I have made a few postings about some of the changes I have been making here
on 160 meters. For many years, my transmit antenna was a single tower, with
a gamma match and about 45 radials over good ground. A couple of years ago,
I started playing with parasitic reflectors and got some useful pattern to
enhance my performance to Europe.
However, I found myself wanting more - and the limited frequency range of
the parasitics were a problem for me - as I really needed good performance
over about 50 kHz of the band (as I have to spend time up the band a lot
in contests due to QRM).
I finally decided to try the "impossible" - getting a two element phased
array to work with gamma matched towers. I say "impossible" - because that
is pretty much what the ON4UN book has to say on the subject.
However, I knew they could work under the right circumstances. Back around
1975, I dropped in on Earl, K6SE, after a 160 meter contest and he
demonstrated his phased array. He had a pile of coax (RG59?) laying on the
floor and as he moved a connector from one place to another, the signal we
were listening to changed strength. He had two towers, one with a 2 element
shorty 40 and another with some kind of tribander - and they were gamma
matched.
You can see the construction of the two elements in this shot:
http://www.kkn.net/~tree/gallery/Tower2009/Image003
The two towers are spaced 125 feet apart in a line going north/south. This
allows me to go end fire over the pole (or south) - or bi-directional
east/west. These are pretty useful configurations for contests as I often
want to be heard on the east coast and Japan at the same time.
One of the "secrets" to make this work was to make the new tower as
similar to the first one (electrically) as possible. I did this by
estimating that the tower height would have to be taller (the loading of a
5 element 20 and 5 element 15 isn't as much as a tribander). I ended up
with the tower being about 10 feet taller. I then put the same exact
gamma match on the second tower and looked to see where the best SWR was.
This test showed me that the tower wasn't tall enough yet - and I added
two wires coming off the top of it (opposite directions) and kept making it
longer until the SWR was best at the same frequency as the first tower. I
ended up with about 45 feet of wire off both sides as I remember.
Once this was done, I used a hyrbid coupler for phasing and then measured
the current amplitude and phase and found them to be pretty much what I
had hoped for. Doing antenna pattern tests driving around a few mile from
the antenna showed a nice pattern.
For awhile, I could detune the second element and go back to my single
vertical for comparison. Once I got comfortable that I indeed had gain
over the single yagi - I removed that feature.
Over the past few months, the performance of the antenna has been proven
in my operation on the air. When I test the pattern with DX stations, I
get very good results.
One of the "unexpected" benefits of this - is that having directivity with
the TX antenna enables me to use the diversity reception feature of the K3
to its fullest ability. I am getting 15 db of F/B which really cuts down
the noise coming from the south when listening to Europe over the pole.
Sometimes, signals are coming in better on the TX antenna than my EU
beverage.
You can listen in stereo to how this sounds here:
http://www.kkn.net/~tree/160/W7LRandSM4CAN_2Jan2010.mp3
If I get the time, I should probably write this up for the NCJ or something,
but wanted to get topband readers the scoop in case someone else is thinking
of doing this kind of thing. It can be done and it works great!! Feel free
to ask me about any details you might need more information on.
73 Tree N6TR/7
Boring, OR
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|