At 05:28 AM 11/10/96 +0000, Casey Conway <email@example.com> wrote:
>I am not too versed on radials for verticals. In installing my HF6V on
>the roof(chimney mount) some of the radials will not be laid flat on the
>roof but rather elevated because the roof peak is higher than the chimney
>mount of the antenna. The specs say you can have the radials lower than
>the antenna (45-90 degrees) but what happens if they are higher ? Any
>help would be appreciated.
Reply privately and I will pose a summary if anyone is interested.
I want to reply on the reflector since I hope I can share some of my
experience with others who may be lurking here and in need of the same help
you've been looking for.
I'm not immediately familiar with the HF6V but ALL verticals with radials
work the same. The vertical portion of the antenna is an electrical 1/4
wavelength on each band. Any single additional radial is also about a 1/4
wavelength on each band. (The radials may actually be sligltly longer- say 5
to 10%). Anyway, it would be helpful to visualize the "vertical" as a
horizontal dipole antenna, rotated 90 degrees into the vertical position and
the lower half of the antenna is now being extended in the horizontal direction.
Let's assume for a moment that your dipole is horizontal. We'll further
assume that at operating height the feedpoint impedance is 72 ohms. (This is
going to vary depending on the antenna's height above actual electrical
ground and several other factors. But let's not belabor the point.)
Now by rotating the horizontal dipole into the vertical position the
feedpoint impedance stays about the same. Interesting!
Now, if you insert and impedance bridge at the feedpoint and make some
measurements you'd find that as you raise the lower portion of the dipole
approaching 90 degrees (straight out horizontal) that the feedpoint
impedance keeps getting lower. At about the 45 degree position from the
bottom you will be very close to a 50 ohm match. As you continue on to about
the horizontal position, if memory serves me correctly, you may get close to
20 ohms- but here I'm not so sure. (If instead of using radials you mounted
the vertical portion on a very large metal sheet that was at least a 1/4
wavelength in radius, the 20 ohm feedpoint number may be accurate.)
Anyway, most groundplane installations call for at least three radials per
band. Obviously, if they're all not at the same angle to the vertical
element the feedpoint impedance (on that band) will be some kind of average
between all three angles. I rather doubt that raising a radial above the
horizontal position will do anything useful for you- but detrimental, I have
So, what's a body to do? Easy! If you're using a chimney mount- a) inspect
the mortar for cracking and age. b) inspect the chimney for rigidity, c) if
a) and b) are satisfactory- get a longer and sturdier mast that elevates the
base of the vertical higher or...
Do yourself a big favor and get as far away from that chimney as possible.
The effluent won't help your antenna any and the added side loads acting
through a longer arm of a taller mast won't do much for the life of the
chimney or the roof below it when it falls.
Instead I recommend a simple peaked roof mount- but not the small one Radio
Shack sells. There are bigger ones that don't cost much more that are
better. I've used all kinds of verticals- trapped and otherwise for over
forty years and
in spite of the humerous nickname used here on the reflector of "dummy load
on a stick"- when properly installed, you'll get a lotta bang for your buck.
Now go have fun! I hope I've been helpful.
Roger Elowitz, K2JAS
Morganville, NJ (about 35 mi. SW of NYC)
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