Tom,
Built an interesting Labor Day special antenna...
Sort of an W8JK....
Two 140ft dipoles mounted 20ft above the ground and spaced 70ft apart. I feed
them in phase with HB open wire line. The antenna and feed line is made from 10
guage solid wire. I used 5.5inch spacing for the feedline (I had a box of new
old stock Johnson ceramic insulators that were begging to be put into use).
At the center of the feedline (35ft) I have a 20ft drop of the same open wire
line that terminates into a HB ultimate transmatch. I matched the feedling
using my MFJ259B at the input to the matcher (saw 52ohms with 0 xl and SWR
1:1at 7.220. The SWR at 7.130 and 7.310 is just under 2:1. So, it's a
set-it-forget-it antenna for my SSB phone operations.
I have an 80m dipole at 55ft that is also fed with open wire and I have been
using it on 40m for the past two years. It has been a super antenna. Reports
from 80m dipole on 40m have always been very good. Always better than my
full-wave loop in the city.
I read that this low to earth array would be better for short skip on 40m which
is what I was looking for since I work a lot of mobiles and nets on 40m. It's
called an NVIS (Near Vertical Incidense Skywave) antenna.
The NVIS array seems to work even better than I could have imagined. Typically,
the difference between the two systems for close-in stations (within 400miles)
is remarkable. The phased dipoles are about 5 -10bd better than the 80m dipole.
However, at 900miles out the dipole is "S"9 +10 and I can hardly be heard on the
NVIS (in the noise). I've never seen such dramatic differences in low band HF
antennas but it sure is fun to be a witness of very high angle radiation versus
lower radiation patterns.
I wonder if EZNEC would have predicted these results? I don't have any computer
modeling software so I have to wing it and believe me after spending 20hrs
hauling all that 10 guage wire around my property in the country I was pretty
anxious about the results, HAHA.
thoughts,
dave
wa3gin
-------------
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 96 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|