Bud, I had a real estate-constrained location when I was in UK. Solution
on 80m was 2 Force12 EF180BVs (37 foot tall, linear loaded) spaced 1/8
wavelength i.e. 30 feet. Didn't have room for extensive radials.
What I found was that laying a good number of radials between the bases of
the verticals in a diamond pattern (imagine the radials leaving the
vertical bases only in the 180 degree sector that lies in the direction of
the other vertical) gave me a 2-ele array that worked well. I did have a
ground line, bonded to the phasing box, to which I terminated the
between-the-verticals radials. Later I added some really short radials
(varying from 10-20 feet long) in the 'outer' 180 degree sectors and
noticed some improvement in performance. Total area occupied around 60
feet by 40 feet - so a 40 meter version would fit into the smallest of
lots.
Performance? Consistent top 5 finishes in CQWW single band 80 and 285
countries in ca. 18 months. (No beverages, loops etc on receive - just the
array). Great f/b to nullout Europe on the short path to the US. The
array wasn't great in the broadside direction, but still allowed me to work
all the countries I need off the side. I'd guess 3-9dB down.
A couple of details - I used the Lewallen method per ON4UN, with
guesstimates of the C and L values as I had no access to a vector analyser
or accurate noise bridge. The MFJ 259 was unhelpful, except as rough
esimate that each vertical had a similar impedance - the measured values
did not yield the right answers for the phasing componets. My technique
was to buid a xtal oscillator for the band, connect it to a mobile whip
antenna and park my truck about 5 wavelengths away in line with the
verticals. Then connected a trusty IC-735 at the phasing box and tweaked
the C and L for minimum S meter reading with the array firing away from the
truck/oscillator. A little crude, but effective. Similar should work with
the Christman method, but adjustment is tricky. Just using ON4UN's example
lengths of coax won't cut it - the impedances in your array will inevitably
be different than his, so you'll have some cut-and-try to do. That's why I
used the Lewallen, wth an Airdux coil/alligator clip and a couple of big
variable C's (don't forget to replace the clip with a hard-soldered
connection after setup if you pln to use power - I fried mine first time
out! And if as I did you use receive type variable Cs, replace them with a
combination of fixed Cs to the same as the adjusted value - and use
doorknobs or similar, there's a lot of current circulating.)
Biggest losses in the array are close in to the base of the verticals - I
used 3ft x 3ft sheets of aluminum at the bases, with holes every couple of
inches around the edge through which I bolted s/s hardware with solder tags
to attach the radials. I worked on the principle that since I could only
have short radials, I'd better have many of them - ended up with about 80
per vertical, all of 20ga wire - just kept adding them when I had spare
time in the yard.
Hope this helps
73
Dave
WJ6O
G3WGN, 80m op at M6T
----------
From: Bud Semon N7CW <n7cw@san.rr.com>
To: 'towertalk@contesting.com'
Subject: [TowerTalk] Advice on 2 el 40M Vertical Array
To: <towertalk@contesting.com>
Date: 16 January 2002 18:17
I'm looking for a small improvement in my 40M signal and have the following
thoughts and constraints. I'm currently using an 80/40M trapped vertical
(similar to the BattleCreek Special, without the 160M section) over a
limited
set of radials. The yard is fairly small and 80M radials can only cover
one
90 degree quadrant.
For the moment, I will do without 80M. I can squeeze in 2 40M verticals
with
1/4 wave spacing, but the radials would only cover about 270 degrees around
each vertical. If I use 1/8 wave spacing, I can pretty much cover 360
degrees.
Since I am mostly a DXer, with limited contesting and my location is in San
Diego, the most important directions for me are NE and SW, which is
convenient
for locating the verticals. I haven't seen a way to phase 1/8 wave
verticals
for a broadside pattern, but I don't really care too much about those
directions. Also, being in the most southwestern corner of the country, I
have
no QRM from the SW when I point NE (the ocean is out there) and little QRM
from
the NE when I point SW (this is the LP and the sun is up to the east of
me).
Consequently, I think working hard for the last couple of dB of F/B is not
worth it.
So....looking through ON4UN's book, the 1/8 wave spaced array with the
Christman coax feed method seems like a good fit. Does anyone have a
better
suggestion?
I also have a couple of specific questions. Are the impedance measurements
I
can make with the MFJ259 accurate enough for proper phasing calculations
(including the mutual impedance)? For radials that run between the 2
verticals, should I terminate them in a ground line that bisects the
verticals?
Thanks in advance for any thoughts and 73, Bud N7CW
n7cw@arrl.net
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
-----------------------------------------------------------------------
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
________________________________________________________________________
Where do you get ICE bandpass filters & beverage matching boxes? The
same place that pays for the hosting of this list: The eHam Store.
Order online at http://store.eham.net.
-----------------------------------------------------------------------
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|