Team
I have had excellent success on DXpeditions by simply spacing a pair of
verticals a quarter wavelength apart and connecting them with a 3/4
wavelength (at the frequency of interest) 50 ohm coax. Please note that a
length of coax which produces only one electrical 1/4 wave length wont work
because its *physical length* in the real world will be less than the 1/4
wavelength real world physical length needed to actually connect to the two
verticals. The shortest length of coax in the real world that will work for
a pair of verticals spaced 1/4 wavelength apart and provide the needed 90
degree phase shift is 3/4 wavelength. This wasn't obvious to me 40 years ago
until I actually tried it :-)
In the most simple phased vertical set up wherein the two 1/4 wavelength
spaced verticals are excited at one vertical and both verticals are
connected together with a 3/4 wavelength coax they will produce a cardioid
pattern in one direction along the line formed by the verticals. When the
array is fed from the other vertical it will form a cardioid pattern in the
opposite direction. If both verticals are fed with equal lengths of coax and
the 3/4 wavelength phasing line between the two is disconnected then the
array will exhibit a figure of eight pattern with the lobes orthogonal to
the direction of the cardioid patterns. This simple setup worked great for
me when I was chasing European DX from California on 80M years ago and I was
primarily interested in one direction.
There are better (but more complicated) ways to feed and to change phasing
for a pair of driven phased verticals using relays, multiple phasing lines,
tuned elements etc but the above technique is pretty simple and has worked
well for me to get enhanced propagation in desired directions and to reduce
interference from undesired directions.
By keeping the array relatively simple everything can fit in a two suit
suitcase for budget mini-DXpeditions if for example one uses pairs of mobile
whips and RG-58. Some assembly required. When operating from V2A, 8P6, ZF2,
VP9, VK3 etc over the years with elevated pairs of phased verticals and
elevated radials I was able to easily differentiate between stations in the
US and Europe for example using simple pairs of phased verticals and phasing
lines. I was able to run 500 watts into the phased verticals with my ancient
modest DXpedition Heathkit SB-200 using just lengths of RG-58 and pairs of
Hustler standard resonators on a pair of Hustler mobile mounts. Years ago
when chasing 5BDXCC I was also able to relatively easily work and confirm
over 100 countries on 80 meters both SSB and CW from California by using a
pair of elevated Butternut HF-2V verticals spaced a quarter wavelength apart
and having 3/4 wavelength sections of RG-8 connecting them. The difference
for me between the phased vertical performance and that of my 80M inverted V
performance with its apex at 72 feet was usually that of working European DX
stations on 80M with just one or two calls or not working them at all during
grey line. Working European stations on 80m from California was a challenge
for me.
I hope this is helpful. Enjoy!
73, Ted, K6XN
Ps FYI we are constructing a four square steerable phased array for 75M at
our Sierra summer home in CM99 this summer and we hope to have it
operational by the end of the summer. All the antennas and phasing lines and
radials and RG-25G tower sections etc are on site but we still have more
trees to clear and concrete bases to put in. We are using four modified
Butternut HF-2V vertical antennas each elevated 20 feet above the forest
floor. Each Butternut HF-2V vertical antenna will have four elevated radials
approximately 67 feet long. The last 75M four square array we built at a
different location using elevated vertical antennas and elevated radials
worked *great*. With each vertical individually tuned to 3.6 MHz the entire
array of four elevated verticals resonated at approximately 3.8 MHz and
performed even better than expected. I will provide the group with more info
on this phased array once we have it up and operational. Right now there is
still 7 feet of snow on the dirt road to our summer wilderness QTH in the
Sierras.
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Lux
Sent: Thursday, April 06, 2006 10:55 AM
To: Bill; towertalk@contesting.com
Subject: Re: [TowerTalk] No Explanation
At 07:30 AM 4/6/2006, Bill wrote:
>Tor N4OGW Stated
>"Also note that the phasing for the cardioid array shown in that
>report is incorrect. You can't just add a "delay" line of 1/4 wavelength
>to get 90 degree phasing." and
>
>"Two verticals, 1/4 wave spacing, 90 degrees out of phase."
>
>withour any explanaiton.
>
>Please explain Tor. It has worked for me.
>
>Thanks
>Bill W7VP
There's some examples of this in the ARRL antenna book, with lots of
examples of what kind of pattern you get with "causually designed feed
networks". Forward gain isn't much affected, but the depth of the null is.
If you have two verticals a quarter wave apart, and you feed one with a
transmission line that is 90 degrees longer than the other, you *might* get
the right relative current phases and magnitudes and you might not. The
antennas are close enough that the mutual impedance is almost as big as the
self impedance. There IS a cookbook length for quarter wave spaced
verticals that achieves the right phasing and magnitudes called the
Christman phasing. Using 50 ohm feedlines, one antenna gets a 84 degree
long line and the other gets a 161 degree long line, they're joined at a
T. However, I suspect that if your antennas are non-ideal, it might not
work as well. For instance, if the loss is higher, the effect of the
mutual coupling is less.
Jim
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