[AMRadio] Loop antenna feed question


Jim Candela jcandela at prodigy.net
Sun Dec 4 17:43:03 EST 2005


John,

   I to am a novice at this stuff. I'm having trouble
visualizing your antenna. You said, 

>"the fence - of about 
> 450 feet diameter, and nearly (but not quite)
> square."

Could you clarify this to me?

  I too have a horizontal loop. Mine is about 15' off
the ground, and has about 240' of wire (roughly 4 X
60'). I feed it with 450 ohm ladder line to the
ground. My antenna is resonant at about 3.9 Mhz, and
the impedance at SWR minimum (1.5:1) is about 80 ohms.
I measure this right on the ground between the ladder
line and my DPDT knife switch (I ground it out when
not in use). I  then use a 1:1 current balun, and
RG-8X coax to the shack.

   My first try was to convert from 450 to 300 ohm tv
twin lead at the hole in the wall, and go straight to
a Johnson 275 matchbox with 300 ohm twin lead. This
was dismal since the Johnson matchbox doesn't seem to
have a wide tuning range to feed 80 ohms balanced. It
did work unbalanced however, but poorly. So I switched
to coax after a 1:1 balun for the run inside. If I do
it again, and intend to feed this antenna at
resonance, 75 ohm coax straight all the way might be
an option.

   This loop loads up like a SOB, and absolutely NO RF
in the shack. The problem though is that my signal
reports are usually poor (often) to great (once in a
while). It seems that for this to work efficiently as
a low horizontal loop (high angle radiator) for short
skip, I need  good soil conductivity, or counterpoise.

Stations beyond 500 miles can hardly copy me, and yet
I can sometimes be full scale at 150 to 200 miles out
early in the evening.

Regards,
Jim
WD5JKO

--- John Lawson <jpl15 at panix.com> wrote:

> 
> 
>    Well - my Valiant is putting out stable rated
> power on all bands - this 
> morning I'll suss out where the audio is
> disappearing to - should be ready 
> for Air then...  now to turn to the Radiator.
> 
> 
>     I'm still in the 'steep' part of the EasyNEC
> learning curve - and I've 
> gone through the several ARRL antenna books - so I'm
> properly confused.
> 
>   I have a loop of 14ga hard copper wire around the
> rear of my propery, 
> attached (with insulators spaced at abt 8' apart) to
> the fence - of about 
> 450 feet diameter, and nearly (but not quite)
> square. It is about 5' above 
> the ground, except for one leg which is attached to
> the house, where it is 
> about 7'.  I am feeding it with 450-ohm ladder-line.
> My Question to the 
> Antenna Gurus is this:  At the feed-point, should I
> maintain the 
> ladder-line spacing where it joins the loop ends, or
> should there be an 
> angle, ie. should the loop ends be wider than the
> feedline pitch - and if 
> so, what should the included angle of the feedline
> 'transistion' be...?
> 
>    I've approached this from two or three analytical
> directions, and of 
> course have different (and somewhat exclusive)
> "answers"....  so I'm 
> Learning, which is a good thing.
> 
> 
>     Cheers
> 
> John  KB6SCO
> 
> 
>
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