentioned I lost my NNW beverage due to the neighbors taking
> down the trees. Well the two hundred feet of coax running West is still
> out there buried in the ground. I hooked it up to the R8 and guess what?
> It has a very strong lobe to the North!! Why I haven't a clue. Infact
> during the day CFUN 1410 Vancouver is listenable on it. No other antenna
> picks it up that well. I notice it on other Vancouver stations too.
> However, there is one big problem with it. There is a very high
> "whoosing" noise level on the bottom part of the band up to about 1200
> khz. It sounds like a bad ground type thing. Not man made noise. It is
> like one of the conductors is not connected type noise. A solid noise
> that does not vary in strength. Now at night I don't notice it as the
> signal levels over power the noise.  I would really love to use this
> coax antenna. Do you know how I can get rid of that bad ground type
> noise? I wonder if I plugged in a matching transformer at the end would
> take care of it? Any thoughts. It sounds rather interesting as the
> signal levels are pretty decent if it wasn't for the background noise. I
> find the coax running West would be that directional to the North, but
> also my 40 foot vertical seems to have some Northerly directional
> properties too.  Any thoughts? Thanks.
>
> Patrick
>
> Patrick Martin
> Seaside  OR
> KAVT Reception Manager



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