By ZL1ALZ, John - New Zealand
As I pointed out to Don,
N4UJW, when he suggested this article to be published, I'm very much
a "monkey see monkey do" amateur. Show me a picture, or a diagram and I'll
make it work. If It works I'm chuffed, if it doesn't I'm lost. Then I go
back to the technical guys with heaps of dumb questions.
This vertical started life as a tilt
over pole to support two inverted Vees at 55 feet.
I found a simple to use calculator at: http://www.csgnetwork.com/antennagpcalc.html
And settled on 7.060Mhz which calculated to a length of 10.102m.
The two galvanized water pipes I
used were sleeved together, the outer at 75mm OD and the inner at
65mm OD, not a tight fit, with an overlap of 1900mm. I drilled three
holes spaced around the top and three at about 1500mm down the outer pipe,
then welded nuts to the holes and simply clamped the pipes together with
bolts. I pre measured and marked graduations on the inner pipe for
quick adjustments later on. I now had two ways to adjust the
vertical. Firstly, the overall length by sliding the pipes in or
out, then clamping in position with the bolts. Secondly, the height
above ground at the feed point by raising or lowering the point at which
the vertical pivots.
I found many references as to
what should be used for radials. Insulated or bare wire? How long?
What size wire? In the ground? How deep? All very
confusing. So I went with what I had available. Being a
contractor on building sites, I use a good number of extension leads,
which, once damaged or cut, can't be used. I had 6 of the damaged
Eyes were soldered to the wires
and screwed to a copper strip with stainless screws.
The radials were laid on top of the ground initially. Once satisfied with the set up, I scalped the grass with the mower and cut a track with the spade, leaving the wire about 50mm below the surface.
A few rides on the ride-on
should cover them up in time.
This raised a number of possible scenarios: Overall length incorrect. Faulty coax. The extra length of the radials. The pipe overlap. All the steel in the frame. The bottom insulator (RF proof). Height above ground of feed point.
The length, coax and insulator were easily checked and confirmed, but I was not keen to cut the pipe or reduce the radials at this time. There was nothing I could do about the steel frame.
Making a major length adjustment had minimal affect on the readings. Attaching the steel frame to the earth/radials had no effect. Lowering the feed point 600mm to within 100mm above ground put me in the 30m band! I gave up on the TS440 and SWR meter and borrowed the MFJ analyzer. In desperation I coiled about 8 turns of the coax in a rough bunch, returned the pipe length to 10.200m. I now have the SWR of 1.6 at 7.030 MHz and 50ohms. By shortening the pipe this was raised to 7.060 MHz.
A search on the net for more information on how to feed the vertical lead me to N4UJW's site at: http://www.hamuniverse.com/balun.html
The HF Air Core 1:1 Choke or Ugly Balun was made with about 20 feet on a 100mm PVC former. The stand offs are 40mm PVC seen below in picture.
I have since buried the radials, which changed the SWR from 1.6 to 1.7. What made the profound change from being out of band to close the target frequency, the Balun or lowering the feed point? Maybe a combination of both? I'm sure someone will correct me, but I'm convinced 'ground mounted' means ground mounted. I still have to reduce surplus coax by about 10m inside the attic behind the shack, what affect will this have? This may be an over kill for a simple vertical. But I still have the option to use the support for other projects. There is about 8m of coax at the Balun and feed point, 28m under ground to the WARC band tower, 4m up the tower and a further 20m on a sky wire to the house with 3-4m in the roof space.
Firstly, a bit about the QTH. Area is not an issue as we live on 13 acres, the house and gardens are pushed into a 2-acre corner and I'm reluctant to go outside these two acres; the retirement fund. It's a rural area about 50km south from Auckland City. Unfortunately the main power supply for the city is almost on top of us. Wedged in between two main lines. One 400m to the south and the other 400m to the north, in fact the cables span the property. Noise on the lower bands is impossible at times and S9 is the norm.
I have been using an 80m dipole with 450-ohm open wire and 4-1 Balun for both 80 and 40 slung below the WARC beam.
On a receive comparison it was difficult to pick any noticeable noise level increase, how much worse can it get? However the signal strength did appear stronger on the dipole. I'm not really set up for switching between antennas on the same band as all the switching and tuning is through the fully automatic amp. During the CQWW SSB I was able to work at the high end of 40m for the first time. I have always been a bit of an "alligator station" on 40m, all mouth and no ears.
The next project is a receiving antenna for 40m. 73....John ZL1ALZ
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