On 09/06/2013 09:26 AM, ZR wrote:
I would think that at 6-12' spacing from the tower it would minimize
interaction on 160 or 80?
I don't know, Carl. I'll leave it to the experts. What I do know is I have
made several attempts to erect a vertical for 80 meters near my 160 meter
tower, using the same radial system. At 10 foot spacing from the tower,
the base resistance of an 80 meter quarter wave vertical was less than 5
ohms. That to me suggests significant interaction with the tower. At 5
foot spacing the base resistance was less than 2 ohms!
Well let's think about that.
10 feet on 160 is exactly like 1.5 inches on 2 meters.
Anyone really think a two meter whip 1.5 inches away from another antenna
or tower would have minimal interaction?
Actually, 10 feet isn't even minimal interaction on six meters, let alone on
160 meters.
I've been all through this collinear stuff and skirt collinear vertical
stuff trying to multiband vertical antennas. I even had a 100 foot insulated
base tower with a 33 foot skirt, trying to feed it like a collinear on 40
meters. I tried things with WXEZ FM when they had a 355 foot tower in a
swamp, I tried things on my towers.
All of that stuff goes into the bag of "it was a good sounding idea, but a
big waste of time". The reality check, even if you have an insulated tower,
is:
1.) Really low wave angles stink on 160 meters most of the time
2.) There isn't that much gain there, because we are forcing the field down
against earth. Saltwater would be a different thing, as would freespace.
3.) There isn't that much gain there, because the extra antenna is forcing a
null (removing power to use at low angles) where a null already exists
4.) As I recall, the optimum current ratio in my 40 meter skirt collinear
vertical was around 1.5 or 2:1, with most of the current in the 33 foot
skirt. Despite all the effort, it worked much worse than a 40 meter dipole
at 80 feet.
5.) My insulated base 318-foot tower was such a useless 160 antenna, I
removed and discarded the insulator. It's just a tower now, to hold up
horizontal and VHF/UHF antennas. This is the same stuff that happened to
W8LT with their balloon or kitetoon antennas over acres of aluminum sheet
groundplane.
73 Tom
_________________
Topband Reflector
|