[AMRadio] Loop antenna feed question


W5OMR/Geoff w5omr at satx.rr.com
Sun Dec 4 20:56:10 EST 2005


crawfish wrote:

>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 
power down.

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,
-Geoff/W5OMR




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