|[AMRadio] Loop antenna feed question|
w5omr at satx.rr.com
Sun Dec 4 20:56:10 EST 2005
>LB Cebik, W4RNL, on his web site, talking about Cloud Warmers, indicates 35
>feet as the accepted height for a 75m loop. I am planning a loop for 160m at
>just a little over that height.
> Joe W4AAB
Does he say -why- having the antenna no more than 1/8th wave above
ground is better than having it a quarter wave up?
I had it explained to me one time like this (strictly dipole speaking)
Picture in your mind, a conventional half-wave, center fed dipole.
Using each end as the outside points, draw a circle around the antenna.
The dipole ends make the width across the circle.
The height of the circle, from ground, would be the same as the length -
1/2 wavelength long. In the neighborhood of 3.880, that length
calculates out to 120' (468 / 3.9Mc = 120')
So, with the picture of the circle around the dipole in your mind,
mentally change the circle to a 'wheel', that is your signal. If your
antenna isn't *at least* 1/4 wavelength above ground, your wheel is
stuck in the dirt, trying to radiate freely.
Makes sense to me, that way - and as long as any part of your antenna is
up at 60' or above, you'll find you've got a better-than-average signal
coming from the antenna with comparable power levels.
So, I've said all that, to ask this question:
What's the difference in half-wave dipoles and their height above
ground, and a full wave loop
( L(ft) = 1005 / f(Mc)
Using 75m as an example:
L(ft) = 1005 / 3.9
L(ft) = 257.7ft )
operating at half-height, as a cloud warmer?
I Have a loop up, except that this loop started life as an inverted Vee
on 75m. The Apex of the antenna is around 60'. This is not where the
feed point is. Because my 51' crankup Tri-Ex tower is buried in the
ground in the -front- yard (I inheirited it from my dad, who set it up
when I was still living on the Left coast) in order to fit the antenna
in the lot, the feed point has to be pretty much over the house, and the
antenna support off-center. After changing from coax to ladder line, it
was just a matter of closing in the bottom of the 75m Inverted Vee. The
rope that holds the apex of the antenna in the air (via a pulley
attached to a 40' push-up mast, in the middle of the tower) angles down
to the yard-separating fence, which pulls the bottom part of the antenna
away from the tower by about 10'. Over all length of the antenna is
somewhere around 240 and 260'. I didn't measure. 60' of open-wire
feed-line forgives many measuring sins. I am using a Heathkit SA-2060A
2kW impedance matching device, which has a nice, -big- torroid balun
that directly feeds the open-wire output.
It is curious to note that I now find I don't need to run more than
around 100w to be only 3db weaker than those stations running 500w or
more, of carrier output. 200w of carrier from my rig into this antenna
has me a bit better than equal signal strength in the Northeast
(according to WA1HLR) as compared to the 'power-houses' on 3.885 in the
EARLY mornings. Since I have a bad case of neighboritis, I keep the
This loop is known to be called a Delta loop (looks like a big
triangle), with the apex of the antenna around 60', the southwest end
supported about 20~25 off the ground, and the northeast end about 10~15
off of the ground, and it is a full-wave loop antenna for 75m, except
that it's on a Vertical plane, as opposed to a big ol' square cloud-warmer.
I like to describe it best, by saying "If you've got room for a 75m
Inverted vee, then you've got room for a delta loop antenna"
The angle of radiation is -much- lower, but theres still enough
high-angle stuff to work the close-in stuff, and because it -is- a loop,
it's quieter than a dipole.
The futzing and fretting over whether or not the ladderline legs should
be spread out or not? pshaw... it don't need to be perfect - it just
needs to have power fed to it.
What I'd -really- like to try, is to utilze my neigbors two 70' tall
Pecan trees, run a straight line between them, then bring the two lines
down in an inverted delta position, and feed it where the antenna points
to the ground. But as I've stated... I have neighboritis. I can't
convince them that by raising the lightbulb further away from the area
that's being illuminated, the less light there will be to shine on the
affected area. *shrug* The same argument could be used against
Covenance cases... the higher the antenna, the less likely it will
interfere with something, close-in. Another good argument for having
your half-wave dipole at least 1/4w above ground...
Try it. You might be surprised.
73 = Best Regards,
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